Description
In this assignment you will load RV32I binary executable file into your memory simulator and
decode/disassemble the instructions.
This is the second of a multi-part assignment involving the creation of a machine capable
of executing programs compiled with g++. The purpose of this assignment gain a thorough
understanding of the RV32I machine instructions.
1 Problem Description
Disassemble an executable binary file by loading it into a simulated memory of sufficient size and
then decode each 32-bit instruction one-at-a-time starting from address zero and continue through
to the end of the simulated memory.
2 Files You Must Write
You will write a C++ program suitable for execution on hopper.cs.niu.edu (or turing.cs.niu.edu.)
Your source files MUST be named exactly as shown below or they will fail to compile and you will
receive zero points for this assignment.
Create a directory named a4 and place within it a copy of all the the source files from Assignment
3 plus the rv32i_decode files defined below.
main.cpp Your main() and usage() function definitions will go here.
hex.h The declarations of your hex formatting functions will go here.
hex.cpp The definitions of your hex formatting functions will go here.
memory.h The definition of your memory class will go here.
memory.cpp The memory class member function definitions will go here.
rv32i decode.h The definition of a class named rv32i_decode will go here.
rv32i decode.cpp The definitions of any member functions of class rv32i_decode will go here.
2.1 main.cpp
You must provide a main() function that is implemented as shown below plus a new utility function
to disassemble the memory contents. This version of main() differs from the that in Assignment 3
by replacing the memory test function calls with the use of the new rv32i_decode class and the
addition of a disassemble() helper function:
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CSCI 463 Assignment 4 – RV32I Disassembler
static void disassemble(const memory &mem)
{
…
}
int main(int argc, char **argv)
{
uint32_t memory_limit = 0x100; // default memory size = 256 bytes
int opt;
while ((opt = getopt(argc, argv, “m:”)) != -1)
{
switch (opt)
{
case ’m’:
{
std::istringstream iss(optarg);
iss >> std::hex >> memory_limit;
}
break;
default: /* ’?’ */
usage();
}
}
if (optind >= argc)
usage(); // missing filename
memory mem(memory_limit);
if (!mem.load_file(argv[optind]))
usage();
disassemble(mem);
mem.dump();
return 0;
}
The disassemble() function is what will decode and print each instruction in a loop.
For each 32-bit word in the simulated memory:
• Call rv32i_decode::decode() to format/render one instruction mnemonic and return it as
a std::string.
• Print the memory address, instruction hex value, and the instruction mnemonic.
See the example output files below and on the course web page for details on how to align and
space the output.
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CSCI 463 Assignment 4 – RV32I Disassembler
2.2 hex.h and hex.cpp
Same as Assignment 3 plus the following additional methods:
• static std::string to_hex0x20(uint32_t i);
This function must return a std::string beginning with 0x, followed by the 5 hex digits
representing the 20 least significant bits of the i argument. Implement this method in a
similar fashion as hex::to_hex8() but without the static_cast.
This new method will be used when formatting the lui and auipc instructions.
• static std::string to_hex0x12(uint32_t i);
This function must return a std::string beginning with 0x, followed by the 3 hex digits
representing the 12 least significant bits of the i argument. See to_hex0x20().
This new method will be used when formatting the csrrX() instructions.
2.3 memory.h and memory.cpp
Same as for Assignment 3.
2.4 rv32i decode.h and rv32i decode.cpp
Your rv32i_decode class must, at least, include the members discussed below plus appropriate
documentation.
Failure to follow this design will cause significant problems with future assignments implemented
by extending this assignment.
Note that all of the members of this class are static and none will actually “print” anything to
the screen.
The rv32i_decode class is a subclass of the hex class. Therefore it will inherit all the hex formatting
methods.
2.4.1 rv32i decode Member Constants
• static constexpr uint32_t XLEN = 32;
XLEN represents the number of bits in RV32I CPU registers.
• You will find it useful to define a number of constants that represent the binary values of the
opcode, funct3, and funct7 fields of each of the instructions as well as some useful widths for
formatting the returned string values. For example:
static constexpr int mnemonic_width = 8;
static constexpr uint32_t opcode_lui = 0b0110111;
static constexpr uint32_t opcode_auipc = 0b0010111;
…
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CSCI 463 Assignment 4 – RV32I Disassembler
static constexpr uint32_t funct3_beq = 0b000;
static constexpr uint32_t funct3_lb = 0b000;
static constexpr uint32_t funct3_sll = 0b001;
…
Note that these opcode constants are expressed with binary literals. This notation was added
in C++14. (Make sure you use the proper compiler flags.)
