TRS-80 Question

[Bill Gunshannon]

On 4/11/22 14:09, Eric Dittman via cctalk wrote:
> On 4/11/22 11:56 AM, Bill Gunshannon via cctalk wrote:
>>
>> Here's one for whatever TRS-80 gurus still hang out.
>>
>> The Mode 4 had 64K RAM and pretended to be a Model III by loading
>> an image of the Model III Rom and then running it.  Would it be at
>> all possible to do the same thing with an image from the Model I
>> and thus make a Model 4 capable of running Model I programs?
> 
> The Model 4 had the Model III ROM included.  It was the Model 4P
> that had to load the Model III ROM image from disk before operating
> in Model III mode.

Not surprised I got that part wrong.  But then, I use my 4P all the
time and haven't touched a normal 4 or 3 in quite a while.

> 
> There were some hardware differences between the Model I and Model
> III, such as the disk interface being memory-mapped in the Model I
> and port-mapped in the Model III, but if the Model I programs don't
> use anything that's different in the Model III then the Model 4 in
> Model III mode should be able to run them.  So a game that's loaded
> from cassette should work but a game that runs from disk and writes
> to the disk won't work.

Actually, the big difference is the memory map.  Most Model 1
programs load right where the top off the Model III ROM sits
so they can not be used on the 3 or 4.  That's why I was hoping
the ROM image trick could be used.  I guess it was just wishful
thinking. Thanks for the info,

bill