I don't know that it is particularly well documented. Secrets of the Rom Revealed covered this for M100. I think if you look in the T200 rom you will find this code is used for bank switching, and also during boot up to check for option rom presence.
I use it alot in REX. On Thursday, January 29, 2026, B9 <[email protected]> wrote: > Thanks! Where is that documented? > > I was getting lost looking at the T200's Technical Reference which was way > too low level (there was a diagram for the banks, but it showed setting > AD0, AD1, AD2 while asserting WR and Y, or some such silliness). > > --b9 > > > > On January 29, 2026 5:13:03 AM PST, Stephen Adolph <[email protected]> > wrote: > >> yes, you need to bank-switch. >> >> * first, disable interrupts and keep them off >> * then send a specific byte to a specific IO port to see multiplan >> >> * access the memory, etc >> >> * then send a specific byte to a specific IO port to get back to the >> main rom >> * enable interrupts >> >> IN 0D8h >> ani 00001100b ; keep the ram bits, zero out the rom bits >> ori 00000001b; enable multiplan rom >> OUT 0D8H >> >> >> IN 0D8h >> ani 00001100b ; keep the ram bits, zero out the rom rom bits >> OUT 0D8H >> >> think this should do it. >> >> On Thu, Jan 29, 2026 at 4:29 AM B 9 <[email protected]> wrote: >> >>> What with talk of uploading the ROMs from Model-T computers, I spent >>> some time learning how to implement a tiny CRC-16 checksum in machine >>> language (using the Xmodem polyomial and a seed of 0). It works great on >>> the Model 100 and 102. However, I just realized I have no idea how to read >>> the entire ROM from a Tandy 200. >>> >>> I've heard it said that the Tandy 200 has 72K of ROM. I believe 40K is >>> visible from BASIC and the other 32K is visible when Multiplan is launched. >>> >>> Does anyone know how I can read the Multiplan ROM from either BASIC or >>> assembly language? >>> >>> Thanks! >>> >>> —b9 >>> >>