A file with the above and other useful constants will be made available on the course web
page.
2.4.2 rv32i decode Member Functions
• static std::string decode(uint32_t addr, uint32_t insn);
It is the purpose of this function to return a std::string containing the disassembled instruction text.
It must be capable of handling any possible 32-bit insn value (regardless of it representing a
useful/legal instruction or not.)
Note that the memory address parameter addr, from which the instruction has been fetched,
is passed to decode() so that it may be used to calculate the PC-relative target address shown
in the J-type and B-type instructions.
You can implement this software decoder by using a switch statement with the value of the
opcode field by extracting it from insn by calling get_opcode(insn).
For each case, call a function to format/render the instruction and its arguments (when
the opcode is specific enough to do so such as for the lui instruction) or use a sub-switch
statement to further decode any additionally required fields (such as funct3 and/or funct7)
and then call a function to format the instruction as shown in section 7.
Note that, most of the instructions are in groups such that the disassembled instruction
format is the same. For example, all of the store instructions differ only in their mnemonic.
Take advantage of this by factoring your std::string formatting logic to minimize the
amount of replicated code. To do this, add formatting helper-functions like these to the
rv32i_decode class as needed:
static std::string render_illegal_insn();
static std::string render_auipc(uint32_t insn);
static std::string render_jal(uint32_t addr, uint32_t insn);
static std::string render_btype(uint32_t addr, uint32_t insn, const char *mnemonic);
static std::string render_itype_load(uint32_t insn, const char *mnemonic);
static std::string render_itype_alu(uint32_t insn, const char *mnemonic, int32_t imm_i);
static std::string render_stype(uint32_t insn, const char *mnemonic);
…
The simplest of these is the renderer for invalid instructions:
std::string rv32i_decode::render_illegal_insn()
{
return “ERROR: UNIMPLEMENTED INSTRUCTION”;
}
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CSCI 463 Assignment 4 – RV32I Disassembler
See the render_lui() example in section 7 for a more interesting example.
These above render_X() functions should also use helper-functions like those below to eliminate messy replicated code:
– static std::string render_reg(int r);
Render, into a std::string, the name of the register with the given number r.
For example, render_reg(23) would return a string with the value: x23
Calling this member is the one and only way that any code may format the name of a
register into a string (for subsequent printing by the program.)
– static std::string render_base_disp(uint32_t register, int32_t imm);
Use this to render, into a std::string, the operands of the form imm(register) for
those instructions that have such an addressing mode.
Note that, in this case, the imm value is printed in decimal. The register should be
rendered into a string by render_reg().
Calling this member is the one and only way that any code may format an imm(register)
operand in your application.
– static std::string render_mnemonic(const std::string &mnemonic);
Render, into a std::string, the given instruction mnemonic with the proper space
padding on right side to make it mnemonic_width characters long.
By using this to make all mnemonics a uniform width when printing instructions that
have operands, aligning the operands will be much easier.
Note that ecall and ebreak are the only instructions that do not have operands and
that must therefore not be rendered with padding on the right.
You should feel free to alter or add more helper-functions for rendering that you find useful.
The following helper-functions for extracting the fields from a 32-bit instruction will make the
instruction decoding logic easier to write, debug, and understand:
• static uint32_t get_opcode(uint32_t insn);
Extract and return the opcode field from the given instruction. A suitable implementation
is:
return (insn & 0x0000007f);
• static uint32_t get_rd(uint32_t insn);
Extract and return the rd field from the given instruction as an integer value from 0 to 31.
• static uint32_t get_rs1(uint32_t insn);
Extract and return the rs1 field from the given instruction as an integer value from 0 to 31.
• static uint32_t get_rs2(uint32_t insn);
Extract and return the rs2 field from the given instruction an integer value from 0 to 31.
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CSCI 463 Assignment 4 – RV32I Disassembler
• static uint32_t get_funct3(uint32_t insn);
Extract and return the funct3 field from the given instruction as an integer value from 0 to
7.
• static uint32_t get_funct7(uint32_t insn);
Extract and return the funct7 field from the given instruction as an integer value from 0x00
to 0x7f.
• static int32_t get_imm_i(uint32_t insn);
Extract and return the imm_i field from the given instruction as a 32-bit signed integer as
shown in RVALP.
• static int32_t get_imm_u(uint32_t insn);
Extract and return the imm_u field from the given instruction as a 32-bit signed integer as
shown in RVALP.
• static int32_t get_imm_b(uint32_t insn);
Extract and return the imm_b field from the given instruction as a 32-bit signed integer as
shown in RVALP.
