(Sorry for the random seeming reply. I had thought I had sent that earlier.)
On Wed, May 25, 2022 at 4:49 PM B 9 <[email protected]> wrote: > 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 <[email protected]> > 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 <[email protected]> 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 <[email protected]> >>> 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 <[email protected]> wrote: >>>> >>>>> Is there an equivalent to the Model 100 REXCPM rom for the Model 200? >>>>> >>>>> >>>>>
