Race Conditions

This quiz is based on HW-ThreadsIntro.

The idea is to gain familiarity with threads by seeing how they interleave; the simulator, x86.py, will help you in gaining this understanding.

The simulator mimics the execution of short assembly sequences by multiple threads. Note that the OS code that would run (for example, to perform a context switch) is *not* shown; thus, all you see is the interleaving of the user code.

The assembly code that is run is based on x86, but somewhat simplified. In this instruction set, there are four general-purpose registers (%ax, %bx, %cx, %dx), a program counter (PC), and a small set of instructions which will be enough for our purposes.

Here is an example code snippet that we will be able to run:

.main
mov 2000, %ax # get the value at the address
add $1, %ax # increment it
mov %ax, 2000 # store it back
halt

The code is easy to understand. The first instruction, an x86 "mov", simply loads a value from the address specified by 2000 into the register %ax. Addresses, in this subset of x86, can take some of the following forms:

2000 -> the number (2000) is the address
(%cx) -> contents of register (in parentheses) forms the address
1000(%dx) -> the number + contents of the register form the address
10(%ax,%bx) -> the number + reg1 + reg2 forms the address

To store a value, the same "mov" instruction is used, but this time with the arguments reversed, e.g.:

mov %ax, 2000

The "add" instruction, from the sequence above, should be clear: it adds an immediate value (specified by $1) to the register specified in the second argument (i.e., %ax = %ax + 1).

Thus, we now can understand the code sequence above: it loads the value at address 2000, adds 1 to it, and then stores the value back into address 2000.

The fake-ish "halt" instruction just stops running this thread.

We will run two threads, thread 0 and thread 1, with context switches occurring at random, indicated points.  You should report the value held in memory and the value held in the %ax register after each instruction.