How to add a value in register bh to ecx in NASM assembler?

Question

Suppose I have a number in bh, how do I add it to ecx? I cannot just

add ecx, bh

since it will cause an error:

invalid combination of opcode and operands

because ecx is a 32 bit register and bh is an 8 bit register.

Answer 1

If it's ok to modify ebx, then for unsigned add:

    movzx   ebx,bh
    add     ecx,ebx

and for signed add:

    movsx   ebx,bh
    add     ecx,ebx

if it's not ok to modify ebx, then for unsigned add:

    add     cl,bh
    jnc     nocarry0
    add     ecx,0100h
nocarry0:

or just use the first two examples preceded by push ebx, and followed by pop ebx.

Answer 2

You can't add values of different sizes (the CPU doesn't support it). Instead, try something like this:

    movzx ebx,bl        ;Zero-extend BL
    add ecx,ebx

Note: For signed numbers you can also use movsx to sign-extend.

Answer 3

unsigned add:

add cl,bh
adc ecx,0100h