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Article #295Re: ADFS disk format (BBC Master)
From: I.D. Griffiths
Date: Mon, 12 Oct 1992 16:23
Date: Mon, 12 Oct 1992 16:23
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In article <1992Sep23.145723.23231@infodev.cam.ac.uk>, amh15@cus.cam.ac.uk (A.M. Hart) writes: |> |> |> HELP! |> |> I need a small but vital piece of information about the BBC Master ADFS. |> |> Does anyone out there know how the checksum at the end of each of the FS |> map sectors (0 and 1 I think) is calculated? Yup, had hours of fun with this a couple of years ago. After disassembling bits of the ADFS, it transpires that the checksum is done by summing each byte into the accumulator but not bothering to clear the carry, so each overflow adds one to the total. Clear carry before the first add. I assume this was done to make it a few cycles faster.... Hope this helps. Ian
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Newsgroups: comp.sys.acorn.tech Path: arc1-ca.pugleaf.net!sparky!uunet!pipex!pavo.csi.cam.ac.uk!cam-cl!idg10 From: idg10@cl.cam.ac.uk (I.D. Griffiths) Subject: Re: ADFS disk format (BBC Master) Message-ID: <1992Oct12.162334.2336@infodev.cam.ac.uk> Sender: news@infodev.cam.ac.uk (USENET news) Nntp-Posting-Host: hall.cl.cam.ac.uk Reply-To: idg10@cl.cam.ac.uk (I.D. Griffiths) Organization: U of Cambridge, England References: <1992Sep23.145723.23231@infodev.cam.ac.uk> Date: Mon, 12 Oct 1992 16:23:34 GMT Lines: 21