ooops. 64 characters per line on the 1. Maybe it was Osborne 1 with 51?

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. 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. 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 me. 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? -tony

Exactly. The switching relay in the expansion interface. Did anyone ever use this feature? -tony

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