Re: CD-ROM 512b block size

[ignatios%cs.uni-bonn.de@localhost]

hi,

On Sat, Mar 05, 2022 at 06:51:26PM +0100, Martin Husemann wrote:
> On Sat, Mar 05, 2022 at 07:30:56PM +0300, Dima Veselov wrote:
> > Greetings!
> > 
> > Some SCSI CD-ROMs have a switch setting from 2048-byte block to
> > 512 for older systems like Irix. Will NetBSD work with 512-byte
> > CD-ROM?
> 
> I'm not sure about a (manual) switch or jumper, but I have used various
> scsi CD drives in Sun gear that used 512 byte blocks by default but could
> be told to switch to 2048 (or simmilar) when using them to grab audio CDs.

would be 2352 for audio. But yes, you could switch them to 2048
for data, maybe. Details are fuzzy in my mind. I wonder whether I
still have a working drive of that age somewhere - I think I switched
to faster drives decades ago everywhere.

Note that the 512-byte sectors where only quarters of 2048-byte
data sectores (which where one level(?) of forward error correction
inside of the 2352 byte sectors used for audio mode).

There were some drives that can be switched between sectore sizes by
a dip switch, and some that can be switched by a MODE SELECT.
See https://www.sun3arc.org/FAQ/cdrom/backgrnd.htm or 
http://www.verycomputer.com/39_7b08394b5e15a473_1.htm for more information.

From reading the code, our scsi cd path (sys/dev/scsipi/cd.c) has
some code to work with the blocksize the disk reported, or to
enforce 2048 bytes when the disk seems to report a broken (<512 or
>16384 bytes) size.

	-is