Project 2a: The Unix Shell


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To implement a command line interpreter or, as it is more commonly known, a shell. The shell should operate in this basic way: when you type in a command (in response to its prompt), the shell creates a child process that executes the command you entered and then prompts for more user input when it has finished.

#### Project 2b: xv6 Scheduler
To implement a simplified multi-level feedback queue (MLFQ) scheduler in xv6.

The basic idea is simple. Build an MLFQ scheduler with four priority queues; the top queue (numbered 0) has the highest priority and the bottom queue (numbered 3) has the lowest priority. When a process uses up its time-slice, it should be downgraded to the next (lower) priority level. The time-slices for higher priorities will be shorter than lower priorities.

## My work

#### Linux: my shell

I put most of the parsing, internal command, and fork() into a function called
runCommand for the convenience of implementing the ! internal command. Circular array
is used to store the history commands, of which the maximum size is 20. If there
is redirection, the command stores in the tokens global variable and the redirection
file stores in the outputs global variable.

#### xv6: MLFQ scheduler

To make a MLFQ scheduler, I mainly modified the codes in two functions, alloproc()
and scheduler(). To store the processes, I built up 4 queues representing four
levels using singly linked list with head and tail pointer. In alloproc(), I
enqueued the processes, initialized them and put them at the end of the top level
priority queue. In scheduler(), I searched from the start of the top level queue
and as soon as I found a RUNNABLE proc, I chose it to run. After running, if the
timer ticks of the proc in this level exceeded this time slice of this level,
dequeued it and added it to the end of the next level ( the lowest level is
different, needed to add the end of the same level) and then researched RUNNABLE
proc from the start of the top level queue. Besides, if a UNUSED proc is met in the
searching, dequeued it.