This M100 mouse driver gives an example of knowledge that was once known but now seems lost:
https://archive.org/details/P100-Magazine/1990-02/page/10/mode/1up?q=ff8dh At some point folks knew that FF8Dh was the serial mask. They even had a name for it (SERMASK) and a T200 equivalent (FCFCh). --b9 On May 4, 2026 8:08:14 PM PDT, 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
