将 1 个字节的立即数添加到 2 个字节的内存位置 [英] add 1 byte immediate value to a 2 bytes memory location

问题描述

来自

注意我突出显示的两条说明.

我在 NASM 中尝试了以下代码(符合第一条突出显示的指令):

添加 WORD [myvar], BYTE 0xA5

但我收到以下错误:

<块引用>

警告:有符号字节值超出范围

我做错了什么?

解决方案

8 位立即数操作数(这里用 imm8 表示)被符号扩展为 16(或 32)位以匹配另一个操作数的大小(分别为 r/m16 或 r/m32).

因此，只能表示 -128 到 127 之间的值，这就是您从汇编程序收到此警告的原因.

对于值 0xA5，您需要使用 WORD 立即数 (imm16):

添加 WORD [myvar], WORD 0xA5

(尽管 WORD 在源操作数上是可选的，因为常量的大小暗示了它).

The add instruction documentation from this page says the following:

Notice the two instructions that I highlighted.

I tried the following code in NASM (which conforms with the first highlighted instruction):

add WORD [myvar], BYTE 0xA5

But I got the following error:

warning: signed byte value exceeds bounds

What am I doing wrong?

解决方案

The 8-bit immediate operand (denoted here by imm8) is sign-extended into 16 (or 32) bits to match the size of the other operand (r/m16 or r/m32, respectively).

Thus, only values between -128 and 127 can be represented, which is why you receive this warning from the assembler.

For the value 0xA5, you need to use a WORD immediate (imm16):

add WORD [myvar], WORD 0xA5

(although the WORD is optional on the source operand, since it is implied by the constant's size).

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