[Simh] Unix v0 hardware

[Bob Supnik]

I'm fairly sure the disk in the "v0" listings is a precursor of the 
RB09, with half the capacity. The read vs write bit is the correct 
location. The code to convert from a disk block to a disk address first 
divides by 80 (the RB09 has 80 sectors per track) and then "BCD-izes" 
both the quotient (the track number) and the remainder (the sector 
number). The words/sector is 64, which matches the RB09 as well. The 
IOTs in the listing also match the RB09. The transfer count is always 
-64 and is loaded with IOT 707124. The transfer address is loaded with 
IOT 707142.

The "half capacity" comes from the limit check on the disk address, 
which is 2 <= da <= 7999. An RB09 has 200*80 = 16000 sectors. The disk 
in "Unix v0" has only 8000. Based on the way the code does the BCD 
conversion, it is expecting only two decimal digits for the track number 
and the sector number. This seems entirely plausible, because the RB09 
was a head-per-track disk with heads on both sides of the platter. A 
predecessor (or lower cost model) with heads on only one side would be 
8000 sectors.

There is no standard disk with the PDP-7 simulator, but it's trivial to 
add an "RB07" and provide a half size option: a hell of a lot easier 
than it probably was in real life. Information on the RB09 is scarce 
(just a section in the PDP-9 Handbook), but it was clearly a buyout 
product, not a DEC design. DEC never did a BCD-addressed disk in its 
history.

The PDP-7 for Unix v0 must have an EAE, but it does not require 
automatic priority interrupts. The PI chain is a bit difficult to 
decode, because of all the cross-outs and hand-written notes, but it 
appears to be:

clsf - clock
dssf - disk
ksf - console keyboard
tsf - console printer
rsf - paper tape reader
psf - paper tape punch
crsf - card reader

So a Unix v0 emulation apparently will need a card reader but not 
DECtape. Both PDP-7 card readers used the same instructions; they varied 
only in performance.

The system call is the CAL instruction.

Unix v0 does not, surprisingly, appear to use the extended memory 
function or the protection trap that came with it. EEM (enter extend 
mode) isn't even defined. This would seem to imply it ran in 8KW of memory.

/Bob Supnik