assembly - NASM 中的 "push BYTE 0x80"和 "warning: signed byte value exceeds bounds"

Question

当我尝试组装以下代码时:

push BYTE 0x80

NASM 显示以下警告:

warning: signed byte value exceeds bounds

这是什么意思？请注意，低于 0x80 的值不会导致警告。

Answer 1

PUSH imm8实际上并没有将一个字节压入堆栈。它至少压入一个字，该字将从字节操作数进行符号扩展。因此，操作数为 0x80你实际上最终推了 0xff80 , 0xffffff80或 0xffffffffffffff80 ，这很可能是 NASM 警告您的内容。

这在 Intel's manual 中有描述:

The D flag in the current code-segment descriptor determines the default operand size; it may be overridden by instruction prefixes (66H or REX.W).
The operand size (16, 32, or 64 bits) determines the amount by which the stack pointer is decremented (2, 4 or 8).
If the source operand is an immediate and its size is less than the operand size, a sign-extended value is pushed on the stack.