CS333 Programming Project 5: User-Level Processes

Description

Overview and Goal
In this project, you will explore user-level processes. You will create a single process, running in its
own address space. When this user-level process executes, the CPU will be in “user mode.”
The user-level process will make system calls to the kernel, which will cause the CPU to switch into
“system mode.” Upon completion, the CPU will switch back to user mode before resuming execution of
the user-level process.
The user-level process will execute in its own “logical address space.” Its address space will be broken
into a number of “pages” and each page will be stored in a frame in memory. The pages will be resident
(i.e., stored in frames in physical memory) at all times and will not be swapped out to disk in this
project. (Contrast this with “virtual” memory, in which some pages may not be resident in memory.)
The kernel will be entirely protected from the user-level program; nothing the user-level program does
can crash the kernel.
Download New Files
The files for this project are available in:
http://www.cs.pdx.edu/~harry/Blitz/OSProject/p5/
Please retain your old files from previous projects and don’t modify them once you submit them.
You should get the following files:
Switch.s
Runtime.s
System.h
System.c
Project 5 Operating Systems
Page 2
List.h
List.c
BitMap.h
BitMap.c
makefile
FileStuff.h
FileStuff.c
Main.h
Main.c
DISK
UserRuntime.s
UserSystem.h
UserSystem.c
MyProgram.h
MyProgram.c
TestProgram1.h
TestProgram1.c
TestProgram2.h
TestProgram2.c
The following files are unchanged from the last project and you should not modify them:
Switch.s
Runtime.s
System.h
System.c — except HEAP_SIZE has been modified
List.h
List.c
BitMap.h
BitMap.c
The following files are not provided; instead you will modify what you created in the last project. Copy
these files to your p5 directory, so that you keep the previous p4 versions in your p4 directory, and
modify the new copies.
Kernel.h
Kernel.c
Merging New “File Stuff” Code
For this project, we are distributing additional code which you should add to the Kernel package.
Please add the material in FileStuff.c to the end of file Kernel.c. It should be inserted directly before
the final endCode keyword.
Project 5 Operating Systems
Page 3
Also, please add the material in FileStuff.h to the end of file Kernel.h. It should be inserted directly
before the final endHeader keyword.
This code adds the following classes:
DiskDriver
FileManager
FileControlBlock
OpenFile
You will use these classes, but you should not modify them.
There will be a single DiskDriver object (called diskDriver) which is created and initialized at start-up
time. There will be a single FileManager object (called fileManager) which is created and initialized
at start-up time. The new main function contains statements to create and initialize the diskDriver and
the fileManager objects.
FileControlBlock and OpenFile objects will be handled much like Threads and
ProcessControlBlocks. They are a limited resource. A limited supply is created at start-up time and
then they are managed by the fileManager. There is a free list of FileControlBlock objects and a free
list of OpenFile objects. The fileManager oversees both of these free lists. Threads may make
requests and may return resources, by invoking methods in the fileManager.
The diskDriver object encapsulates all the hardware specific details of the disk. It provides a method
that allows a thread to read a sector from disk into a memory frame and it provides a method that writes
a frame from memory to a sector on disk.
Other Changes To Your Kernel Code
Please make the following changes to your copy of Kernel.h:
Change
NUMBER_OF_PHYSICAL_PAGE_FRAMES = 27 — for testing only
to:
NUMBER_OF_PHYSICAL_PAGE_FRAMES = 100 — for testing only
Change
–diskDriver: DiskDriver
–fileManager: FileManager
to:
diskDriver: DiskDriver
fileManager: FileManager
Project 5 Operating Systems
Page 4
Add a function prototype for the function InitFirstProcess. You can add it after the other function
prototypes:
Change
ProcessFinish (exitStatus: int)
to:
ProcessFinish (exitStatus: int)
InitFirstProcess ()
Please make the following changes to your copy of Kernel.c:
Change the DiskInterruptHandler function from:
FatalError (“DISK INTERRUPTS NOT EXPECTED IN PROJECT 4”)
to:
currentInterruptStatus = DISABLED
— print (“DiskInterruptHandler invoked!\n”)
if diskDriver.semToSignalOnCompletion
diskDriver.semToSignalOnCompletion.Up()
endIf
Task 1:
Your first task is to load and execute the user-level program called MyProgram. Since the user-level
program must be read from a file on the BLITZ disk, you’ll first need to understand how the BLITZ disk
works, how files are stored on the disk, and how the FileManager code works.
MyProgram invokes the SystemShutdown syscall, which you’ll need to implement.
Task 2:
Modify all the syscall handlers so they print the arguments that are passed to them. In the case of
integer arguments, this should be straightforward, but the following syscalls take a pointer to an array of
char as one of their arguments.
Exec
Create
Open
This pointer is in the user-program’s logical address space. You must first move the string from userspace to a buffer in kernel space. Only then can it be safely printed.
Project 5 Operating Systems
Page 5
Also, some of the syscalls return a result. You must modify the handlers for these syscalls so that the
following syscalls return these values. (These are just arbitrary values, to make sure you can return
something.)
Fork 1000
Join 2000
Exec 3000
Create 4000
Open 5000
Read 6000
Write 7000
Seek 8000
For this task, you should modify only the handler methods (e.g., Handle_Sys_Fork, Handle_Sys_Join,
etc.) You should not modify SyscallTrapHandler or the wrapper functions in UserSystem.
Task 3:
Implement the Exec syscall. The Exec syscall will read a new executable program from disk and copy
it into the address space of the process which invoked the Exec. It will then begin execution of the new
program. Unless there are errors, there will not be a return from the Exec syscall.
The User-Level View
First, let’s look at our operating system from the users’ point of view. User-level programs will be able
to invoke the following kernel routines:
Exit
Shutdown
Yield
Fork
Join
Exec
Create
Open
Read
Write
Seek
Close
(This is the grand plan for our OS; these system calls will not be implemented in this project.)
Project 5 Operating Systems
Page 6
These syscalls are quite similar to kernel syscalls of the same names in Unix. We describe their precise
functionality later.
A user-level program will be written in KPL and linked with the following files:
UserSystem.h
UserSystem.c
UserRuntime.s
We are providing a sample user-level program in MyProgram.h / .c.
The UserSystem package includes a wrapper (or “jacket”) function for each of the system calls. Here
are the names of the wrapper functions. There is a one-to-one correspondence between the system calls
and the wrapper functions.
System call Wrapper function name
Exit Sys_Exit
Shutdown Sys_Shutdown
Yield Sys_Yield
Fork Sys_Fork
Join Sys_Join
Exec Sys_Exec
Create Sys_Create
Open Sys_Open
Read Sys_Read
Write Sys_Write
Seek Sys_Seek
Close Sys_Close
(In Unix, the wrapper function often has the same name as the syscall. All wrapper functions have
names beginning with Sys_ just to help make the distinction between wrapper and syscall.)
Each wrapper function works the same way. It invokes an assembly language routine called DoSyscall,
which executes a “syscall” machine instruction. When the kernel call finishes, the DoSyscall function
simply returns to the wrapper function, which returns to the user’s code.
Arguments may be passed to and from the kernel call. In general, these are integers and pointers to
memory. The wrapper function works with DoSyscall to pass the arguments. When the wrapper
function calls DoSyscall, it will push the arguments onto the stack. The DoSyscall will take the
arguments off the stack and move them into registers. Since it runs as a user-level function, it places
them in the “user” registers. (Recall that the BLITZ machine has a set of 16 “system registers” and a set
of 16 “user registers.”)
Each wrapper function also uses an integer code to indicate which kernel function is involved. Here is
the enum giving the different codes. For example, the code for “Fork” is 4.
