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Re: [Libcdio-devel] Distraction over Wikipedia topic, was Rock Ridge and

From: Natalia Portillo
Subject: Re: [Libcdio-devel] Distraction over Wikipedia topic, was Rock Ridge and libisofs/xorriso 'AL' extension
Date: Mon, 31 Jul 2017 12:15:32 +0100


The specified command predates SCSI-3, so check on SBC or SCSI-2, cant tell you 
exactly where after reading all specs its all a mess in mu head :/ seems to have the 
exact command.

When we are giving block access to an iso file we are basicallu emulatinh a CD 
drive, so should we emulate that behaviour is a legitimate question.

In discimagechef I've chose to not do that for simplicity doing the sector cut 
in 5 filesystems instead than in 35 disc image drivers, but maybe someone 
thinks the correct is to do like the cd drives and offer different block sizes.

The Apple CD300 and Apple CD600 are the same Sony drives used by Sun and SGI 
and the CD300 supports that command (is the only drive without a jumper that 
can boot in my SparcStation 10). Afaik no ATAPI drives suport it at all, it was 
something very specifically asked for first drives before systems where adapted 
to variable block sizes.

It's a mess how 512 bytes is hardcoded all over the industry, in buffer sizes, 
ignored "sector size" fields, etc, etc.

E.g. Apple specifically says (IM:IV) minimal sector size for their boot 
descriptor ('ER' predating APM, that's in IM:V) is 256 bytes, yet the boot file 
length is hardcoded to 512 bytes units.

Natalia Portillo

Sent from my iPhone

> On 31 Jul 2017, at 11:55, Thomas Schmitt <address@hidden> wrote:
> Hi,
> Natalia Portillo wrote:
>> The MODE SELECT is that basically you can change "sector size" on
>> several SCSI devices.
> Indeed ? Which Mode Page does this ?
> I am looking at MMC-1 and MMC-5 and only see in Mode Page 0x05 the field
> Data Block Type. It selects one of the various block write modes which
> expect from the burn program to send 2048 to 2448 of the payload bytes
> of a CD sector.
> (Not applicable to DVD or BD media.)
> SPC specs mention the command MODE SELECT but delegate the specification
> of mode pages to the specialized device class documents. For optical drives
> that would be MMC.
>> I also have some Yamaha drives with that jumper.
> My last Yamaha died in 2003. :(...
> Methusalem on my desk is an LG Blu-ray burner of 2008.
>> So there's the question, anything that emulates an ISO with those boot
>> sectors, should suppor the "MODE SELECT" command somehow or expect the
>> applications it is feeding to do the sector cutting.
> Nowadays i expect block size 2048 with all optical media and am prepared
> to convert adresses which are intended for 512 byte blocks before reading
> blocks from optical media.
> Vice versa, the 2048 byte block addresses need to be converted to the
> block size of a 512 byte device, like a USB stick.
> Within the ISO 9660 specs for optical media, i only know of the El Torito
> boot image size field, which counts in 512 byte blocks.
> All other filesystem and Boot Record aspects are for 2048 bytes.
> In general, ISOs have emancipated themselves from CD-ROM.
> They now live on DVD and BD (where the proprietary software community wanted
> to enforce UDF), on USB sticks, and even in data files which get submitted
> to virtual machines as virtual CD or hard disk.
> So i would not assume that SCSI hardware commands are specially related to
> interpreting the content of ISO 9660 filesystems.
> Have a nice day :)
> Thomas

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