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

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 :-( Actually, the M3 has a double-density disk controller (1793 IIRC), and by defualt it used double-density encoding on the disks. It can do single density as well, though (although, IIRC, TRS-DOS 1.3 uses double density only). So you want SSDD or DSDD blank disks. The sort of disks that are called '360K' on a PC work fine. Again, IRIC, TRS-DOS 1.3 pretty much restircts you to 40 tracks single-sided. Other OSes let you do anything the drives are capable of. I assume this is a 'flippy' disk. The M3 is double density, with about 180K per side. Not all of that space is avaiallbe for user programs, of course. -tony