Project 5 Operating Systems
Page 7
enum SYSCALL_EXIT = 1,
SYSCALL_SHUTDOWN,
SYSCALL_YIELD,
SYSCALL_FORK,
SYSCALL_JOIN,
SYSCALL_EXEC,
SYSCALL_CREATE,
SYSCALL_OPEN,
SYSCALL_READ,
SYSCALL_WRITE,
SYSCALL_SEEK,
SYSCALL_CLOSE
These code numbers are used both by the user-level program and by the kernel. Consequently, there is
an identical copy of this enum in both Kernel.h and UserSystem.h. (You should not change the system
call interface, but if one were to change these code numbers, it would be critical that both enums were
changed identically.)
As an example, here is the code for the wrapper function for “Read.” It simply invokes DoSyscall and
returns whatever DoSyscall returns.
function Sys_Read (fileDesc: int,
buffer: ptr to char,
sizeInBytes: int) returns int
return DoSyscall (SYSCALL_READ,
fileDesc,
buffer asInteger,
sizeInBytes,
0)
endFunction
Here is the function prototype for DoSyscall:
external DoSyscall (funCode, arg1, arg2, arg3, arg4: int) returns int
The DoSyscall routine is set up to deal with up to 4 arguments. Since the Read syscall only needs 3
arguments, the wrapper function must supply an extra zero for the fourth argument.
DoSyscall treats all of its arguments as untyped words (i.e., as int), so the wrapper functions must
coerce the types of the arguments if they are not int. Whatever DoSyscall returns, the wrapper function
will return.
DoSyscall is in UserRuntime.s, which will be linked with all user programs. The code is given next.
It moves each of the 4 arguments into registers r1, r2, r3, and r4. It then moves the function code into
register r5 and executes the syscall instruction. It assumes the kernel will place the result (if any) in r1,
so after the syscall instruction, it moves the return value from r1 to the stack, so that the wrapper
function can retrieve it.
Project 5 Operating Systems
Page 8
DoSyscall:
load [r15+8],r1 ! Move arg1 into r1
load [r15+12],r2 ! Move arg2 into r2
load [r15+16],r3 ! Move arg3 into r3
load [r15+20],r4 ! Move arg4 into r4
load [r15+4],r5 ! Move funcCode into r5
syscall r5 ! Do the syscall
store r1,[r15+4] ! Move result from r1 onto stack
ret ! Return
Some of the kernel routines require no arguments and/or return no result. As an example, consider the
wrapper function for Yield. The compiler knows that DoSyscall returns a result, so it insists that we do
something with this value. The wrapper function simply moves it into a variable and ignores it.
function Sys_Yield ()
var ignore: int
ignore = DoSyscall (SYSCALL_YIELD, 0, 0, 0, 0)
endFunction
Here is a list of all the wrapper functions, including their arguments and return types.
Sys_Exit (returnStatus: int)
Sys_Shutdown ()
Sys_Yield ()
Sys_Fork () returns int
Sys_Join (processID: int) returns int
Sys_Exec (filename: String) returns int
Sys_Create (filename: String) returns int
Sys_Open (filename: String) returns int
Sys_Read (fileDesc: int, buffer: ptr to char, sizeInBytes: int)
returns int
Sys_Write (fileDesc: int, buffer: ptr to char, sizeInBytes: int)
returns int
Sys_Seek (fileDesc: int, newCurrentPos: int) returns int
Sys_Close (fileDesc: int)
In addition to the wrapper functions, the UserSystem package contains a few other routines that support
the KPL language. These are more-or-less duplicates of the same routines in the System package.
Likewise, some of the material from Runtime.s is duplicated in UserRuntime.s. This duplication is
necessary because user-level programs cannot invoke any of the routines that are part of the kernel.
For example the functions print, printInt, nl, etc. have been duplicated at the user level so the userlevel program has the ability to print.
[Note that, at this point, all printing is done by cheating, using a “trapdoor” in the emulator. Normally, a
user-level program would need to invoke syscalls (such as Sys_Write) to perform any output, since
user-level programs can’t access the I/O devices directly. However, since we are not yet ready to
address questions about output to the serial device, we are including these cheater print functions, which
rely on a trapdoor in the emulator.]
Every user-level program needs to “use” the UserSystem package and be linked with the
UserRuntime.s code. For example:
Project 5 Operating Systems
Page 9
MyProgram.h
header MyProgram
uses UserSystem
functions
main ()
endHeader
MyProgram.c
code MyProgram
function main ()
print (“My user-level program is running!\n”)
Sys_Shutdown ()
endFunction
endCode
Here are the commands to prepare a user-level program for execution. The makefile has been modified
to include these commands.
asm UserRuntime.s
comp UserSystem -unsafe
asm UserSystem.s
comp MyProgram -unsafe
asm MyProgram.s
lddd UserRuntime.o UserSystem.o MyProgram.o -o MyProgram
Note that there is no connection with the kernel. The user-level programs are compiled and linked
independently. All communication with the kernel will be through the syscall interface, via the wrapper
functions.
This is exactly the way Unix works. For user-level programs, library functions and wrapper functions
are brought into the “a.out” file at link-time, as needed. This explains why a seemingly small “C”
program can produce a rather large “a.out” executable. One small use of printf in a program might pull
in, at link-time, more output formatting and buffering routines than you can possibly imagine.
When an OS wants to execute a user-level program, it will go to a disk file to find the executable. Then
it will read that executable into memory and start up the new process.
In order to execute MyProgram, we need to introduce the BLITZ “disk.” The disk is simulated with a
Unix file called “DISK.” After the user-level program is compiled, it must be placed on the BLITZ disk
with the following Unix commands:
diskUtil -i
diskUtil -a MyProgram MyProgram
The first command creates an empty file system on the disk. The second command copies a file from
the Unix file system to the BLITZ disk. It creates a directory entry and moves the data to the proper
place on the simulated BLITZ disk. Commands to initialize the BLITZ disk have also been added to the
makefile.
Project 5 Operating Systems
Page 10
Once the kernel is running, it will read the file from the simulated BLITZ disk and copy it into memory.
The Syscall Interface
In our OS, each process will have exactly one thread. A process may also have several open files and
can do I/O via the Read and Write syscalls. The I/O will go to the BLITZ disk. For now, there is no
serial (i.e., terminal) device.
Next we describe each syscall in more detail.
function Sys_Exit (returnStatus: int)
This function causes the current process and its thread to terminate. The returnStatus will be
saved so that it can be passed to a Sys_Join executed by the parent process. This function never
returns.
function Sys_Shutdown ()
This function will cause an immediate shutdown of the kernel. It will not return.
function Sys_Yield ()
This function yields the CPU to another process on the ready list. Once this process is scheduled
again, this function will return. From the caller’s perspective, this routine is similar to a “nop.”
function Sys_Fork () returns int
This function creates a new process which is a copy of the current process. The new process will
have a copy of the virtual memory space and all files open in the original process will also be
open in the new process. Both processes will then return from this function. In the parent
process, the pid of the child will be returned; in the child, zero will be returned.
function Sys_Join (processID: int) returns int
This function causes the caller to wait until the process with the given pid has terminated, by
executing a call to Sys_Exit. The returnStatus passed by that process to Sys_Exit will be
returned from this function. If the other process invokes Sys_Exit first, this returnStatus will
be saved until either its parent executes a Sys_Join naming that process’s pid or until its parent
terminates.
function Sys_Exec (filename: String) returns int
This function is passed the name of a file. That file is assumed to be an executable file. It is read
in to memory, overwriting the entire address space of the current process. Then the OS will
Project 5 Operating Systems
Page 11
begin executing the new process. Any open files in the current process will remain open and
unchanged in the new process. Normally, this function will not return. If there are problems, this
function will return -1.
function Sys_Create (filename: String) returns int
This function creates a new file on the disk. If all is okay, it returns 0, otherwise it returns a nonzero error code. This function does not open the file; so the caller must use Sys_Open before
attempting any I/O.
function Sys_Open (filename: String) returns int
This function opens a file. The file must exist already exist. If all is OK, this function returns a
file descriptor, which is a small, non-negative integer. It errors occur, this function returns -1.
function Sys_Read (fileDesc: int, buffer: ptr to char, sizeInBytes: int) returns int
This function is passed the fileDescriptor of a file (which is assumed to have been successfully
opened), a pointer to an area of memory, and a count of the number of bytes to transfer. This
function reads that many bytes from the current position in the file and places them in memory.
