I have the following code snippet:
mov $0, %eax
mov $0x2023, %ax
mov $1, %ah
What happens when mov $0x2023, %ax is executed?
I expected %eax=0x20231 but it is 0x123. I've used a debugger and saw that %ax was 0x2023, but %ah becomes 20 and %al becomes 23. Why does this happen? Below is how I thought the code will run.
mov $0, %eax # eax = (AX)0000000000000000 (AH)00000000 (AL)00000000
mov $0x2023, %ax # eax = (AX)0010 0000 0010 0011 (AH)00000000 (AL)00000000
mov $1, %ah # eax = (AX)0010 0000 0010 0011 (AH)00000001 (AL)00000000
EAX is not composed of AX, AH, and AL the way you think!
See How do AX, AH, AL map onto EAX?
high end low end
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v v
<--- EAX --->
mov $0, %eax 00 00 00 00
<-AX->
mov $0x2023, %ax 00 00 20 23
AH AL
mov $1, %ah 00 00 01 23
<-??->
No name was given to the ?? part that runs from bit 16 to bit 31.
Personally, I regularly call it AXE.