• static int32_t get_imm_s(uint32_t insn);
Extract and return the imm_s field from the given instruction. A suitable implementation is:
int32_t rv32i_decode::get_imm_s(uint32_t insn)
{
int32_t imm_s = (insn & 0xfe000000) >> (25-5);
imm_s |= (insn & 0x00000f80) >> (7-0);
if (insn & 0x80000000)
imm_s |= 0xfffff000; // sign-extend the left
return imm_s;
}
Note that the shifting distances in the above example have been specified as a calculation
using the values shown in the castellated diagrams in RVALP. Showing the shift distance
values such as: 25-5 is more clear than simply 20 because it a) illustrates your intent and b)
prevents a silly subtraction mistake from creating a problem.
• static int32_t get_imm_j(uint32_t insn);
Extract and return the imm_j field from the given instruction as a 32-bit signed integer as
shown in RVALP.
3 Input
Your program will accept the same command-line arguments as Assignment 3.
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CSCI 463 Assignment 4 – RV32I Disassembler
The optional [-m hex-mem-size] argument is a hex number representing the amount of memory
to simulate. When not given, the default value must be 0x100.
The last argument is the name of a file to load into the simulated memory. In this assignment, this
will be the name of a RV32I binary executable program.
You will be provided with a number of suitable executable test programs and associated output
files.
4 Output
Your program’s output will be a disassembly of the executable binary file followed by a dump of
the simulated memory.
The disassembled instructions are rendered with a mix of decimal and hex values:
• The far left column is the 32-bit address printed in hex.
• The second column is the 32-bit hex full-word representation of the instruction that was
fetched from your memory at the address in column 1.
• The third column is the instruction mnemonic.
• The presence and format of the fourth column depends on the type of the instruction.
– Register numbers are printed as an ’x’ followed by the decimal number of the register
(0-31) as in x12, x0, and x31.
– Hexadecimal literals are printed with a leading 0x as in 0xabcde.
– Decimal literals are printed with the first character being an optional negative sign ‘-‘
followed by one or more digits (0-9) as in 1234 and -58.
It seems unfortunate that the letter x was chosen for the register indicator since it is so close to
(and could be confused with) the indicator of hexadecimal numbers 0x. Be careful!
See below for a list of every instruction and the proper disassembly format. (A copy of this output
and others are available on the course web site.) Your program must match the reference output
precisely (including whitespace.)
winans@hopper:~$ ./rv32i -mc0 testdata/allinsns4.bin
00000000: abcde237 lui x4,0xabcde
00000004: abcde217 auipc x4,0xabcde
00000008: 004000ef jal x1,0x0000000c
0000000c: 00808267 jalr x4,8(x1)
00000010: ff808267 jalr x4,-8(x1)
00000014: 00000263 beq x0,x0,0x00000018
00000018: fe001ee3 bne x0,x0,0x00000014
0000001c: 00004263 blt x0,x0,0x00000020
00000020: 00005263 bge x0,x0,0x00000024
00000024: 00006263 bltu x0,x0,0x00000028
00000028: 00007263 bgeu x0,x0,0x0000002c
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CSCI 463 Assignment 4 – RV32I Disassembler
0000002c: 4d200203 lb x4,1234(x0)
00000030: b2e01203 lh x4,-1234(x0)
00000034: 4d202203 lw x4,1234(x0)
00000038: 80004203 lbu x4,-2048(x0)
0000003c: 00005203 lhu x4,0(x0)
00000040: 4c400923 sb x4,1234(x0)
00000044: 4c401923 sh x4,1234(x0)
00000048: 4c402923 sw x4,1234(x0)
0000004c: 4d228213 addi x4,x5,1234
00000050: fff3a313 slti x6,x7,-1
00000054: 8304b413 sltiu x8,x9,-2000
00000058: 0015c513 xori x10,x11,1
0000005c: 7d06e613 ori x12,x13,2000
00000060: 4d27f713 andi x14,x15,1234
00000064: 00c89813 slli x16,x17,12