If there are not enough bytes between the current position and the end of the file, then a lesser
number of bytes are transferred. The current file position will be advanced by the number of
bytes transferred.
If the input is coming from the serial device (the terminal), this function will wait for at least one
character to be typed before returning, and then will return as many characters as have been
typed and buffered since the previous call to this function.
This function will return the number of characters moved. If there are errors, it will return -1.
function Sys_Write (fileDesc: int, buffer: ptr to char, sizeInBytes: int) returns int
This function is passed the fileDescriptor of a file (which is assumed to have been successfully
opened), a pointer to an area of memory, and a count of the number of bytes to transfer. This
function writes that many bytes from the memory to the current position in the file.
If the end of the file is reached, the file’s size will be increased.
The current file position will be advanced by the number of bytes transferred, so that future
writes will follow the data transferred in this invocation.
The output may also be directed to the serial output, i.e., to the terminal.
This function will return the number of characters moved. If there are errors, it will return -1.
Project 5 Operating Systems
Page 12
function Sys_Seek (fileDesc: int, newCurrentPosition: int) returns int
This function is passed the fileDescriptor of a file (which is assumed to have been successfully
opened), and a new current position. This function sets the current position in the file to the
given value and returns the new current position.
Setting the current position to zero causes the next read or write to refer to the very first byte in
the file. If the file size is N bytes, setting the position to N will cause the next write to append
data to the end of the file.
The current position is always between 0 and N, where N is the file’s size in bytes.
If -1 is supplied as the new current position, the current position will be set to N (the file size in
bytes) and N will be returned.
It is an error to supply a newCurrentPosition that is less than -1 or greater than N. If so, -1 will
be returned.
function Sys_Close (fileDesc: int)
This function is passed the fileDescriptor of a file, which is assumed to be open. It closes the
file, which includes writing out any data buffered by the kernel.
Asynchronous Interrupts
From time-to-time an asynchronous interrupt will occur. Consider a DiskInterrupt as an example.
When this happens, an assembly routine called DiskInterruptHandler in Runtime.s will be jumped to.
It begins by saving the system registers (after all, a Disk Interrupt might occur while a kernel routine is
executing and we’ll need to return to it). Then DiskInterruptHandler performs an “upcall” to the
function named DiskInterruptHandler in Kernel.c. Perhaps it is a little confusing to have an assembly
routine and a KPL routine with the same name, but, oh well…
The high-level DiskInterruptHandler routine simply signals a semaphore and returns to the assembly
DiskInterruptHandler routine, which restores the system registers and returns to whatever code was
interrupted. All the time while these routines are running, interrupts are disabled and no other interrupts
can occur.
Also note that the interrupt handler uses space on the system stack of whichever thread was interrupted.
It might be that some unsuspecting user-level code was running. Although the interrupt handler will use
the system stack of that thread, the thread will be none-the-wiser. While the interrupt handler is running,
it is running as part of some more-or-less randomly selected thread. The interrupt handler is not a thread
on its own.
Project 5 Operating Systems
Page 13
Error Exception Handling
When a runtime error is detected by the CPU, the CPU performs exception processing, which is similar
to the way it processes an interrupt. Here are the sorts of runtime errors that can occur in the BLITZ
architecture:
Illegal Instruction
Arithmetic Exception
Address Exception
Page Invalid Exception
Page Read-only Exception
Privileged Instruction
Alignment Exception
As an example, consider what happens when an Alignment Exception occurs. (The others are handled
the same way.)
The CPU will consult the interrupt vector in low memory (see Runtime.s) and will jump to an assembly
language routine called AlignmentExceptionHandler. The assembly routine first checks to see if the
interrupted code was executing in system mode or not. If it was in system mode, then the assumption is
that there is a bug in the kernel, so the assembly routine prints a message and halts execution.
However, if the CPU was in user mode, the assumption is that the user-level program has a bug. The
OS will need to handle that bug without itself stopping. So the assembly AlignmentExceptionHandler
routine makes an upcall to a KPL routine with the same name.
The high-level AlignmentExceptionHandler routine simply prints a message and terminates the
process. Process termination is performed in a routine called ProcessFinish, which is not yet written.
(For now, we’ll assume that user-level programs do not have any bugs.)
When ProcessFinish is implemented in a later project, it will need to return the ProcessControlBlock
(PCB) to the free pool. It will also need to free any additional resources held by the process, such as
OpenFile objects. Of course, any open files will need to be closed first. Finally, ProcessFinish will
call ThreadFinish and will not return.
(Note that a Thread object cannot be added back to the free thread pool by the thread that is running.
Instead, in ThreadFinish the thread is added to a list called threadsTobeDestroyed. Later, after
another thread begins executing (in Run) the first thing it will do is add any threads on that list back to
the free pool by calling threadManager.FreeThread.)
Project 5 Operating Systems
Page 14
Syscalls
When a user-level thread executes a syscall instruction, the assembly routine SyscallTrapHandler in
Runtime.s will be invoked. The assembly routine will then call a KPL routine with the same name.
The assembly routine does not need to save registers because the interrupted code was executing in user
mode and the handler will be executed in system mode.
Recall that just before the syscall, the DoSyscall routine placed the arguments in the (user) registers r1,
r2, r3, and r4, with an integer indicating which kernel function is wanted in register r5. The
SyscallTrapHandler assembly routine takes the values from the user registers. Since it is running in
system mode, it must use a special instruction called “readu” to get values from the user registers. It
pushes them on to the system stack so that the high-level routine can access them. Then it calls the
high-level SyscallTrapHandler routine. When the high-level routine returns, it takes the returned
value from the stack and moves it into user register r1, using an instruction called “writeu,” and then
executes a “reti” instruction to return to the interrupted user-level process. Execution will resume back
in DoSyscall directly after the “syscall” instruction.
The high-level routine called SyscallTrapHandler simply takes a look at the function code and calls the
appropriate routine to finish the work. For every kind of syscall, there is a corresponding “handler
routine” in the OS.
System call Handler function in the kernel
Exit Handle_Sys_Exit
Shutdown Handle_Sys_Shutdown
Yield Handle_Sys_Yield
Fork Handle_Sys_Fork
Join Handle_Sys_Join
Exec Handle_Sys_Exec
Create Handle_Sys_Create
Open Handle_Sys_Open
Read Handle_Sys_Read
Write Handle_Sys_Write
Seek Handle_Sys_Seek
Close Handle_Sys_Close
It is these routines that you will need to implement, in this and other projects.
Note that interrupts will be disabled when the SyscallTrapHandler routine begins. The first thing the
high-level routine does is set the global variable currentInterruptStatus to DISABLED so that it is
accurate. In fact, all the interrupt and exception handlers begin by setting currentInterruptStatus to
DISABLED for this reason.
