Re: A86: Interrupt questions
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Re: A86: Interrupt questions
A checksum is something that verifies things... For the TI85, the string (and i
believe all of them) files had checksums, just as the user routines do. There are
different ways that they work. Basically, though, it just adds up certain bytes
(whether that be every $40th byte, every byte, every otehr byte) and then takes
the mod of it (if you're using the add opcode, you obviously don't have to worry
about this one, at least if you have only a one-byte checksum) Anyway, you take
the mod of the sum so that it fits in a previously determined # of bytes (1 for
the user routines, not sure how many for the 85s files, i know my virus checker
uses like 4 or 8 or something) into some location (separate file/variable or the
1st couple bytes in the file/var). That way, if the file/var gets corrupted, it
checks the checksum before running it, so that it won't crash. for the user
routines, it's just to make sure a concious effort was made in putting it in and
setting the flag.
Dux Gregis wrote:
> I was looking at $66 and it is possible for that call to evade the exx and di,
> reti instructions, if it reads the right value from the on port (port 3). I
> can't tell if it's forced to return or not: it may just have something do with
> the checksum.
>
> Btw, what is the checksum? How does any checksum work?
--
Stephen Hicks
mailto:shicks@mindspring.com
ICQ:5453914
IRC:Kupopo
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