XOR/AND not clearing out the OF flag when needed

[ssm951]

According to the CS:APP book 3rd edition, section 3.6.1, it is stated "For the logical operations, such as XOR, the carry and overflow flags are set to zero". To my understanding, other than not having the carry flag, y86 should have the same behavior on condition codes as x86.

Currently, if you do something like an addq that sets the overflow flag, the xorq instruction does not clear it. This is an easy fix in the instr.js file, line 81, where you can first set OF to zero before going into the switch statement.

Here is an example of a .ys code that reflects the discrepancy with the book stated above:

irmovq scratch, %rax
rrmovq %rax, %rbx
rmmovq %rbx, 0x8(%rax)
irmovq 0x9, %rax
irmovq 0x7fffffffffffffff, %rbx
addq %rbx, %rax
xorq %rax, %rax 
halt

scratch:

[ssm951]

Fixed by the approval of this pull request: #7

Thanks @boginw for keeping up with this project. We use this in our university to teach students about assembly code.