Path: utzoo!attcan!uunet!lll-winken!lll-lcc!ames!oliveb!amiga!neil
From: neil@amiga.UUCP (Neil Katin)
Newsgroups: comp.sys.amiga.tech,comp.sys.amiga
Subject: Re: What is the checksum used for the Amiga's track & sector information
Keywords: Checksum, Disk, Sector
Message-ID: <2378@amiga.UUCP>
Date: 12 Jun 88 06:38:17 GMT
References: <3845@pasteur.Berkeley.Edu>
Reply-To: neil@spam.UUCP (Neil Katin)
Organization: Commodore-Amiga Inc, Los Gatos CA
Lines: 20
Xref: utzoo comp.sys.amiga.tech:929 comp.sys.amiga:19871

In article <3845@pasteur.Berkeley.Edu> koster@cory.Berkeley.EDU.UUCP (Kevin Oster) writes:
>   The RKM volume 2 tells a little about the format. It says there are two
>32 bit checksums, one for the header information and one for the data area.
>How do I compute them? These checksums are on the raw data after MFM decoding,
>even lower level than trackdisk. Also is there anything special about the
>$4489 matchword? Couldn't it be found by chance in a sector's data area?
>($4489 is the MFM encoded version of $A1)

The checksum is a simple XOR of the data bits (the clock bits are
masked off), for both the header and the data portions.

The 4489 pattern is can never be a legal data pattern: A1 encodes
to 44A9; it is the dropped clock bit that makes it possible to find
these sector headers.

I feel a bit "at risk" with this answer since I did the trackdisk
and disk resource; If we can just get you to control your "murderous
tendencies" (:->).

	Neil Katin
	{oliveb,pyramid}!amiga!neil