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A couple of small points on a quick review: Both the call and the return potentially change the RAM mapping. Now I admit that I did not specify that it should be preserved, but to be completely generic it should be. Your return code also potentially changes CRNTPG (0xFAD3). I am sure you won't find ...

A little puzzle to exercise your grey cells during these strange times :) One of the issues when writing code to go into an MTX ROM is calling a subroutine which happens to be in a different ROM. The standard MTX ROMs have a number of solutions for special cases, but the challenge here is to write a...

Should anybody be interested, attached is my disassembly of the SDX high memory. There are two major areas of unused memory at: EC5F - F5B0 F5EF - FA52 Much of this space is used by data structures for the NODE RING if that is installed. Adopting the principle of overwriting unused parts of the BDOS...

It is perhaps significant that back in the day I did not even know I had SDX ROM BASIC. Whenever I wanted BASIC with disk support I used FDXB. I think I even asked Memotech about it when I first got the drive, as I was not anticipating the extra cost of having to buy an 80 column monitor needed for ...

Looking at the ROM source there is a USER SYSCOPY command. This works by making a clean copy of the system tracks on a source disk, and then writing them back to the destination disk. Only issue is that the copy of the system tracks is placed at 0x8000, potentially overwriting any loaded program.

Thinking about this has raised a side question: Was there ever a manual for SDX BASIC? I never had one, and I don't see one on Dave's site. There is documentation for FDXB, and mostly you can just replace "DISC" by "USER". But FDXB relies upon CP/M to do disk formatting. So there are some commands t...

It turns out that functions #25 and #28 are implemented in CP/M 2.2, see the source here . Presumably just not documented. I suppose what I am really questioning is why SDX makes use of the system tracks from the drive rather than just implementing everything in ROM. Was it really much quicker to sp...

Agree, not all the BDOS is required for the SDX, but it is an interesting compromise: The simplest solution would have been to load the BDOS/BIOS in full, and place the file control blocks and other data structures below that. This would however use the maximum amount of high memory. Putting these d...

I have been trying to improve my understanding of exactly what use SDX BASIC makes of high memory (above 0xC000). The first thing I found is that only part of high memory is initialised when you first start the system. A lot more is loaded into high memory when you issue a USER command. To investiga...

Finally found the first issue with my USB Blaster clone. The ten wire data cable had been assembled with one of the connectors reversed :o I did not have a spare connector, and these IDC connectors are not meant to be removable. However, with some careful levering I was eventually able to remove the...