TRS80, other than model 1

tony duell ard at
Sat Jun 13 15:08:19 CDT 2015

> In addition to the model 1, . . .
> The model 1 line was converted from a "component" system to a single box.
> For those not familiar, it resembled a Northstar Dimension.  Keyboard,
> screen, two 5.25" floppies in a terminal like case.  That was the model 3.
> Default disk format was double density.  Since the WD 179x controller
> could not write a truly exact model 1 format single density disk (did not

The data address markers on the directory cylinder of a M1 TRS-DOS disk 
are one of the odd ones that only a 1771 can write.  IIRC if you do a double
density upgrade to an M1, you end up with both 1771 and 179x disk controller
chips so that you can still write original M1 TRS-DOS disks. The M3 only has the
179x controller. 

There were many other changes between the M1 and M3 hardware. Most noticeable
is that many I/O devices were memory mapped on the M1 (in the area between the ROMs
and RAM), but were mapped as I/O ports on the M3. 

One odditiy was the Centronics printer port. Memory mapped on the M1 (write to the
address to send a character to the printer, read it to check printer status) and I/O mapped
on the M3. But because so much software directly read that address to check a printer was
present and ready, the M3 (and M4) allows a read from the same memory address to check
the printer status. But AFAIK you have to use the I/O address to output a character. 

There were enough changes between the M1 and M3 that in general M1 disk software will
not run on an M3 and vice versa.

> support certain data address marks), there were some kludges and changes
> in TRS-DOS, so there were minor glitches in the "upgrade path".
> Radio Shack eventually cut a deal to market L-DOS as TRS-DOS 6.
> ("And Randy Cook is now finally collecting royalties!")

TRS-DOS 6 (LDOS 6, LS-DOS 6) was a M4 operating system. However, IIRC the hard
disk OS for the M1 and M3 was LDOS 5.1.x and was sold by Radio Shack.

> Soon, the model 4 was made available - same basic machine with 80x24 video
> (V 51x16), green screen, a few missing keys provided, and ability to
> remap the ROM from the memory map.  Finally, CP/M was available without

The M1 and M3 were limited to 48K RAM (64K memory map space, 12K or 14K for ROM, the 
rest for video, keyboard, I/O devices). The M4 could take 128K RAM, there was bank switching
logic to map RAM, ROM, video, etc into the 64K CPU memory map. You could have the plain
64K RAM that CP/M liked.

Note that a M4 can almost perfectly emulate an M3, and will boot M3 OSes, run M3 software, etc
If you do this you lose the M4 enhancements (you have 48K RAM, ROM at location 0, 64*16 video
etc). But it lets you use the older software if you want to.

The M4 uses the same disk system (and RS232 interface board) as the M3. Disk machines have 
a single-bit sound output device, quite why this wasn't fitted to cassette-based machines is beyond

> Model 4P was luggable version, vaguely resembling early Compaq.

One incompatibilty here, the 4P has no cassette port.

Trivia question : What feature was present on all disk based M1s (in fact any M1 with an expansion
interface) but not on any M3 or M4 machines?


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