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 DIP-8: 8-bit TTL computer 
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Joined: Sat Nov 19, 2022 7:42 pm
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Location: Europe/London
Yes, I agree - and I forgot that in C, "char" on its own can be signed or unsigned. If I pass -unsigned-char to vbcc, then char is unsigned by default, as it should be, and my string code isn't trying to sign-extend all the time.

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Thu Jan 19, 2023 12:50 pm

Joined: Wed Nov 20, 2019 12:56 pm
Posts: 92
dip16 wrote:
(As well as making comparisons signed, I realised that "sig" could more generally act as a "signal" to modify the behaviour of other instructions - but I've not found another use for it yet.)


One thing I used my "sgn" modifier for is making my right-shift instruction copy the msb instead of shifting in zeroes - avoiding the need for both "lsr" and "asr" instructions - maybe something worth applying to your "ror" instruction?

Also the sig modifier might be useful for "sub" as well as "cmp" - assuming sub modifies the flags?


Thu Jan 19, 2023 1:05 pm

Joined: Mon Oct 07, 2019 2:41 am
Posts: 619
robinsonb5 wrote:
dip16 wrote:
(As well as making comparisons signed, I realised that "sig" could more generally act as a "signal" to modify the behaviour of other instructions - but I've not found another use for it yet.)


One thing I used my "sgn" modifier for is making my right-shift instruction copy the msb instead of shifting in zeroes - avoiding the need for both "lsr" and "asr" instructions - maybe something worth applying to your "ror" instruction?

Also the sig modifier might be useful for "sub" as well as "cmp" - assuming sub modifies the flags?


I still use 1960's computing ideas, no flags just Link/Carry bit. Branches test a register for a + - or zero.
I have no CMP instruction, but have adx,sbx ( add index , subtract index ) that does not affect the carry bit.
I my case adding a real flag register, and a CMP is more pain than it is worth.


Thu Jan 19, 2023 4:14 pm
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