Re: Twos complement subtraction

[Michael J. Mahon <mjmahon@aol.com>]

"ict@ccess" <gids.rs@sasktel.net> wrote:
> On Mar 22, 4:17 am, roughana <andrew.roug...@writeme.com> wrote:
>> I have a situation where I am modifying existing code that uses twos
>> complement subtraction in 65816 assembly.
>> 
>> Here's the original code:
>> 
>> TYA ; Length in Y
>> EOR #-1
>> SEC
>> ADC :PTRlength ;ZP address
>> STA :PTRlength ;ZPaddress
>> 
>> Now it turns out that we need to subtract one more than the length in
>> Y.
>> Here's one solution:
>> 
>> TYA
>> INC
>> EOR #-1
>> SEC
>> ADC :PTRlength ;ZP address
>> STA :PTRlength ;ZPaddress
>> 
>> and a second solution which is less cycles. But will this do the trick
>> in all cases?
>> 
>> TYA
>> EOR #-1
>> CLC
>> ADC :PTRlength ;ZP address
>> STA :PTRlength ;ZPaddress
>> 
>> I stepped through it and the second one worked with the case I tried.
>> Both resulted in carry set at the end.
>> 
> 
> 
> Here's another if you want the carry to be clear
> 
> TYA
> SEC
> SBC PTRlength
> EOR #-1
> STA PTRlength

Or, if you know that Carry is set, use the above code without the SEC. If
you know Carry is set, use the earlier code, but omit the CLC. ;-)

-michael - NadaNet 3.1 and AppleCrate II: http://home.comcast.net/~mjmahon