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From: ewhac@well.sf.ca.us (Leo 'Bols Ewhac' Schwab)
Newsgroups: comp.sys.amiga,comp.sys.amiga.tech
Subject: Re: AmigaDOS Filing systems questions
Message-ID: <19479@well.sf.ca.us>
Date: 9 Aug 90 09:44:44 GMT
References: <2729@mindlink.UUCP>
Reply-To: ewhac@well.UUCP (Leo 'Bols Ewhac' Schwab)
Followup-To: comp.sys.amiga.tech
Organization: Klein Bottle Wanted; Inquire Within.
Lines: 48
Quote: "Gorbachev sings tractors:  Turnip!  Buttocks!"  -- Pravda

[ This has been redirected to comp.sys.amiga.tech. ]

In article <2729@mindlink.UUCP> a275@mindlink.UUCP (Travers Naran) writes:
>I have a problem, I was looking at my AmigaDOS Tech Ref. Manual and studying
>the Filing System. No where does it mention what format is the "Bitmap" [ ... ]
>What is the format of the Bitmap [ ... ] ?

	I don't know how to get tell AmigaDOS to update the bitmap short of
making it do a full re-validate.

	The following is a comment in a very old program I wrote called
'dm', which shows sector allocation on an OFS floppy.  I can send you the
whole program if you like, or you can dig it off one of the older Fish
Disks:
--------
	/*
	 * At this point, we now have the bitmap in the disk buffer.
	 * This is what the first longword in the bitmap represents:
	 *
	 *	333322222222221111111111
	 *	32109876543210987654321098765432	Sector #
	 *	--------------------------------
	 *	10100101001010010010010001010011	Longword
	 *	MSB			     LSB
	 *
	 * If the bit is set, the sector is free.
	 *
	 * Sectors 0 and 1 contain the boot block.  Thus, the DOS is not
	 * allowed to walk on them, and they are permanently allocated.
	 * However, the bitmap does not reflect this i.e. the bitmap starts
	 * at sector 2, which is indicated by bit 0, longword 1.  This means
	 * we have to offset everything by two.  Can you say "crock?"
	 *
	 * Actually, I suppose this is good, since it prevents some would-be
	 * smart person from un-allocating those sectors and messing things
	 * up badly.
	 *
	 * Bitmap starts at longword 1; longword 0 appears to be some kind
	 * of significant garbage (checksum?)
	 */
--------
	I was a lot younger back then...

_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_
Leo L. Schwab -- The Guy in The Cape	INET: well!ewhac@ucbvax.Berkeley.EDU
 \_ -_		Recumbent Bikes:	UUCP: pacbell > !{well,unicom}!ewhac
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