我正在阅读syscall的下列定义:
.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
我看到有人解释说,cmpq $-4095 %rax
这行代码是用来判断%rax是否包含处于-1和-4095之间的值。它是如何判断的?cmpq指令具体是做什么的?