Tarbell is making me insane

Tony Duell ard at p850ug1.demon.co.uk
Thu Nov 15 15:04:59 CST 2007

> > Then they switched to selling a sandwich board that added a 179x for
> > double density, while still retaining the 1771, for the oddball DAMs th=
> at
> > it could provide (needed for TRS80)
> What was oddball about DRAM (Is that what you meant?) for those machines?

Not DRAM, DAM. DAM = Data Address Marker, a pattern written to the disk to
indicate the start of a sector. It is a pattern that couldn't occur
anywhere else in the track, it's written, IIRC, with some clock pulses

Most controllers can write/read 2 ifference DAMs -- normal DAM and 
'deleted data DAM' (the names I believe relate to their original usage on 
the IBM 3740). The 1771 controller could handle 4 differnt DAMs, and the 
TRS-80 Model 1 TRS-DOS (and IIRC LDOS) used one of the 'extra' ones on 
the driectory track (so as to indentify said track very clearly). 

The problem is that while the double density controllers can read such 
disks, then can't correctly write them. Which means a 1793 (or whatever) 
double-density controller can't write Model 1 TRS-DOS disks. 

TRS-DOS would man bitterly (I can't rememebr whuite how) if it had the 
'wrong' DAM on the directory cylinder. So Model 1s that had a 
double-0esnisty upgrade kept the 1771 chips as well so as to be able to 
use the origianl disks.


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