Re: A86: Interrupt questions

To: <assembly-86@lists.ticalc.org>
Subject: Re: A86: Interrupt questions
From: "Trey Jazz" <joemama@minot.com>
Date: Wed, 22 Apr 1998 02:28:35 -0500
Delivered-To: assembly-86-outgoing@towerguard.unix.edu.sollentuna.se
Reply-To: assembly-86@lists.ticalc.org


i think the zshell did the checksum different, i tried to change some of the
strings with the mid commands so it was the same length as the original and
i would get checksum errors

yes all files to have checksums, i think the os fixes them if they dont
match the data (on some files at least...like programs)



>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.