Author Topic: Another question..  (Read 8101 times)

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Offline TiAddict

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Re: Another question..
« Reply #15 on: May 27, 2011, 04:40:44 pm »
umm sorry but what is "sbc"?

Offline Deep Toaster

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Re: Another question..
« Reply #16 on: May 27, 2011, 04:46:19 pm »
SBC is SuBtract with Carry. It's like SUB, but it also subtracts one if the carry flag is set. It's the only way to do 16-bit subtraction/comparison since SUB can only operate on A.
« Last Edit: May 27, 2011, 04:46:22 pm by Deep Thought »




Offline TiAddict

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Re: Another question..
« Reply #17 on: May 27, 2011, 04:47:36 pm »
oh i see. thanks Deep Thought! :D

Offline thepenguin77

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Re: Another question..
« Reply #18 on: May 27, 2011, 04:50:34 pm »
Subtract with carry. It subtracts DE and the carry flag.

SBC is usually used when you need to do subtraction with 4 bytes and such.
Code: [Select]
;we will subtract big num 1 from big num 2

ld hl, (bigNum2) ;hl = $0201
ld de, (bigNum1) ;DE = $0605
or a ;this time we don't want the carry flag
sbc hl, de
ld (result), hl ;DE was bigger than HL, so the carry flag is set

ld hl, (bigNum2+2)
ld de, (bigNum1+2)
sbc hl, de ;since the last operation carried, (remember 2nd grade math)
ld (result+2), hl ;we had to subtract an extra 1 from this result

bigNum1:
.db 5, 6, 2, 3
bigNum2:
.db 1, 2, 8, 9
result:
.db 0, 0, 0, 0

This is what SBC is used for. But, we are forced to use it because SUB HL, DE doesn't exist. :P

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Offline TiAddict

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Re: Another question..
« Reply #19 on: May 27, 2011, 04:58:25 pm »
hahah thanks you for that! xD