问题描述
我在读下面定义的系统调用：
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
我看到它解释说，该行 cmpq $ -4095％RAX 确定％RAX是否包含-1和-4095之间的值。它是如何做到的？究竟该cmpq指令做什么呢？
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?
推荐答案
cmpq $ -4095，RAX％ 64位寄存器％RAX与即时值进行比较 -4095 - 值是符号扩展为64位，用于比较的目的。即 -4095 有64位2的补再presentation： FFFF FFFF FFFF F001
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 指令设置的标志从第二个操作数的注册，因为它会为分（减）第一 - '第二'和'第一'被扭转了AT＆amp; T公司的语法。 或（RAX + 4095）（ - - （4095）RAX）：在效果标志根据结果集C>，是在2的补数是相同的。
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.
一组标志是的执行的标志（CF），这是在（符号）溢出设置。在宰指令（跳，如果在上面的或相等）实际上是 JNC （跳一个别名 - 如果 - 不进位）。换句话说，该分支机构采取了（RAX + 4095）做的不的携带。在2的补数，这将是在范围 RAX 的价值观真： [ - 4095，-1] 。 （牢记2的补数算术环绕方式）。
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).
该指令，包括 CMP 和宰（或 J＆LT; COND＆GT; ）的描述：Intel® 64和IA-32架构软件开发人员手册，第2卷。
The instructions, including cmp and jae (or j<cond>) are described in: Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 2.
在[E]标志位寄存器（什么算术标志表示）在Intel® 64和IA-32架构软件开发人员手册，第1卷。
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.
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