Re: A83: SBC

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Re: A83: SBC


Awww... SUXX.. I wrote this _looong_ reply, then I must've pressed Cancel or
something cuz it disappeared... But why don't you try around a bit with
_dispHL.

Linus

On 17-Jun-98, James Matthews wrote:

>Thanks Linus...I understand, but kinda...

>Could you (or anyone) give me an example.  Like take two numbers and
>subtract them using SUB, then using SBC.  Thanks.

>James.

>> Normally, SBC and ADC are used when adding bigger numbers than 8-bit
>ones.
>> E.g. if two three byte numbers should be added, you first add the least
>> significant bytes. This addition sets the carry flag if the additiongot
>past
>> ff. Next, you add the middle bytes _together with the carry flag_. If
>there
>> was a carry, that carry will be added! AND the carry flag will be
>updated, so
>> that you can add the most significant byte directly (with adc of course).
>> 
>> When sgs designed their z80, they ran out of instruction codes. They had
>to
>> remove some instructions, and they thought that 16 bit subtraction didn't
>need
>> two separate instructions (sub and sbc). So they removed sub, meaning
>that the
>> programmer would have to clear the carry flag manually to do a sub. This
>is
>> done with e.g. "or a".
>> 
>> There's still both add and adc, though.
>> 
>> Linus
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