what does the cmpq instruction do?
assembly, x86, x86-64
Solution
`cmpq $-4095, %rax` compares the 64-bit register %rax with the immediate value `-4095` - the value is sign-extended to 64-bits for the purposes of the comparison. i.e., `-4095` has the 64-bit 2's complement representation: `ffff ffff ffff f001`
`cmp` instructions set the flags register as it would for a `sub` (subtract) of the second operand from the first - 'second' and 'first' being reversed in AT&T syntax. In effect the flags are set according to the result of: `(RAX - (- 4095))` or `(RAX + 4095)`, being the same in 2's complement.
One of the flags set is the carry flag (CF), which is set on (unsigned) overflow. The `jae` instruction (jump-if-above-or-equal) is actually an 'alias' for `jnc` (jump-if-not-carry). In other words, the branch is taken if `(RAX + 4095)` does not carry. In 2's complement, this will be true for values of `RAX` in the range: `[-4095, -1]`. (Keeping in mind how 2's complement arithmetic wraps).
The instructions, including `cmp` and `jae` (or `j<cond>`) are described in: Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 2.
The [E]FLAGS register (and what the arithmetic flags denote) are described in section 3.4.3 of Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 1.
Problem
I was reading the following definition for syscall: ``` .text .globl syscall .type syscall,%function .align 16 syscall: movq %rdi, %rax /* Syscall number -> rax. */ movq %rsi, %rdi /* shift arg1 - arg5. */ movq %rdx, %rsi movq %rcx, %rdx movq %r8, %r10 movq %r9, %r8 movq 8(%rsp),%r9 /* arg6 is on the stack. */ syscall /* Do the system call. */ cmpq $-4095, %rax /* Check %rax for error. */ jae __syscall_error /* Branch forward if it failed. */ ret /* Return to caller. */ .size syscall,.-syscall ``` I saw it explained that the line `cmpq $-4095 %rax` determines whether %rax contains a value between -1 and -4095. How does it do that? What exactly does the cmpq instruction do?