Let me see if I understand correctly. The main problem is that CPM requires the entire 64K of address space to be RAM. For the Model 100, REXCPM somehow is able to replace both the builtin ROM (0-7FFF) and RAM (8000-FFFF) with its own System RAM. That method will not work for the Model 200 in which the ROM goes from 0 to 9FFF and the RAM is in three banks at A000-FFFF.
I tried to look up how REXCPM is able to switch out the ROM and RAM to see if I could find an analogous solution for the Model 200, but unfortunately REXCPM's Technical Information simply says, "Work in progress!!!" <https://bitchin100.com/wiki/index.php?title=REXCPM#Technical_Information>. I'm guessing that REXCPM is able to replace the lower 32KB on a Model 100 simply by virtue of being an "OPTION ROM". I have no idea how it replaces the RAM. Despite the Model 200 BASIC ROM now going to 40KB, the service manual states that OPTION ROMs are limited to 32KB. I see that only fifteen address lines (32KB) go to the socket. But is that insurmountable? The diagram for the Bank Selection Circuit (SLA5080F0U) appears to select the ROM for addresses up to 9FFF, regardless of whether the ROM is bank 1 (BASIC) or bank 3 (OPTION ROM). So, any OPTION ROM would already be filling in that hole from 8000 to 9FFFF, but with the data from 0 to 1FFF since it is missing *A₁₅*, the high bit of the address. Right? Correct me if I'm wrong — and I probably am — but theoretically couldn't one create an option ROM chip that has an extra input pin for *A₁₅* and have it work to replace all 40KB of ROM with RAM? I'm imagining a blue wire for *A₁₅* connecting to, perhaps, pin 27 on chip M13, the supplemental 8KB of BASIC ROM. —B9 On Sun, May 15, 2022 at 2:35 PM Stephen Adolph <twospru...@gmail.com> wrote: > There would have to be a convenient plug and play ideally way to decode > that. > From what is present in the ram module compartment, I dont see a good > solution. > I'll take another look. > > On Sunday, May 15, 2022, B 9 <hacke...@gmail.com> wrote: > >> I, too, would love to see RexCPM for the Tandy 200. >> >> I apologize for my ignorance, but why is there a hole? For ROM? Would it >> be possible to bank out 8000 to 9FFF and replace it with RAM when running >> CPM? >> >> —B9 >> >> >> On Sun, May 15, 2022 at 9:44 AM Stephen Adolph <twospru...@gmail.com> >> wrote: >> >>> Hi >>> >>> At this time, no. It would require some changes due to the unique >>> memory map in the T200 relative to the M100. >>> >>> I think the biggest issue is that an all ram mode is not clearly >>> possible. There would be a hole fro. 8000 to 9FFF. That's not great for >>> CPM. >>> >>> Steve >>> >>> On Saturday, May 14, 2022, Hiraghm <hira...@hotmail.com> wrote: >>> >>>> Is there an equivalent to the Model 100 REXCPM rom for the Model 200? >>>> >>>> >>>>
