为什么NA​​SM在使用有效的指令助记符作为操作数中的符号名称时不会遇到麻烦? [英] Why doesn't NASM have trouble with valid instruction mnemonics as symbol names in operands?

问题描述

我编写了以下简单程序，但是nasm拒绝对其进行编译.

I wrote the following simple program, but nasm refuses to compile it.

section .text
    global _start

_start:
    mov rax, 0x01 
    mov rdi, 0x01
    mov rsi, str
    mov rdx, 0x03
    syscall

    mov rax, 60
    syscall

segment .data
    str db 'Some string'


nasm -f elf64 main.asm
main.asm:15: error: comma, colon, decorator or end of line expected after operand

我在此 answer 中读到的原因是str是指令助记符.因此，我在str中添加了一个冒号，现在可以正常编译了.但是那行

As I read in this answer this is because str is an instruction mnemonic. So I added a colon to str and now it compiles fine. But what about the line

mov rsi, str

str是此处的指令助记符，但仍可以正常编译.为什么?

str is an instruction mnemonic here, but it still compiles fine. Why?

推荐答案

如 NASM手册所述，除了宏定义和指令，NASM源代码行的格式具有以下四个字段的组合:

As the NASM manual explains, other than macro definitions and directives, the format of a NASM source line has some combination of these four fields:

label:    instruction operands        ; comment

将mov视为助记符后，便不再将其余标记视为可能的指令助记符.汇编语言严格来说是每个语句仅使用一条指令.

After it sees mov as the mnemonic, it's no longer considering the remaining tokens as possible instruction mnemonics. Assembly language is strictly one instruction per statement.

如果您想将编码str ax指令的字节作为mov -sign-extended-imm32的立即操作数，则必须自己使用数字常量来完成. NASM语法没有为您执行此操作的方法，因此一旦发现助记符，其解析器就无需递归到操作数中.

If you wanted the bytes that encode an str ax instruction as the immediate operand for mov-sign-extended-imm32, you'd have to do that yourself with a numeric constant. NASM syntax doesn't have a way to do that for you, so its parser doesn't need to recurse into the operands once its found a mnemonic.

或者使用db发出mov指令的字节，而不是手动编码str.

Or instead of encoding str manually, use a db to emit the bytes of the mov instruction.

db 0x48, 0xc7, 0xc6    ; REX.W prefix, opcode for mov r/m64,imm32,  ModR/M = rsi destination
      str  [rax+1]     ; a disp8 makes this 4 bytes long.


;; same machine code as
mov rsi, strict dword 0x0148000f    ; str [rax+1]

;; nasm optimizes it to mov esi, imm32 without strict dword.
;; I guess I should have used that 5-byte instruction for the DB version...

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