3812Strange discovery with stop + reset, run and Bytesaver
- Jul 25, 2014Hi All,
First some background, over the last few weeks I've spent some time on a mini project to reconfiguring my 8k Bytesaver card to run 8k Basic. For a while now I've been using John Garza's 2k Altair Rom monitor, which I really like, it has some great features. But I've wanted to cut it down to 1k and re-address it, so I could fit 8k Basic on the other 7 roms in the Bytesaver. I spent some time with the source code and managed to remove some features that I don't use and get it to assemble at just under 1k. I used CP/M running under the SIMH AltairZ80 emulator to re-assemble the monitor and I used mload to create the stand alone binary. All worked well! I now have the 1k Altair Rom addressed at E000 and 8k Basic addressed from E400 to FFFF. And using the block move and run feature of the monitor I can transfer Basic to ram and run it instantly.
Now to the strange discovery that I made with my machine. Since addressing the monitor to E000 which corresponds to the first slot of the 8k Bytesaver, whenever I turn on the Altair and simply hit stop, reset and run (all address switches down), I can run the monitor at address E000. I only just discovered this, usually I would examine address E000 manually and then hit run.
Now, how is it possible that I can simply hit reset and run from a cold boot and the monitor at address 56k will run?? I've never read that the 8k Bytesaver has any feature like this, so I'm pretty sure it's not that. I've also checked that the Bytesaver is addressed properly and not somehow overlapping ram, and it's addressed properly at E000.
Once observation I've made is that my particular Altair has a non standard jumper wire installed on the back of the front panel, the wire is connected to the run switch. Until now I've never had any idea why that wire was, but I suspect it may be wired to jump to the rom card and run from there. If so it's an amazing co-incidence. Can anyone shed any light on how my machine is capable of jumping to the rom card?
A while back I posted a picture of the back of my front panel on the VCF, you can see the jumper wire here:
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