OpenEuphoria

Re: Not too important asm beginner question.... (high and low

• Posted by Pete Eberlein <xseal at HARBORSIDE.COM> Apr 13, 1998
• 701 views

Eduardo Uemura Okada wrote:
> What are the low and high bytes of a register ?
>
> Thanks.
>
> Eduardo Uemura Okada

The low and high bytes are the lower 8-bits and upper 8-bits of a 16-bit
register.  Clear as mud, right?  Here are some examples:

say AX = #10FF
the high byte AH is #10, which is the same as floor(AX/#100)
and the low byte AL is #FF, which is the same as and_bits(AX,#FF)

now say you have a low byte AL = #20 and high byte AH = #4C
AX = AH*#100 + AL
   = #4C*#100 + #20
   = #4C00 + #20
   = #4C20

But if you're dealing with 32-bit registers, the lower byte is the same,
but the upper byte isn't the upper-most anymore.  If EAX=#10FF09C2, then
AH=and_bits(floor(EAX/#100),#FF) = #09.  As before, AL =
and_bits(EAX,#FF).

So, EAX = and_bits(EAX,#FFFF0000) + AH*#100 + AL

To access the upper word in EAX, you may have to use a shift operation to
move the bytes into AH and AL.  ex. SHR EAX,16

Note: these examples also work with the other general registers
(BX,CX,DX or EBX,ECX,EDX with BH,BL,CH,CL,DH,DL)

I hope this helped.

Ask me more questions like this and I might have enough content to put
together an asm tutorial :)
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