TRS-80 Question

[Eric Dittman]

On 4/11/22 1:33 PM, Bill Gunshannon via cctalk wrote:
> 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,

There's a 2K hole in the Model I memory map above the ROM, this
space is used by the Model III ROM (12K ROM on the Model I, 14K
on the Model III).  After that you have memory-mapped I/O on
the Model I.  Both of them then have the keyboard mapped at
3800H and the video at 3C00H.  RAM starts at 4000H on both of
them.

How the first section of RAM is used depend on whether you are
using disk or cassette and if you are using disk then which OS
you use.  There's a good breakdown of how the RAM is used here:

https://www.trs-80.com/wordpress/zaps-patches-pokes-tips/ram-addresses-and-routines/
-- 
Eric Dittman