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Why does adc behave as it does?


minerscale
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Hi guys, I'm just starting 2600 after making a game for the genesis in C. I'm quite enjoying the ridiculous challenge of doing it and i'm interested in the properties of adc and the carry flag.

 

Why does adc add one more to the accumulator when the carry flag is set? It seems quite random that it would be doing that. It's sort of useful but a pain in the ass at the same time. I end up having to call clc wasting 2 cycles in the process.

 

I'm very confused as to why. So any insight into that would be great. :???:

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If you want to add numbers that are longer than 8-bits it is very useful. A number larger than 8-bits is represented using multiple 8-bit values. To add them together, you clear the carry and start adding them together from the lowest pair. If the addition overflows, the carry-out becomes the carry-in for the next addition. The code below shows how to add two 16-bit numbers. Notice that there is no CLC after the first addition as the carry flag needs to be preserved:

add16:
    clc				; clear carry
    lda num1lo
    adc num2lo                  ; add low numbers
    sta reslo			; store low sum
    lda num1hi
    adc num2hi		        ; add high numbers with carry
    sta reshi			; store high sum
Edited by cd-w
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