Also note that after the handler routines return to the interrupted routine, interrupts will be re-enabled.
Why? Because the Status Register in the CPU will be restored as part of the operation of the reti
instruction, restoring the interrupt (and paging and system mode) status bits to what they were when the
Project 5 Operating Systems
Page 15
interrupt occurred. (Note that we do not bother to change currentInterruptStatus to ENABLED
before returning to user-level code, because any re-entry to the kernel code must be through
SyscallTrapHandler, or an interrupt or exception handler, and each of these begins by setting
currentInterruptStatus.)
Implementing the Shutdown syscall is straightforward. The handler should call FatalError with the
following message:
Syscall ‘Shutdown’ was invoked by a user thread
The BLITZ Disk
The BLITZ computer includes a disk which is emulated using a file called DISK on the host computer.
In other words, a write to the BLITZ disk will cause data to be written to a Unix file and a read from the
BLITZ disk will cause a read from the Unix file. The emulator will simulate the delays involved in
reading, by taking account of the current (simulated) disk head position. When the I/O is complete—
that is the simulated time when the emulator has calculated the disk I/O will have completed—the
emulator causes a DiskInterrupt to occur.
To interface with the BLITZ disk, we have supplied a class called DiskDriver, which makes it
unnecessary for you to write the code that actually reads and writes disk sectors. You can just use the
code in the class DiskDriver. There is only one DiskDriver object; it is created and initialized at
startup time.
class DiskDriver
superclass Object
fields
…
semToSignalOnCompletion: ptr to Semaphore
semUsedInSynchMethods: Semaphore
diskBusy: Mutex
methods
Init ()
SynchReadSector (sectorAddr, numberOfSectors, memoryAddr: int)
StartReadSector (sectorAddr, numberOfSectors, memoryAddr: int,
whoCares: ptr to Semaphore)
SynchWriteSector (sectorAddr, numberOfSectors, memoryAddr: int)
StartWriteSector (sectorAddr, numberOfSectors, memoryAddr: int,
whoCares: ptr to Semaphore)
endClass
This class provides a way to read and write sectors synchronously as well as a way to read and write
sectors asynchronously.
To perform a disk operation without blocking the calling thread, you can call StartReadSector or
StartWriteSector. These methods are passed the number of the sector on the disk at which to begin the
transfer, the number of sectors to transfer and the location in memory to transfer the data to or from.
Project 5 Operating Systems
Page 16
These methods are also passed a pointer to a Semaphore; upon completion of the operation (possibly in
error!) this semaphore will be signaled with an Up() operation. This is exactly the semaphore that is
signaled whenever a DiskInterrupt occurs. So to perform asynchronous I/O, the caller will invoke
StartReadSector (or StartWriteSector) giving it a Semaphore. Then the caller can either do other
stuff, or wait on the Semaphore.
Since it may be a little tricky to manage asynchronous I/O correctly, the DiskDriver class also provides
a couple of methods to make it easy to do I/O synchronously.
When you call SynchReadSector or SynchWriteSector, the caller will be suspended and will be
returned to only after a successful completion of the I/O. These routines will deal with transient errors
by retrying the operation until it works. Other errors (such as a bad sectorAddr or bad memoryAddr)
will be dealt with by a call to FatalError.
In order to implement these methods, the DiskDriver contains a mutex called diskBusy and a
semaphore called SemUsedInSynchMethods. Each synch method makes sure the disk is not busy with
I/O from some other thread and, if so, waits until it is completed. This is the purpose of the diskBusy
mutex. After acquiring the lock, each synch method will call StartReadSector (or StartWriteSector)
supplying the semaphore. The synch method will then wait until the disk operation is complete. The
calling thread will remain blocked for the duration.
The “Stub” File System
As a later project, you might want to implement a full file system, more like the one used by Unix
systems. For now, you are supplied with a very minimal file system, called the “stub” file system. In
Unix, directories are structured in a tree shape and there are lots of complexities concerning how files
are stored on the disk.
In the stub file system, the disk will contain only one directory, and several files. The directory is
limited in size to one sector and is kept in sector 0 of the disk. The exact number of files that can be
accommodated depends on how long the file names are.
Each file has a name and a file length (in bytes). Each file is stored on disk in a sequence of consecutive
sectors. Once a file is placed on the disk, and more files are added after it, it is impossible to increase
the size of the file. Each file is allocated an integral number of sectors. (Since the last sector in each file
may be only partially full, it would be possible to increase the size of a file up to the next sector
boundary. However, it is not worth the effort. Instead, the solution is to design a better file system!)
For now, the directory is read-only, so files may not be created and the size of files may not be changed.
The classes FileManager, FileControlBlock, and OpenFile are provided for you, to make it easier to
use the file system from within the kernel.
Project 5 Operating Systems
Page 17
The “diskUtil” Tool
The BLITZ tool called diskUtil can be used to create a file system on the BLITZ disk, to add files to the
disk, to remove files, and to print out the directory.
The BLITZ DISK is organized as follows. The disk contains a single directory and this is kept in sector
0. The files are placed sequentially on the disk, one after the other. Each file will take up an integral
number of sectors. Each file has an entry in the directory. Each entry contains
(1) The starting sector
(2) The file length, in bytes (possibly zero)
(3) The number of characters in the file name
(4) The file name
The directory begins with three numbers:
(1) Magic Number (0x73747562 = “stub”)
(2) Number of files (possibly zero)
(3) Number of the next free sector
These are followed by the entries for each file.
Once created, a BLITZ file may not have its size increased. When a file is removed, the free sectors
become unusable; there is no compaction or any attempt to reclaim the lost space.
Each time the diskUtil program is run, it performs one of the following functions:
Initialize set up a new file system on the BLITZ disk
List list the directory on the BLITZ disk
Create create a new file of a given size
Remove remove a file
Add copy a file from Unix to BLITZ
Extract copy a file from BLITZ to Unix
Write write sectors from a Unix file to the BLITZ disk
The following command line options tell which function is to be performed by diskUtil:
-h
Print help info.
-d DiskFileName
The file used to emulate the BLITZ disk. If missing, “DISK” will be used.
Project 5 Operating Systems
Page 18
-i
Initialize the file system on the BLITZ “DISK” file. This will
effectively remove all files on the BLITZ disk and reclaim all available
space.
-l
List the directory on the BLITZ disk.
-c BlitzFileName SizeInBytes
Create a file of the given size on the BLITZ disk. The BLITZ
disk must not already contain a file with this name. Only the
directory will be modified; the actual data in the file will be
whatever bytes happened to be on the disk already.
-r BlitzFileName
Remove the file with the given name from the directory on the BLITZ disk.
-a UnixFilename BlitzFileName
Copy a file from Unix to the BLITZ disk. If BlitzFileName already
exists, it must be large enough to accommodate the new data.
-e BlitzFileName UnixFileName
Extract a file from the BLITZ disk to Unix. This command will copy
the data from the BLITZ disk to a Unix file. The Unix file may or may
not already exist; its size will be shortened or lengthened as necessary.
-w UnixFileName SectorNumber
The UnixFileName must be an existing Unix file. The SectorNumber is an
integer. The Unix file data will be written to the BLITZ disk, starting
at sector SectorNumber. The directory will not be modified.
We are providing a DISK file which should be large enough, but if you want, you may create a new
BLITZ disk file of a different size. The new disk file must also be initialized properly; it can be created
and initialized with the format command in the BLITZ emulator. For example:
% blitz
…
> format
…
The name of the disk file is “DISK”.
The file “DISK” did not previously exist. (It could not
be opened for reading.)
