I have spreadsheets to contribute. Happy to share those. Steve On Monday, May 4, 2026, B9 <[email protected]> wrote:
> Wow, great sleuthing. Which program are you referring to that operates at > non-standard rates? > > In trying to decipher the old listings, I've repeatedly wondered if people > had a different ROM disassembly available to them in the past. Of course, > it's totally possible that folks were independently figuring things like > this out and they simply didn't share the knowledge. > > By the way, I'm starting a side-project to gather all the known ROM/RAM/IO > addresses for the Tandy 200 into a single machine and human readable file. > The idea is to have a format that can be used for both disassembling > existing code (for example, as "labels" in Ghidra) and as an equiv > "library" for writing new assembly language programs. > > --b9 > > > > > > > On May 4, 2026 1:20:19 PM PDT, Peter Noeth <[email protected]> wrote: > >> Group, >> >> I have been doing some work on a BASIC program I downloaded from the >> old club100 library that uses the RS232 port at a non-standard baud rate. >> >> The BASIC program also POKES the value 31 into memory location 65421 >> (FF8Dh). Since there is no documentation in the code or .DO file, I have >> been researching this location. According to Ken Pettit's M100 ROM >> disassembly (dated 2013), this memory location is only used twice, once in >> the "UART config from text string" routine (17E6h) at 1832h and again at >> the "RS232 Receive Interrupt" routine (6DACh) at location 6DB9h. >> >> The memory location FF8Dh is noted as "RS232 Control Byte" in both >> locations in the disassembly, but in reality, is a "read data mask". >> Microsoft included this ability, but who knows why. Without the original >> commented ROM listing we will never know. >> >> In the "UART Config From Text String" routine, the value 255 (FFh) is >> always written to memory at FF8Dh, which couldn't be a shadow of the RS232 >> Control Byte (baud rate, parity, etc.). And in the "RS232 Receive >> Interrupt" routine, after data is read from the UART Receive Data Register >> (C8h), it is ANDED with the byte at FF8Dh before being stored in the C >> Register. Definitely a "mask" operation. >> >> The BASIC program is putting a value of 31 (00011111b) in FF8Dh, >> masking whatever data is received to a value between 0 and 31. This would >> make sense to the BASIC program which is configuring the RS232 port for >> 5-bit operation. But I would think it is not necessary. Unfortunately, I >> can't contact the originator of the BASIC program, to find out why he did >> this and how he found this memory location in RAM as I have found no Model >> 100 documentation that references it (I own most all of the books written >> about the Model 100). >> >> So, just a "word to the wise" about the function of memory location >> 65421 (FF8Dh). Maybe Ken and others who have worked on commenting the >> disassembly of the Model 100 ROM, will update their listings. >> >> Regards, >> PeterN >> >
