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assemblyx86stackcpu-registers

Clarify subsections of 32-bit registers x86

I have a question regarding the subsections of the 32-bit registers in x86 assembly: Are the lower subsections of a register (when used) treated like a part of the corresponding 32-bit register?

ror cl,2    
ror cl,2
push ecx
pop eax

Take for example the code section above - since CL is a subsection of the ECX register, when I push the contents of the register onto the stack does what was in the CL subsection get pushed onto the stack as well? Or are they treated separately?

And suppose if I then popped the stack onto the EAX register does what was in the CL register from before go into the EAX register?

Solution

  • The ECX register contains CX in its lower part, which contains CH and CL as represented in the picture below. When modifying the value stored in CL, you also change CX and ECX, but not CH.

    To answer your questions:

    Q: When I push the contents of the register [ECX] onto the stack does what was in the CL subsection get pushed onto the stack as well? Or are they treated seperatley?.
    A: All in one shot. CL is part of ECX. CL is just a way provided by the processor to access a part of ECX.

    Q: And suppose if I then poped the stack onto the EAX register does what was in the CL register from before go into the EAX register?.
    A: Yes. More precisely, what was in CL would go into AL.

    x86-32 register board

    Source: A tutorial on Register Allocation by Puzzle Solving (on compilers.cs.ucla.edu)