[A86] Re: TI-86 menu routine

[Prev][Next][Index][Thread]

[A86] Re: TI-86 menu routine


>ld b,0xh
>                           ; here i need to check if a < b

 cp b ; update flags
 jr c,CutNow ;if a<b goto CutNow, (or jP if the lable is farther away)
 sub b

>                           ; if a < b, then jp CutNow
>                           ; if a is not less than b, then sub b
>jp Loop


A carry occurs when one number subtracted from the other number has been 
'flipped' around (in other words 2-3 = -1 but one byte can only represent 0-255 
so the carry flag tells when the number subtracted from the other is greater 
than or less than the other.

NC (no carry) is the same as >= (greater than or equal to). It happens when 
either the number your comparing is of equal value (resulting subtraction = 0) 
or it is greater than the number.

If you want to compare just > (greater than but not equal to) then increase the 
number youre comparing a to by 1.

ex.
;a=6
 cp 5
 jr nc,aisgreaterthan5
 
;a=5
 cp 5
 jr nc,aisgreaterthanorequalto5

;a=4
 cp 5
 jr c,aislessthan5

;a=4
 cp 4
 jr c,aislessthanorequalto4
 jr z,aislessthanorequalto4



The cp command does not change the register values but it does perform the same 
operation as the sub command. Basically, it subtracts the number from the other 
(a-number_comparing_to or hl-number_comparing_to) then it updates the flags. 
The register contents at this point are not changed. The sub command will do 
the same thing but the contents of the registers are changed. So if you plan on 
doing a 'sub b' after up there and if you are planning on doing a sub b at 
CutNow then you could optimize the above by using sub b instead of cp b because 
the flags are still updated and you wouldnt need the extra sub b's.

I also wrote a piece about comparing signed numbers (negative 8bit numbers)
on this mailing list. So if you want to search for that to understand the p and 
m flags for negative comparisons go right ahead :)


Hope this helps,

Ricky Cobb
http://tip.ti-programmers.com/
arcadesdude@intercom.net
[IM]
icq 41440378
msn arcadesdude
y!  arcadesdude
aim arcadesdude

8/24/02 8:31:51 AM, Michael Williams <spacex@williams-net.org> wrote:

>
>Hey all!
>
>    I havent said anything so far, but I have learned alot from you 
>guys. Thanks!
>Anyways, I have a problem, Im trying to make a TI-86 assembly menu routine
>that does the same thing as the Menu( basic command.  I've run into a 
>little hiccup though.
>I need to find out if register a is less than register b.  How do I do this?
>Thanks a lot for your help.
>
>Michael Williams
>
>PS. This is what it looks like now
>
>CutString:
>ld de,hl-01h        ;hl -> string to be cut
>ld a,17h              ; 17h is the number of pixels we have
>Loop:
>inc de
>ld b,(de)
>cp b,                  ; All the characters that would cause the string 
>to terminate
>jp z, CutNow
>cp b,                  ; All the characters that are six pixels wide
>jp z, Minus6
>
>....
>....
>
>CutNow:
>ld (de), 00h
>ret
>
>Minusx:               ; this is the same func for all the different 
>pixelwidths, just cut and pasted
>                           ; x is between 1 and 6
>ld b,0xh
>                           ; here i need to check if a < b
>                           ; if a < b, then jp CutNow
>                           ; if a is not less than b, then sub b
>jp Loop
>
>
>
>
>
>
Follow-Ups: References: