在Intel 80386的NASM程序中，如何无移位/旋转设置GPR特定位的进位标志？

Absolutely! You can set the Carry Flag (CF) based on a specific bit in a general-purpose register (GPR) without using any shift or rotate operations, while preserving all other bits in the register. Here are two reliable approaches for NASM on the 80386:

Approach 1: Using a Temporary Register (Most Efficient) #

If you have an unused GPR (like ecx in this example) to spare, this method avoids stack operations entirely:

; Example: Set CF to the value of bit 5 in EAX (0x20 = 2^5)
mov ecx, eax       ; Save a copy of EAX's original value
and ecx, 0x20      ; Mask out everything except the target bit (bit 5)
neg ecx            ; Set CF based on the masked value:
                   ; - If the target bit was 1, ecx is non-zero → CF = 1
                   ; - If the target bit was 0, ecx is 0 → CF = 0
; EAX remains completely unchanged throughout this process

Approach 2: Using the Stack (No Temporary Registers Needed) #

If all GPRs are in use, you can use the stack to preserve the original register value:

; Example: Set CF to the value of bit 3 in EBX (0x08 = 2^3)
push ebx           ; Save EBX's original value to the stack
and ebx, 0x08      ; Isolate the target bit (bit 3)
neg ebx            ; Update CF: non-zero → CF=1, zero → CF=0
pop ebx            ; Restore EBX to its original state

Why This Works #

The neg instruction is key here. On the 80386, neg sets the Carry Flag as follows:

• CF = 1 if the operand is non-zero (meaning our target bit was 1, since we masked all other bits out)
• CF = 0 if the operand is zero (meaning our target bit was 0)

This method strictly avoids shift/rotate operations (shl, shr, rol, ror, etc.) and guarantees the original GPR's bits remain untouched.

内容的提问来源于stack exchange，提问作者MaxiMaxi