00000068: 00c9d913 srli x18,x19,12
0000006c: 40cada13 srai x20,x21,12
00000070: 018b8b33 add x22,x23,x24
00000074: 41bd0cb3 sub x25,x26,x27
00000078: 01ee9e33 sll x28,x29,x30
0000007c: 0020afb3 slt x31,x1,x2
00000080: 0020b233 sltu x4,x1,x2
00000084: 0020c233 xor x4,x1,x2
00000088: 0042d1b3 srl x3,x5,x4
0000008c: 4042d1b3 sra x3,x5,x4
00000090: 0020e233 or x4,x1,x2
00000094: 0020f233 and x4,x1,x2
00000098: 000b1a73 csrrw x20,0x000,x22
0000009c: 001cabf3 csrrs x23,0x001,x25
000000a0: 002e3d73 csrrc x26,0x002,x28
000000a4: fff55ef3 csrrwi x29,0xfff,10
000000a8: 10066f73 csrrsi x30,0x100,12
000000ac: 00177ff3 csrrci x31,0x001,14
000000b0: 001ffff3 csrrci x31,0x001,31
000000b4: 00100073 ebreak
000000b8: 00000073 ecall
000000bc: a5a5a5a5 ERROR: UNIMPLEMENTED INSTRUCTION
00000000: 37 e2 cd ab 17 e2 cd ab ef 00 40 00 67 82 80 00 *7………@.g…*
00000010: 67 82 80 ff 63 02 00 00 e3 1e 00 fe 63 42 00 00 *g…c…….cB..*
00000020: 63 52 00 00 63 62 00 00 63 72 00 00 03 02 20 4d *cR..cb..cr…. M*
00000030: 03 12 e0 b2 03 22 20 4d 03 42 00 80 03 52 00 00 *…..” M.B…R..*
00000040: 23 09 40 4c 23 19 40 4c 23 29 40 4c 13 82 22 4d *#.@L#.@L#)@L..”M*
00000050: 13 a3 f3 ff 13 b4 04 83 13 c5 15 00 13 e6 06 7d *……………}*
00000060: 13 f7 27 4d 13 98 c8 00 13 d9 c9 00 13 da ca 40 *..’M………..@*
00000070: 33 8b 8b 01 b3 0c bd 41 33 9e ee 01 b3 af 20 00 *3……A3….. .*
00000080: 33 b2 20 00 33 c2 20 00 b3 d1 42 00 b3 d1 42 40 *3. .3. …B…B@*
00000090: 33 e2 20 00 33 f2 20 00 73 1a 0b 00 f3 ab 1c 00 *3. .3. .s…….*
000000a0: 73 3d 2e 00 f3 5e f5 ff 73 6f 06 10 f3 7f 17 00 *s=…^..so……*
000000b0: f3 ff 1f 00 73 00 10 00 73 00 00 00 a5 a5 a5 a5 *….s…s…….*
5 How To Hand In Your Program
When you are ready to turn in your assignment, make sure that the only files in your a4 directory
is/are the source files defined and discussed above. Then, in the parent of your a4 directory, use
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CSCI 463 Assignment 4 – RV32I Disassembler
the mailprog.463 command to send the contents of the files in your a4 project directory in the
same manner as we have used in the past.
6 Grading
The grade you receive on this programming assignment will be scored according to the syllabus
and its ability to compile and execute on the Computer Science Department’s computer.
It is your responsibility to test your program thoroughly.
When we grade your assignment, we will compile it on hopper.cs.niu.edu using these exact
commands:
g++ -g -ansi -pedantic -Wall -Werror -std=c++14 -c -o main.o main.cpp
g++ -g -ansi -pedantic -Wall -Werror -std=c++14 -c -o rv32i_decode.o rv32i_decode.cpp
g++ -g -ansi -pedantic -Wall -Werror -std=c++14 -c -o memory.o memory.cpp
g++ -g -ansi -pedantic -Wall -Werror -std=c++14 -c -o hex.o hex.cpp
g++ -g -ansi -pedantic -Wall -Werror -std=c++14 -o rv32i main.o rv32i_decode.o memory.o hex.o
Your program will then be run multiple times using different memory sizes and binary test files.
7 Hints
While this is the second part of a multi-part assignment, it too should be written in parts!
• Start by updating main.cpp and stub in the disassemble() method such that it does nothing
but return so you can just get your new framework to compile and verify that is OK by running
it and see that mem.load_file() and mem.dump() still work OK.
• Implement the disassemble() loop to just print the pc and fetched instruction-word values.
• Implement the render_illegal_insn() helper-function and then call it from a first draft
stub version of rv32i_decode::decode() that treats all the instructions as illegal. It might
look something like this:
std::string rv32i_decode::decode(uint32_t addr, uint32_t insn)
{
switch(get_opcode(insn))
{
default: return render_illegal_insn();
}
assert(0 && “unrecognized opcode”); // It should be impossible to ever get here!
