Re: Why no Model II emulator?


"Tom Lake" <tlake@xxxxxxxxxxxx> wrote in message
news:46d717d5$0$18903$4c368faf@xxxxxxxxxxxxxxxxx

"N Morrison" <Another1171@xxxxxxxxx> wrote in message
news:1188497343.934188.156010@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

There are no ROMs - just a small boot ROM to load and execute the boot
sector on the floppy. It's really basically a Z-80 emulator that is
needed.

You'd need to emulate the ROM to some extent to boot a TRSDOS
disk image. At least the part that SEEKs to the proper directory and
loads the boot files. Also the error message or Diskette? message
issued when there is no boot file (or did the Model II just hang like the
Model I?)


The Model II was different and pretty advanced for it's day.

First off, the Model II was 'loadable' in the sense that there was NO
operating ROM in the machine. There was a small boot ROM that
loaded a 'mini' loader in single density from the 8" floppy. Then the
system kicked into more of a 'real' mode in that it used DMA to
read the rest of the diskette double density. Everything was then
loaded into RAM. (The Model II was TOTALLY RAM in operation,
no ROM space at all.) There were a few different versions of the
boot ROM, but they all worked pretty much the same way. Only
adding boot capability like the HD.

Since there wasn't a standard ROM in the Model II, that means you'd
need a lot more than a Z80 emulator, you'd need to also emulate
all the chips in the Model II, since you wouldn't know how the chips
were used and then use that to 'load' whatever version of TRSDOS
you wanted to operate.

On the other hand, it might be a lot easier to write an emulator that
was DOS restricted and at that point would work with the 'DOS'
functionality already 'built in'. That would mean unfortunately that
some software wouldn't work if it went to the hardware in weird
ways, but software that worked that way was rare except for the
backup protected stuff like P&T CP/M. There were diskettes that
were run protected, like the Model II diag disks that kept trying to
force a backup from the original disk, but those used the DOS
sector int calls in TRSDOS 2.0 versions. (Which unfortunately were
gone from the TRSDOS 4.x versions. A really poor decision in my
opinion, but that's just my opinion, a giant step backwards.)

Mike


.

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