TRS-80 Model 1 vs Model III disk/application compatibility

Matthew Reed mkr at
Tue Jan 27 18:33:25 CST 2009

Tony Duell wrote:
> IIRc, the Model 3 can physically read any M1 disk, although it can't
> necessarily write them (The M1 used one of the 1771's 'extra' data markers
> on the directory cylinder, which later disk cotnrollers couldn't write). 
> I cn't rememebr about OS compatibility -- LDOS, which is what I mostly 
> use can read M1 TRS-DOS or LDOS disks on a model 3, but I have an idea 
> TRS-DOS won't. And binary programs generally won't run on the other 
> machine -- almost all the OS routines changed their addresses :-(

The Model III versions of LDOS, DOSPLUS, MULTIDOS, and NEWDOS/80 could 
all read Model I TRSDOS disks. Model III TRSDOS 1.3 couldn't read them 
directly, but it did come with a conversion utility.

Most programs written after the introduction of the Model III worked on 
both computers. There were some programs with separate versions, but 
they tended to be low-level utilities or self-booting games. Programs 
written before the Model III was introduced were another story.

Matthew Reed
mkr at

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