}
. . . and your output would look something like this:
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CSCI 463 Assignment 4 – RV32I Disassembler
winans@ux410ua~$ ./rv32i -m48 testdata/tinyprog.bin
00000000: 00000137 ERROR: UNIMPLEMENTED INSTRUCTION
00000004: 00001137 ERROR: UNIMPLEMENTED INSTRUCTION
00000008: 80000137 ERROR: UNIMPLEMENTED INSTRUCTION
…
00000000: 37 01 00 00 37 11 00 00 37 01 00 80 37 01 00 40 *7…7…7…7..@*
00000010: 37 f1 ff 4f 37 f1 ff ff 37 f0 ff ff 17 01 00 00 *7..O7…7…….*
…
Note that it is OK to have a declaration of a method in your rv32i_decode class without
actually defining it as long as you don’t call it.
Note the use of an assertion to make sure that execution can never fall off the end of the
switch statement without stopping the program. Given the style of this particular switch
statement design that includes a default case with a return and, as will be shown below, any
cases added as the solution is evolved will also include a return statement, execution of the
code can never run past the end of the switch statement. If it does then it means that the
code in the switch statement is broken and the failing assertion will terminate the program
and let us know.
• Add each instruction opcode one at-a-time starting with lui like this:
std::string rv32i_decode::decode(uint32_t addr, uint32_t insn)
{
switch(get_opcode(insn))
{
default: return render_illegal_insn();
case opcode_lui: return render_lui(insn);
}
assert(0 && “unrecognized opcode”); // It should be impossible to ever get here!
}
std::string rv32i_decode::render_lui(uint32_t insn)
{
uint32_t rd = get_rd(insn);
int32_t imm_u = get_imm_u(insn);
std::ostringstream os;
os << render_mnemonic(“lui”) << render_reg(rd) << “,”
<< to_hex0x20((imm_u >> 12)&0x0fffff);
return os.str();
}
The output would then be:
./rv32i -m48 testdata/tinyprog.bin
00000000: 00000137 lui x2,0x00000
00000004: 00001137 lui x2,0x00001
00000008: 80000137 lui x2,0x80000
…
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CSCI 463 Assignment 4 – RV32I Disassembler
• When you write the get_imm_X() and get_functX() methods test them with a series of
useful hard-coded values to make sure you have all the bits repositioned and working correctly.
It can save you a HUGE amount of debugging time if you exhaustively test them with many
bit patterns before using them.
Extract the field data in these methods by using using a combination of the bitwise and &,
or |, and shift operators: >> and << as seen in the get_imm_s() example code seen earlier.
• Implement and test one opcode at-a-time using the reference card and the diagrams showing
how the instruction fields are decoded in “RISC-V Assembly Language Programming” (AKA
RVALP.) The latest version is located here: https://github.com/johnwinans/rvalp/releases/
(Click on the “Assets” menu for the latest/top shown release and open “book.pdf.”)
• Do all the the easy ones first such as the U-type and R-type instructions that appear at the
beginning of the tinyprog.bin example file.
• When implementing instructions that share the same rendering code, use one helper for all
of them. For example, the render_btype() can handle the rendering any of the branch
instructions and a render_itype_alu() helper-function can handle the rendering any of
I-type instructions that perform ALU operations as can be seen below:
std::string rv32i_decode::decode(uint32_t addr, uint32_t insn)
{
switch(get_opcode(insn))
{
default: return render_illegal_insn();
case opcode_lui: return render_lui(insn);
….
case opcode_btype:
switch (funct3)
{
default: return render_illegal_insn();
case funct3_beq: return render_btype(addr, insn, “beq”);
case funct3_bne: return render_btype(addr, insn, “bne”);
…
}
assert(0 && “unrecognized funct3”); // impossible
case opcode_alu_imm:
switch (funct3)
{
default: return render_illegal_insn();
case funct3_add: return render_itype_alu(insn, “addi”, get_imm_i(insn));
…
case funct3_sll: return render_itype_alu(insn, “slli”, get_imm_i(insn)%XLEN);
case funct3_srx:
switch(funct7)
{
default: return render_illegal_insn();
case funct7_sra: return render_itype_alu(insn, “srai”, get_imm_i(insn)%XLEN);
case funct7_srl: return render_itype_alu(insn, “srli”, get_imm_i(insn)%XLEN);
}
assert(0 && “unrecognized funct7”); // impossible
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CSCI 463 Assignment 4 – RV32I Disassembler
}
assert(0 && “unrecognized funct3”); // impossible
case …
…
}
assert(0 && “unrecognized opcode”); // It should be impossible to ever get here!
}
Note that the shamt value passed to the render_itype_alu() helper-function for the shiftimmediate instructions is the same thing as the imm_i value if we ignore all but the leastsignificant XLEN bits. . . which is the remainder if we divide the imm_i by XLEN.
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