[Simh] RX50 disks

[Paul Koning]

> On Jan 21, 2016, at 5:18 PM, Mark Pizzolato <Mark at infocomm.com> wrote:
> 
> I’m guessing here, but consider this:  The SIMH disk image format is one where the bytes of data are completely in “logical” sector order.  All disk images connected to the RQ device (even RX50 floppies) are considered SIMH disk images.  Depending on the underlying media, or the tools used to extract them, the data you’ve got may or may not be in logical sector order.  It may be in physical sector order which might include some form of interleave factor.  Additionally, the underlying disk may or may not have 512 byte sectors.  For instance the physical sectors may be 128 bytes each and they may be interleaved around the disk with 4 of them forming a single logical 512 byte sector.

All those are likely issues.  Another one is the track length.  PC 5.25 inch floppies normally have 9 sectors per track; RX50s have 10.  Linux can be told to set the floppy format to 10 sectors per track.  I haven't tried it recently, but in the past, this was done by adding a line to /etc/fdprm, like this one:

rx50   	         800    10   1  80    0 0x23 0x01 0xDF     0x50

and then setting the drive to use that format (I don't remember how that is done).  There may be newer ways to achieve this, but in any case, the key point is that you need to tell the drive that there are 10 sectors per track.

A correctly recovered bootable disk is pretty easy to recognize.  You can disassemble the boot sector in detail, but a giveaway is the first word, which is octal 240 (the NOP instruction).  That's the PDP11 bootable device rule for all systems except the PRO, where the first word is required to be zero (and the boot code starts at offset 2).

	paul