Enter the number of tracks (e.g., 1000; type 0 to abort):
3
…
Initializing sectors 0 through 47…
Successful completion.
Project 5 Operating Systems
Page 19
Next, we use diskUtil to create a file system, add several files, and print the directory, by typing these
commands at the Unix prompt:
% diskUtil –i
% diskUtil -a temp1 MyFileA
% diskUtil -a temp2 MyFileB
% diskUtil -a MyProgram MyProgram
% diskUtil –l
StartingSector SizeInSectors SizeInBytes FileName
============== ============= =========== =====================
0 1 8192 < directory >
1 1 8192 MyFileA
2 3 17000 MyFileB
5 8 60264 MyProgram
The FileManager
There is only one FileManager object; it is created and initialized at startup time.
We are supplying several methods to help you access files on the “stub” file system; these methods are
located in this class. You’ll need to know how to access files in order to create the first user-level
process. You’ll need to open the executable file, read the bytes from disk, then close the file. You’ll
also need to use the fileManager when you implement the Exec syscall.
Some of the following material pertains more to the next project than this project. Read it all now to get
familiar with the framework. You may want to review it again during the next project.
Associated with the FileManager class, there are two other classes called FileControlBlock and
OpenFile. These two classes contain fields, but do not contain many methods of their own (besides
Init() and Print() methods). Instead, most of the work associated with the file system is done by the
FileManager methods.
The FileControlBlock (FCB) objects and the OpenFile objects are limited resources. The
FileManager maintains a free list for each of these, as well as code to allocate new FCB objects and
new OpenFile objects and maintain the free lists.
The FileManager also deals with opening files. This involves finding the file in the file system, that is,
determining the file’s location on disk. In the “stub” file system this is pretty simple since there is only
one directory and it fits into a single sector. The FileManager—as programmed now—reads the
directory sector (sector 0) into a frame as part of the FileManager.Init method. Subsequent attempts to
open a file require no disk accesses. (Of course, for a “real” file system, things won’t be so simple!)
Project 5 Operating Systems
Page 20
FileControlBlock (FCB) and OpenFile
The semantics of files in the kernel you are building will be similar to the semantics of files in Unix.
Consider the case where one process has opened a file and does a kernel call to read, say, 10 bytes. The
kernel must read the appropriate sector, extract the 10 bytes out of that sector, and finally copy those 10
bytes into the process’s virtual memory space. This requires the kernel to maintain a frame of memory
to use as a buffer; the sector will be read into this buffer by the OS.
If the 10 bytes happen to span the boundary between sectors, the kernel must read both sectors in order
to complete the Read syscall. And of course, during the I/O operations other threads must be allowed to
run.
Now consider what happens when a process wants to write, say, 20 bytes to a file. The kernel will need
to bring in the appropriate sector and copy the 20 bytes from the process’s virtual address space to the
buffer. Should the kernel write the buffer back to disk immediately? No; it is likely that the process
will want to write some more bytes to that very same sector, so it is more efficient to leave the sector in
memory.
When should the kernel write the sector back to disk? When the process closes the file, the kernel must
write it back. Also, other I/O operations on the file may need different sectors, so the kernel should
write the sector back to disk when the buffer is needed for another sector. However, if the buffer has not
been modified, then there is no need to write it back to the disk. Therefore, we associate a Boolean
called bufferIsDirty with each buffer frame. When a buffer is first read in from disk, it is considered to
be “clean,” but after any operation modifies the buffer, it should be marked “dirty.”
Next consider the case in which two processes have both opened the same file. (Let’s call them
processes “A” and “B.”) Any update by process A must be immediately visible to process B. If process
A writes to a file and B reads from that same file, even before A has closed the file, then B should see
the new data. Since the kernel may not actually write to the disk for a long time after process A does the
write, it means that processes A and B must share the buffer.
Also, when one process finally closes a file, the buffer must be written back to the disk. The guarantee
the kernel makes is that once we return from a call to Sys_Close, the disk has been updated. The
program can stop worrying about failures, etc., and can tell the user that it has completed its task. Any
changes the program has made—even if the system crashes in the next instance—will be permanent and
will not be lost. After a Sys_Close, the kernel must not return to the user-level program until the buffer
(or all buffers, if there are more than one) is written to the disk successfully.
The purpose of a FileControlBlock (FCB) is to record all the data associated with a single file. This
includes the buffer and the bufferIsDirty bit. Here is the definition of FCB:
Project 5 Operating Systems
Page 21
class FileControlBlock
superclass Listable
fields
fcbID: int
numberOfUsers: int — count of OpenFiles pointing here
startingSectorOfFile: int — or -1 if FCB not in use
sizeOfFileInBytes: int
bufferPtr: int — addr of a page frame
relativeSectorInBuffer: int — or -1 if none
bufferIsDirty: bool — Set to true when buffer is modified
methods
Init ()
Print ()
endClass
A small number FCBs are preallocated and kept in a table called fcbTable, which is maintained by the
FileManager. The FileManager is responsible for allocating new FileControlBlock objects and for
returning unused FileControlBlock objects to a free pool called fcbFreeList.
The startingSectorOfFile tells where the file is located on the disk. Since all the sectors in a file are
contiguous, the starting address and the length are all we need. The meaning of sizeOfFileInBytes is…
well, obvious. [Descriptive variable names like we tend to use are a HUGE help in understanding and
reading code!] A single memory frame is allocated for each FCB at kernel startup time and bufferPtr is
set to point to that memory region. relativeSectorInBuffer tells which sector of the file is currently in
the buffer and is –1 if there is no valid data in the buffer.
Next consider a process “A” that has opened a file. All of the “read” and “write” operations that the
user-level process executes are relative to a “current position” in the file. Several processes may have
the same file open. All processes that have file “F” open will share a single FCB. However, they will
each have a different “current position” in the file.
To handle the current position, we have the class OpenFile, which is defined as:
class OpenFile
superclass Listable
fields
kind: int — FILE, TERMINAL, or PIPE
currentPos: int — 0 = first byte of file
fcb: ptr to FileControlBlock — null = not open
numberOfUsers: int — count of Processes pointing here
methods
Print ()
ReadBytes (targetAddr, numBytes: int) returns bool — true=All Okay
ReadInt () returns int
LoadExecutable (addrSpace: ptr to AddrSpace) returns int — -1 = problems
endClass
Like the FCBs, there is a preallocated pool of OpenFile objects, which are created at system startup
time. The FileManager is responsible for allocating new OpenFile objects and for returning unused
OpenFile objects to a free pool called openFileFreeList.
Project 5 Operating Systems
Page 22
When process “A” opens a file, a new OpenFile object must be allocated and made to point to an FCB
describing the file. If there is already an FCB for that file, then the OpenFile should be made to point to
it; otherwise, we’ll have to get a new FCB, check the directory, and set up the FCB.
When do we return an FCB to the free pool? When there are no more OpenFiles using it. This is the
reason we have a field called numberOfUsers in the FCB. This field is a “reference counter.” It tells
the number of OpenFile objects that point to the FCB. When a new OpenFile is allocated and made to
point to an FCB, the count must be incremented. When an OpenFile is closed, the count should be
decremented. When the count becomes zero, the FCB must be returned to the free pool.
When a process is terminated, for example due to an error such as an AlignmentException, the kernel
must close any and all OpenFiles the process is using. The process may explicitly close an OpenFile
with the Close syscall. Once a file is closed, the process should attempt no further I/O on the file and if
the process does, the kernel should catch it and treat it as an error (by returning an error code from the
Sys_Read or Sys_Write kernel call).
