[Coco] OS-9 RBF - accessing past what LSN0 says?

[Robert Gault]

Allen Huffman wrote:
> I have a 128MB EZ135 disk that I used with RGB-DOS/OS-9.  I put 255 RGB-DOS drives on it at the end, and used the rest of the space for OS-9 (about 50MB).
>
> The OS-9 section has an LSN0 that says 0313FE (201,726) sectors, and beyond that is the RGB-DOS portion.
>
> If I use dEd, or BASIC09, and try to get past sector 201,726, it won’t let me. In years past, I have gotten around this by modifying LSN0 to something larger, and then I can. That’s what I resorted to doing tonight.
>
> How do utilities like RSDOS and PCDOS work when you put in a disk without an LSN0? I expect the driver itself does not care (if it’s like OSK/OS-9000).
>
> Thanks for any shedding of light you might be able to do…
>
> 		— Allen
>

It is not that difficult but best left to an OS-9 driver designed to access the 
RGBDOS (now HDBDOS) portion. Here is an example of my vemudsk driver which acts 
exactly like emudsk but adds the offset of $5A000 to all LSNs and then adds $264 
for each additional RGBDOS drive.

I use PD.STP in the descriptor to indicate the RGBDOS drive number.
getsect pshs d,x       4 bytes
  lda PD.DRV,y          get drive number requested
  cmpa #2
  bhs driverr           only two drives allowed, return error
  sta >drive            tell hardware
* new code to get RGB Disk Basic drive#. RG
  ldb PD.STP,y          get step value now vdrive#
  clra
  tfr d,x               regx now is vdrive#
  * put VLSN on stack
  ldd vofset+1,pcr      $A000
  pshs d                2 bytes
  ldb vofset,pcr        $05
  pshs b                1 byte    stack now has 7 bytes see structure above
  * Virtual drives are 35-track so LSN <=$276
  cmpx #0               which Vdrive is regX?
  beq gs2               if drive0 branch and use size of vhd
gs1 ldd tlo,s          get lo word of os-9 size
  addd #630             sectors on a 35t disk
  std tlo,s             save new value
  ldb thi,s             get hi byte of os-9 size
  adcb #0               add in any carry from first addition
  stb thi,s             save new value
  leax -1,x             next vdrive
  bne gs1               continue until correct drive# reached
gs2 ldd tlo,s          add in LSN on Vdrive to total offset
  addd rx,s             add lo word of lsn
  tfr d,x
  ldb thi,s             get hi offset byte
  adcb rb,s             add lsn hi byte
  lda ra,s              recover command
* end of new code. rg
  stb >lsn
  stx >lsn+1

You can do the same thing with a SCSI system.

Robert