Re: A83: Boolean operators
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Re: A83: Boolean operators
OK, thanks. I had some idea it had do to with that C flag, but I couldn't
recite the details. I was just programming away when I stumbled on this
problem. I didn't want to leave my concentration on the prog by going on the
net, so I just posted an e-mail. Many thanks however.
-----Oorspronkelijk bericht-----
Van: Sebastian Grössl <sebgroessl@bluewin.ch>
Aan: 'assembly-83@lists.ticalc.org' <assembly-83@lists.ticalc.org>
Datum: dinsdag 5 september 2000 18:40
Onderwerp: Re: A83: Boolean operators
The question isn't that bad!! Ok, look there are various flags on the Z80:
the Zero flag and the Carry flag! I think you know how the zero flag
works... So I can tell you, the Carry flag is just the same! Let's say you
got a number in a and another number in b. You want to compare them:
ld a,FirstNumber
ld b,SecoundNumber
cp b ; compare a with b
jp z,they_are_the_same ; if a-b=0 goto they_are_the_same
jp c,a_is_smaller_than_b ; if a-b produces an overflow --> b > a -->
a-b= -c then jp
a_is_higher_than_b: ; ...
So you see, it's pretty logical, the carry-flag is just like having 9 bits
instead of 8, that means if you add (or substract) a value to another and
there is not enough "space" in a 8 bit register, the 9th bit (= the carry
flag) will also be set. You have to know, that the cp substracts the
accumulator form the value after cp. Afterwards it resets the accumulater to
the value it had before.
You see, this is as simple as possible(, because the Processor can't do
wonders in algebra like us, humens).
Greez
Sebastian
P.s.: I also had such problems when I stated of whith ASM. This methode can
be used also by other operators!
Frank Schoep[SMTP:dsfs98@concepts.nl] wrote:
OK, I might be going for The Worst Question Ever Asked on ASM83 (tm), but I
need to know the following:
how can you check if a number is larger than another (I only need to know
how to do this with 8 bit numbers.)? Is there a way to load the 1st number
in the accumulator and cp/and/or/xor/rr/slr/sla/rl it with another number to
do this?
Please let me know...
Frank