Our file I/O will follow the semantics of Unix. When a process is cloned with the Fork syscall, all open
files in the parent process must be shared with the child process. Consider what happens when a parent
and a child are both writing to the same file, which was originally opened in the parent. Since both
processes share the OpenFile object, they will share the current position. If the child writes 5 bytes, the
current position will be incremented by 5. Then, if the parent writes 13 bytes, these 13 bytes will follow
the 5 bytes written by the child.
In order to implement these semantics, it will be possible for several PCBs to point to the same
OpenFile object. We need to maintain a reference count for the OpenFiles, just like the reference count
for the FCBs. Whenever a process opens a file, we need to allocate a new OpenFile object and set its
count to 1. Whenever a process forks, we’ll need to increment the count. When a process closes a file
(either by invoking the Close syscall or by dying), we’ll need to decrement the count. If the count goes
to zero, we’ll need to return the OpenFile to the free pool and decrement the count associated with the
FCB.
User-level processes must not be allowed to use pointers into kernel memory and cannot be allowed to
touch kernel data structures such as OpenFiles and FCBs. So how does a user process refer to an
OpenFile object? Indirectly, through an integer. Here’s how it works.
Each Process will have a small array of pointers to OpenFiles called fileDescriptor.
class ProcessControlBlock
…
fields
…
fileDescriptor: array [MAX_FILES_PER_PROCESS] of ptr to OpenFile
methods
…
endClass
When a process invokes the Open syscall, a new OpenFile will be set up. Then the kernel will select an
unused position in this array and make it point to the OpenFile. For example, positions 0, 1, and 2
Project 5 Operating Systems
Page 23
might be in use, so the kernel may assign a file descriptor of 3 for the newly opened file. The kernel
must make fileDescriptor[3] point to the OpenFile and should return “3” as the fileDescriptor to the
user-level process.
When the user-level process wants to do an I/O operation, such as Read, Write, Seek, or Close, it must
supply the fileDescriptor. The kernel must check that (1) this number is a valid index into the array,
and (2) the array element points to a valid OpenFile. When closing the file, the kernel will need to
decrement the reference count for the OpenFile object and also set fileDescriptor[3] to null. Then, if
the user process attempts any future I/O operations with file descriptor 3, the kernel can detect that it is
an error.
Since user-level file I/O will not be implemented in this project, you will not need to worry about
fileDescriptors yet.
When a user-level program does a Read or Write syscall—in Unix or in our OS—the data may be
transferred from/to either
• a file on the disk
• an I/O device such as a keyboard or display (these are called “special files” in Unix)
• another process, via a “pipe”
In all three cases, an OpenFile object will be used. The field called kind tells whether the object
corresponds to a FILE, the TERMINAL, or a PIPE. In this project, we will only use OpenFiles to
perform the Exec syscall, so the kind will be only FILE (and not TERMINAL or PIPE).
To Read in an Executable File
To read in an executable file from disk, your code will need to:
• Open the file
• Invoke LoadExecutable to do the work
• Close the file
Read through the code for FileManager.Open:
method Open (filename: String) returns ptr to OpenFile
Open is passed a ptr to array of char; this is the name of the file on the BLITZ disk that you want to
read from. It will allocate a new OpenFile object and a new FCB object and set them up. Then it will
return a pointer to the OpenFile object, which you’ll use when calling LoadExecutable. If anything
goes wrong, Open returns null. The only real danger is getting the filename wrong.
In BLITZ, like Unix, executable files have rather complex format. For details, you can read through the
document titled “The Format of BLITZ Object and Executable Files.” So that you don’t have to write
all this code, we are providing a method called OpenFile.LoadExecutable:
method LoadExecutable (addrSpace: ptr to AddrSpace) returns int
Project 5 Operating Systems
Page 24
Look through LoadExecutable; it will
• Create a new address space (by calling frameManager.GetNewFrames)
• Read the executable program into the new address space
• Determine the starting address (the initial program counter, also called the “entry point”)
• Return the entry point
If there are any problems with the executable file, this method will return –1. Otherwise it will return
the entry point of the executable. This is the address (in the logical address space) at which execution
should begin. Normally, this will be 0x00000000.
User-Level Processes
Each user-level process will have a single thread which will normally execute in User mode, with
“paging” turned on and interrupts enabled.
Each user-level process will have a logical address space, which will consist of
• A Page for “environment” data
• Pages for the text segment
• Pages for the data segment
• Pages for the BSS segment
• Pages for the user’s stack
These are shown in order, with the stack pages in the highest addresses of the logical address space.
The environment page will sit at address 0 and will contain information that the OS wishes to pass to a
new user-level process. This includes userID, working directory, etc. We will not use an environment
page, so the text pages will begin at address 0.
Kernel.h contains this:
const
NUMBER_OF_ENVIRONMENT_PAGES = 0
USER_STACK_SIZE_IN_PAGES = 1
MAX_PAGES_PER_VIRT_SPACE = 20
The text pages contain the program and any constant values.
The data pages will contain the static (global) program variables.
The BSS pages will contain space for uninitialized program variables (such as large arrays). The OS
will set all bytes in the BSS pages to zero. Most KPL programs do not use a BSS segment, so there will
usually be zero BSS pages.
Project 5 Operating Systems
Page 25
The user-level program will have a stack, which will grow downward. Each logical address space will
have a predetermined small number of pages (in our case, this is one page) set aside for its stack. In
Unix, if a user process’s stack grows beyond its initial allocation, more stack pages would be added. In
our OS, if a user process’s stack grows beyond this, it will begin overwriting the BSS and data pages,
and the program will probably get an error of some sort soon thereafter.
As an example, a program might use:
0 environment pages
2 text pages
1 data page
0 BSS pages
1 stack page
This process’s logical address space will have 4 pages. Each page has PAGE_SIZE bytes (8 Kbytes), so
the entire address space will be 32 Kbytes. Any address between 0x00000000 and 0x00007FFF (which
is 32K-1 in hex) would be legal for this program. If the program tries to use any other address, a
PageInvalid Exception will occur.
In Unix, the environment and text pages would be marked read-only and any attempt to update bytes in
those pages would cause an exception. In this project, all pages of the logical address space will be
read-write, so our OS will not be able to catch that sort of error in the user-level program.
Each page in the logical address space will be stored in one frame in memory. The frames do not have
to be contiguous and the pages may be stored in pretty much any order. However, all pages will be in
memory throughout the process’s lifetime.
The page table will keep track of where each page is kept. While the process is executing, “paging” will
be turned on so that the memory management unit (MMU) will translate all logical addresses into
physical addresses. Our example program will not be able to read or write anything outside of its 4
pages.
There may be several processes in the system at any time. Each ProcessControlBlock contains an
AddrSpace, which tells how many pages the process’s address space has and which frame in physical
memory holds each page.
When some process (call it “P”) is ready to be scheduled and given a time-slice, the MMU will be need
to be set up so that it points to the page table for process P. You can do this with the method:
AddrSpace.SetToThisPageTable ()
which calls an assembly routine to load the MMU registers. This method must be invoked before
paging is turned on. When paging is turned off (i.e., whenever kernel code is being executed), the
MMU registers are ignored.
Project 5 Operating Systems
Page 26
Note that each thread will have two stacks: a user stack and a system stack. We have already seen the
system stack; it is used when one kernel function calls another kernel function. The user stack will be
used when the thread is running in user mode. The system stack, which is fairly small, normally
contains nothing while the user-level program is running. In other words, the system stack is completely
empty.
After the user-level program begins executing, execution can re-enter the kernel in only through
exception processing. That is, the only ways to get back into the kernel are:
• an interrupt,
• a program exception, or
• a syscall
In each of these cases, the exact same thing happens: some information is pushed onto the system stack,
the mode is changed to system mode, paging is turned off, and a jump is made to a kernel “handler”
routine.
