[Simh] SCSI-Interface for simh-vax?

[Timothe Litt]

> CD's are normally 1024 byte blocks
2048 Bytes/sector is the ISO std for CD-ROMs (Mode 1).  Mode 2 omits ECC
for2336 B/sector - but I don't know of a case where someone was crazy
enough to use it for data.

> we might have done at DEC was mess with the block size on a CD

DEC does not modify the physical sector format - it is implemented in
the drive.

VMS packs four 512 B logical sectors into one 2048 B physical sector;
the driver handles buffering and provides the illusion of a 512B sector
size.  Most FILES-11 CDs use unmodified ODS-2.   But distribution CDs
would do things like omit (or truncate) the bit table to save space. 
For that reason, ANA/DISK would fail.  There are some CDs that use a
slightly modified HOM block (FILES-11 B Level 0), but it wasn't widely
adopted.

There are other oddities - drives & drivers tell different lies about
the geometry (cyl/track/sector) of a CDROM; multiplying these out
frequently will not match the file system's idea of the volume size. 
(As recorded in the SCB for FILES-11, equivalent for other formats.) 
The lies vary by OS, version, drive & phase of the moon.  The same CD
read under different conditions will report alternative facts.  These
will not trip up a DEC OS on DEC HW - but can create obscure issues with
simulation - especially if you try to pass geometry  from a physical
drive thru SimH.

Writing a CD is rarely supported by a standard driver - typically, CD
writing software issues direct SCSI commands to the drive (encapsulated
in whatever the real transport is).  This may be by direct IO, or via a
class driver.  It can be somewhat tricky - note that most drives can not
tolerate buffer underruns when writing.

> I wasn’t able to figure out how to make it work in RSTS/E.

To be bootable, a CD needs an appropriate boot block (LBN 0).  For VMS,
it's written by 'writeboot' - not initialize.  I don't remember the
details for RSTS - look at SAVRES->RESTORE and BACKUP for
possibilities.    Or wait for Paul K to fill that in.

Also note that dual format CDROMs are possible - 9660 reserves the first
16 sectors for this; thus it's possible to write a disk that is readable
as both FILES-11 and & 9660 (with file data being in the same sectors;
only the metadata differs.)  Such disks were actually created.

On 01-Sep-18 15:28, Clem Cole wrote:
> below...
>
> On Sat, Sep 1, 2018 at 2:39 PM Zane Healy <healyzh at avanthar.com
> <mailto:healyzh at avanthar.com>> wrote:
>
>     Create a virtual disk in SIMH the size of the CD-R blank.  Prep
>     the disk, then burn it to CD-R.  This is how I created my bootable
>     CD’s for RT-11 and RSX-11M+.  I’ve then used those CD’s to do
>     installs on my PDP-11/73.  I wasn’t able to figure out how to make
>     it work in RSTS/E.  I could create the CD-R, but not boot and
>     install RSTS/E from it.
>
>
> Just curious ... aren't there funnies because CD's are normally 1024
> byte blocks and disks are usually 512?    IIRC, there are places that
> store numbers of blocks (not bytes), and you have to be careful.    I
> have >>not<< played in any of that in years.   
>
> IIRC one of things  we might have done at DEC was mess with the block
> size on a CD -- that's a Tim Litt type question.   Those bits are so
> long ago depreciated/garbage collected in my acitve brain cells. ;-)
> ᐧ
>