NetNews Usenet Archive 1992 #31/NN_1992_31.iso/spool/alt/cdrom/5307

Text File | 1992-12-23 | 1.8 KB | 37 lines

Newsgroups: alt.cd-rom
Path: sparky!uunet!spool.mu.edu!agate!iat.holonet.net!bwilliam
From: [e-mail] (Bill Williams)
Subject: Re: What uses mode-2, form-2 (>2048-byte sectors)?
Message-ID: <[e-mail]>
Organization: HoloNet National Internet Access BBS: 510-704-1058/modem
References: <[e-mail]>
Date: Wed, 23 Dec 1992 07:16:27 GMT
Lines: 26

Regarding "interesting" logical block sizes. The Apple CD ROM driver
specification insists that the CD ROM driver support all the common
logical block sizes up to 2048 and including others such as 2340, 2335, etc.

When this is done the driver is not expected to be able to perform duties
as a mounted CD-ROM. normally the computer would use a driver or
sophisticated multimedia Extension that would occaisionally commandeer the
CD-ROM driver, change the logical block size dynamically from 512 (or
2048) to 2340, perform the dirty work in 2340 byte mode on the local
machine, and then set the device back to its oriinal mode. On the
Macintosh their already is a STRICT rule requiring ALL scsi drivers
intended for file system support to communicate in block sizes divisible
by 512. Under these circumstances, the "smart" driver author merely treats
all 2340 byte block addressing as what it really is deep down on the
actual media.... That is a 2340 byte block with only room for 2048 bytes
of "computer" data encapsulated in it. The rest is for CDI, Audio
commentary, various checksums, error correcting, subunit headers,
subheaders, etc.

The moral. Don't worry about what you are missing. In fact Kodak Photo CD
as used on the Macintosh *NEVER EVER* needs any bytes contained outside of
the 2048 bytes of normal data even though all PhotoCD blocks (when on a
CD-ROM) are actually 2340 byte blocks.

Bill Williams