The BLITZ computer has two sets of registers: one for user-mode code and one for system-mode code.
Thus, the user registers do not need to be saved, unless the kernel will switch to another thread. This is
done in the Run method, which contains this code:
if prevThread.isUserThread
SaveUserRegs (&prevThread.userRegs[0])
endIf
…
Switch (prevThread, nextThread)
…
if currentThread.isUserThread
RestoreUserRegs (&currentThread.userRegs[0])
currentThread.myProcess.addrSpace.SetToThisPageTable ()
endIf
If the kernel handler code wishes to return to the same user-level code that was interrupted, it can merely
return to the assembly language handler routine, which will perform a “reti” instruction. The user
registers and the MMU registers will (presumably) be unchanged, so when the mode reverts to “user
mode” and the paging reverts to “paging enabled,” the user-level program will resume execution with
the same values in the user registers and the same logical address space.
Creating a User-Level Process
The main function calls function InitFirstProcess, which you must implement. The first thing you’ll
need to do is get a new thread object by invoking GetANewThread. Since the InitFirstProcess
function should return, you cannot use the current thread. Next you’ll need to initialize the thread and
invoke Fork to start it running. (You can name this new thread something like “UserProgram,” but the
name is only used in the debugging printouts.)
Project 5 Operating Systems
Page 27
The new thread should execute the StartUserProcess function, which will do the remainder of the work
in starting up a user-level process. InitFirstProcess can supply a zero as an argument to
StartUserProcess and can return after forking the new thread.
The first thing you’ll need to do in StartUserProcess is allocate a new PCB (with GetANewProcess)
and connect it with the thread. So initialize the myThread field in the PCB and the myProcess field in
the current thread.
Next, you’ll need to open the executable file. It is acceptable to “hardcode” the filename (e.g.,
“TestProgram1”) into the call to Open, although changing the name of the initial process will require a
recompile of the kernel. If there are problems with the Open, this is a fatal, unrecoverable error and the
kernel startup process will fail.
Next, you’ll need to create the logical address space and read the executable into it. The method
OpenFile.LoadExecutable will take care of both tasks. If this fails, the kernel cannot start up.
LoadExecutable returns the entry point, which you might call initPC.
Don’t forget to close the executable file you opened earlier, or else a system resource will be
permanently locked up.
Next, you’ll need to compute the initial value for the user-level stack, which you might call
InitUserStackTop. It should be set to the logical address just past the end of the logical address space,
since the initial push onto the user stack will first decrement the top pointer. The logical address space
starts at zero. The logical address space contains
addrSpace.numberOfPages
pages. Each page has size PAGE_SIZE bytes.
The StartUserProcess function will end by jumping into the user-level program. This is a one way
jump; execution will never return. (Instead, if the user-level program needs to re-enter the kernel, it will
execute a syscall). As such, nothing on the system stack will ever be needed again. We want to have a
full-sized system stack available for processing any syscalls or interrupts that happen later, so you need
to reset the system stack top pointer, effectively clearing the system stack.
You might call the new value initSystemStackTop. You’ll need to set it to:
& currentThread.systemStack[SYSTEM_STACK_SIZE-1]
Project 5 Operating Systems
Page 28
Next, you’ll need to turn this thread into a user-level thread. This involves these actions:
1. Disable interrupts
2. Initialize the page table registers for this logical address space
3. Set the isUserThread variable in the current thread to true
4. Set system register r15, the system stack top
5. Set user register r15, the user stack top
6. Clear the System mode bit in the condition code register to switch into user mode
7. Set the Paging bit in the cond. code register, causing the MMU to do virtual memory mapping
8. Set the Interrupts Enabled bit in the cond. code register, so that future interrupts will be handled
9. Jump to the initial entry point in the program
Recall that every thread begins life with interrupts enabled, so your StartUserProcess function will be
executing with interrupts enabled. The first step is to disable interrupts, since there are possible race
conditions with steps (2) and (3).
[What is the race problem? Consider what happens if a context switch (i.e., timer interrupt) were to
occur between setting the page table registers and setting isUserThread to true. Look at the Run
method. The MMU registers would be changed for the other process; then when this thread is once
again scheduled, the code in Run will see isUserThread==false so it will not restore the MMU
registers. Merely swapping the order of steps (2) and (3) results in a similar race condition.]
The first 3 steps can be done in high-level KPL code, but steps (4) through (9) must be done in assembly
language.
Read through the BecomeUserThread assembly routine in the file Switch.s., which will take care of
steps (4) through (9). StartUserProcess should end with a call to this routine:
BecomeUserThread (initUserStackTop, initPC, initSystemStackTop)
The BecomeUserThread will change the mode bits and perform the jump “atomically.” This must be
done atomically since the target jump address is a logical address space. (The way it does this is a little
tricky: it pushes some stuff onto the system stack to make it look like syscall or interrupt has occurred,
and then executes a “reti” instruction.)
BecomeUserThread jumps to the user-level main routine and never returns.
Approach to Implementing the Exec Syscall
The sequence of steps in InitFirstProcess and StartUserProcess is very similar to what you’ll need
when implementing the Exec syscall. You should be able to copy much of this code when
implementing Sys_Handle_Exec.
Project 5 Operating Systems
Page 29
One difference is that during an Exec, you already have a process and a thread, so you will not need to
allocate a new ProcesControlBlock, allocate a new Thread object, or do a fork. However, you will
have to work with two virtual address spaces. The LoadExecutable method requires an empty
AddrSpace object; it will then allocate as many frames as necessary and initialize the new address
space.
Unfortunately, LoadExecutable may fail and, if so, your kernel must be able to return to the process
that invoked Exec (with an error code, of course). So you better not get rid of the old address space
until after the new one has been initialized and you can be sure that no more errors can occur.
One approach is to create a local variable of type AddrSpace. Don’t allocate it on the heap, just use
something like:
var newAddrSpace: AddrSpace = new AddrSpace
Then, after the new address space has been set up, you can copy it into the ProcessControlBlock, e.g.,
currentThread.myProcess.addrSpace = newAddrSpace
Don’t forget to free the frames in the previous address space first, or else valuable kernel resources will
remain forever unavailable and the kernel will eventually freeze up!
Another tricky thing is copying the filename string from a virtual address space into the kernel address
space where it can be used. The filename argument is a virtual address, but since the kernel is running
in Handle_Sys_Exec, paging will be turned off.
You’ll need to copy the characters into an array variable, not something newly allocated on the heap. It
is okay to put a maximum size on this array and then check that it is not exceeded. In fact, there is a
constant in Kernel.h for this purpose:
const
MAX_STRING_SIZE = 20
(In a real OS, the maximum string size would be much larger or even nonexistent. Here, we use a small
size to make testing the limits easier.)
Note that the filename pointer is virtual address, which must be translated into a physical address; you
can’t just use it, as is. This requires some code to perform the page table lookup in software.
Furthermore, since the filename string is in virtual space, it may cross page boundaries. (In fact, the test
program contains cases where this happens!)
