Date: Mon, 8 Sep 1997 21:30:15 -0700 (PDT)
Message-Id: <199709090430.VAA27713@adit.ap.net>
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To: djgpp AT delorie DOT com
From: Nate Eldredge <eldredge AT ap DOT net>
Subject: Re: Djgpp inline asm/graphics (hline) question
Precedence: bulk

At 01:38  9/7/1997 -0700, you wrote:
>Jesse Legg wrote:

>>    testl $1, %%edi    ; test for odd number address (help!?? :)
>
>Erm... why not try dividing by two? If you get a result that has a
>fraction, not even, if not, even...? 
That would require using floating point arithmetic, making things MUCH
slower and more complicated. Using IDIV might work, but has problems
including divide-by-zero errors when the dividend is too big, needing
different registers and more of them, and being slower. IMHO, `testl $1,
%edi' is the way to go.
>
>>    shr $1, %%ecx      ; from my real mode tutorial; what is this? :)
>
>I think this means divide by two. Shr ecx,1 (Intel format?) shifts the
>bits in ecx one step to the right, this dividing the number in ecx by
>two. If you shr ecx,2 for instance, you divide by four, then ecx,3
>divides by 8, etc. Shl (if I am not cofusing two) multiplies by powers
>of 2 in the the same manner.
That's correct.

>>    adc $0, %%ecx    ; ditto
>
>I have no idea what this does... why not look in an asm ref?
adc = ADd with Carry. It adds the first operand to the second, and adds one
more if the carry flag was set. Frequently you see this when adding 64-bit
numbers, since the carry out of the low dword (from an `add' instruction')
can be figured in.

Nate Eldredge
eldredge AT ap DOT net