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From: gson@niksula.hut.fi (Andreas Gustafsson)
Newsgroups: comp.sys.handhelds
Subject: CHIP-8 limitations (was: <None>)
Message-ID: <1990Nov15.095509.1001@santra.uucp>
Date: 15 Nov 90 09:55:09 GMT
References: <0471181703FF001A65@gacvx2.gac.edu>
Sender: news@santra.uucp (Cnews - USENET news system)
Reply-To: gson@niksula.hut.fi (Andreas Gustafsson)
Organization: Helsinki University of Technology, FINLAND
Lines: 24

In article <0471181703FF001A65@gacvx2.gac.edu> CW%APG.PH.UCL.AC.UK@pucc.PRINCETON.EDU writes:
>          i) I find moving the sprite to the left is quite difficult
>             since there is no equivalent instruction to Vx=Vx-KK only
>             Vx=Vx+KK. Am I missing something fundamental? Does anyone have
>             a nice little routine or algorhythm (spelling?) to explain
>             a simple way to do it.

Yes, you are missing something fundamental.  Because all arithmetic in
CHIP-8 is done modulo 256, you can subtract a constant simply by
adding the two's complement of that constant.  For instance, the
instruction 7AFF will subtract one from VA.

>        ii)  Can the string that contains the game be any length, or is
>             it REALLY limited to 4K.

Three and a half, actually, because the addresses 000-1FF are
reserved.  But CHIP-8 code is very compact, so you can do quite a lot
in 3.5 k.  You should remember that CHIP-8 was originally designed in
the late seventies, to run on machines that seldom had more that one
or two k of memory.
-- 
Andreas Gustafsson
Internet: gson@niksula.hut.fi
Voice: +358 0 563 5592