Dealing with the filename is fairly complex, but it turns out that we are giving you a method
GetStringFromVirtual (kernelAddr: String, virtAddr, maxSize: int) returns int
which will do most of the work. (GetStringFromVirtual calls CopyBytesFromVirtual to do the
copying.) The GetStringFromVirtual method can be used like this:
Project 5 Operating Systems
Page 30
var
strBuffer: array [MAX_STRING_SIZE] of char
…
i = currentThread.myProcess.addrSpace.GetStringFromVirtual (
& strBuffer,
filename asInteger,
MAX_STRING_SIZE)
if i < 0 ...error... endIf You might think of allocating a temporary buffer on the heap, but remember that we do not want to allocate anything on the heap after kernel start-up. [ Recall that the “alloc” expression in KPL always allocates bytes on the heap. Once the kernel has booted and is running, you must avoid further allocations. Why? One problem is automatic garbage collection like you see in Java; we can’t use automatic garbage collection since it would produce unpredictable delays and might cause the kernel to miss interrupts or, in the case of a real-time system, miss deadlines. Also, there is the possibility that the heap might fill up, and dealing with a “heap full” error in the kernel is difficult. Another option might be to try to manage the heap without automatic garbage collection, but years of C++ experience has taught everybody that this is very difficult to do correctly. This explains why we have gone to the trouble to create classes like ThreadManager and ProcessManager, instead of simply allocating new Thread and ProcessControlBlock objects. ] AllocateRandomFrames The main function includes a function named AllocateRandomFrames, which is aimed only at catching bugs in the kernel. This function will allocate every other frame in the physical memory and never release them, creating a “checkerboard pattern” in memory. Henceforth, no two pages will ever be allocated to contiguous page frames. Large, multi-byte chunks of data in the user-level process’s address space will occasionally span page boundaries. Since these pages may not be in adjacent frames, your kernel will have to be careful about moving data to and from user space. What may appear to the user-level program as a string of adjacent bytes may in fact be spread all over memory. Some of the user-level syscalls pass pointers to the kernel. For example, Open passes a pointer to a string of characters. Keep in mind that this pointer is a logical address, not a physical address. As such, you cannot simply use the pointer as is. Take a look at these methods AddrSpace: CopyBytesFromVirtual (kernelAddr, virtAddr, numBytes: int) returns int CopyBytesToVirtual (virtAddr, kernelAddr, numBytes: int) returns int GetStringFromVirtual (kernelAddr: String, virtAddr, maxSize: int) returns int An invocation of AllocateRandomFrames has been added just after the FrameManager is initialized. Please leave this in and do not modify the AllocateRandomFrames routine. Project 5 Operating Systems Page 31 What to Hand In Please submit hardcopy of your output and hardcopy your new versions of the following files: Kernel.h Kernel.c For both files, please take a colored pen and circle exactly those parts you have added or changed. Please submit all of Kernel.h. For Kernel.c, please submit only the pages that have something circled. In other words, please discard all pages that contain ONLY material that we are distributing. Please keep the remaining pages in order. Also, if you have modified any part of a method or function, please hand in the entire function, so there is a little context surrounding your code modifications. As a second option, which will save on wasted paper, you may perform the deletions with an editor, instead of discarding physical pages. You can cut out large sections of code, but please indicate where material has been clipped out. Please replaced deleted material with a line like this XXXXXXXXXXXXXXXXXXX skipping XXXXXXXXXXXXXXXXXXX to indicate which lines were removed. But please remember to circle your material with a colored pen. For the output, please run TestProgram1 (which exec’s TestProgram2) and submit what it produces. Coding Style For all projects, please follow our coding style. Make your code match our code as closely as possible. The goal is for it to be impossible to tell, just by looking at the indentation and commenting, whether we wrote the code or you wrote the code. (Of course, your code will be circled!) Please use our convention in labeling and commenting functions: Project 5 Operating Systems Page 32 ----------------------------- Foo --------------------------------- function Foo () -- -- This routine does.... -- var x: int x = 1 x = 2 ... x = 9 endFunction Methods are labeled like this, with both class and method name: ---------- Condition . Wait ---------- method Wait (mutex: ptr to Mutex) ... Note that indentation is in increments of 2 spaces. Please be careful to indent statements such as if, while, and for properly. If you follow these conventions, then even if you throw away other pages, it should be clear which class the method belongs to. Sample Output If your program works correctly, you should see something like this: =================== KPL PROGRAM STARTING =================== Initializing Thread Scheduler... Initializing Process Manager... Initializing Thread Manager... Initializing Frame Manager... AllocateRandomFrames called. NUMBER_OF_PHYSICAL_PAGE_FRAMES = 100 Initializing Disk Driver... Initializing Serial Driver... Serial handler thread running... Initializing File Manager... Loading initial program... User-level program 'TestProgram1' is running... ***** Testing Syscall Parameter Passing ***** ***** About to call Sys_Yield... ***** Should print: ***** Handle_Sys_Yield invoked! Handle_Sys_Yield invoked! ***** About to call Sys_Fork... ***** Should print: ***** Handle_Sys_Fork invoked! Handle_Sys_Fork invoked! Project 5 Operating Systems Page 33 ***** About to call Sys_Join... ***** Should print: ***** Handle_Sys_Join invoked! ***** processID = 1111 Handle_Sys_Join invoked! processID = 1111 ***** About to call Sys_Create... ***** Should print: ***** Handle_Sys_Create invoked! ***** virt addr of filename = 0x0000BFF8 ***** filename = MyFileName Handle_Sys_Create invoked! virt addr of filename = 0x0000BFF8 filename = MyFileName ***** About to call Sys_Open... ***** Should print: ***** Handle_Sys_Open invoked! ***** virt addr of filename = 0x0000BFF8 ***** filename = MyFileName Handle_Sys_Open invoked! virt addr of filename = 0x0000BFF8 filename = MyFileName ***** About to call Sys_Read... ***** Should print: ***** Handle_Sys_Read invoked! ***** fileDesc = 2222 ***** virt addr of buffer = 0x0000B0A8 ***** sizeInBytes = 3333 Handle_Sys_Read invoked! fileDesc = 2222 virt addr of buffer = 0x0000B0A8 sizeInBytes = 3333 ***** About to call Sys_Write... ***** Should print: ***** Handle_Sys_Write invoked! ***** fileDesc = 4444 ***** virt addr of buffer = 0x0000B0A8 ***** sizeInBytes = 5555 Handle_Sys_Write invoked! fileDesc = 4444 virt addr of buffer = 0x0000B0A8 sizeInBytes = 5555 ***** About to call Sys_Seek... ***** Should print: ***** Handle_Sys_Seek invoked! ***** fileDesc = 6666 ***** newCurrentPos = 7777 Handle_Sys_Seek invoked! fileDesc = 6666 newCurrentPos = 7777 ***** About to call Sys_Close... ***** Should print: ***** Handle_Sys_Close invoked! ***** fileDesc = 8888 Handle_Sys_Close invoked! fileDesc = 8888 ***** About to call Sys_Exit... ***** Should print: ***** Handle_Sys_Exit invoked! Project 5 Operating Systems Page 34 ***** returnStatus = 9999 Handle_Sys_Exit invoked! returnStatus = 9999 ***** Syscall Test Complete ***** ***** Testing Exec Syscall ***** ***** About to call Sys_Exec with a non-existant file... ***** Should print: ***** Okay Okay ***** About to call Sys_Exec with an overly long file name... ***** Should print: ***** Okay Okay ***** About to perform a successful Exec and jump to TestProgram2... ***** Should print: ***** User-level program 'TestProgram2' is running! User-level program 'TestProgram2' is running! ***** About to call Sys_Shutdown... ***** Should print: ***** FATAL ERROR in UserProgram: "Syscall 'Shutdown' was invoked by a user thread" -- TERMINATING! FATAL ERROR in UserProgram: "Syscall 'Shutdown' was invoked by a user thread" -- TERMINATING! (To find out where execution was when the problem arose, type 'st' at the emulator prompt.) =================== KPL PROGRAM TERMINATION ===================