On Tue, Apr 12, 2022 at 3:46 PM Fred Cisin via cctech <cctech@classiccmp.org>

> >> There's a 2K hole in the Model I memory map above the ROM
> On Tue, 12 Apr 2022, Yeechang Lee via cctech wrote:
> > Is this the hole that causes stock Model I to not run CP/M?
> NO.
> The problem with CP/M on TRS80 is that CP/M expects RAM from location 0 on
> up.

When I was a freshman at Purdue, I lugged my Model III to my dorm room and
connected to the ECN network with a 300 baud modem. I used a local editor
and wrote a Pascal program to upload my Pascal source to the dual processor
VAX-11/780 (Google George Goble), ea or eb, (I don't remember) that was
used by our introductory programming class. The terminal program I had was
something I found in Byte magazine in assembler and I modified it for the
TRS-80. I had BASIC, Assembler and Fortran on the TRS-80 M III, so it was
probably all in assembly.

There was an article in Byte about CP/M for the TRS-80 Model III that
described a hack to swap the ROM for RAM. The idea was to invert bits A15
and A14. That would move the ROM and keyboard from 0000 to C000. There was
a spare bit in some 4 bit register, so all you had to do was cut a couple
traces and insert some XOR gates. I remember doing the modification on a
Saturday while listening to the Purdue football game on the radio. I put it
all back together and it worked. WoHo!

At that point though I had no access to CP/M or where I might get it legal
or otherwise, but I was good to go when I found it.

I still have the computer and I still have the Byte copy. So 37 years later
I should try to complete the project.


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