Eudimorphodon
Veteran Member
Wasn’t this already completely hashed out in a different thread, that at one point even had a link dropped in it to a homebrew board that was basically a copy of an existing Z80 SBC with the CPU replaced by a V20? (Thereby rendering it capable of, yes, running a completely pure CP/M-80 because all the I/O is at 8080-friendly addresses.)
The only peripherals in a standard PC reachable at I/O ports under 100h are the timers/DMA/keyboard/etc, IE, the stuff that’s actually on the motherboard in an IBM 5150. The disk controllers, serial and parallel ports, everything else, are all at higher ports that the 8080 mode of the V20 can’t touch directly.
I would also assume that trying to use the memory mapped video of the PC directly in 8080 mode would be a problem, since most PCs don’t have the text video memory directly adjacent to conventional memory, therefore making it impossible to set up a single 64k segment to hold both your CP/M TPA/BIOS and the 4K of text video buffer, unless NEC added some ability to do “far” memory accesses in 8080 mode without also doing a mode switch.
Edit: https://forum.vcfed.org/index.php?threads/cp-m-80-binaries-on-v20-with-pdos-86-v20.1243908/
The only peripherals in a standard PC reachable at I/O ports under 100h are the timers/DMA/keyboard/etc, IE, the stuff that’s actually on the motherboard in an IBM 5150. The disk controllers, serial and parallel ports, everything else, are all at higher ports that the 8080 mode of the V20 can’t touch directly.
I would also assume that trying to use the memory mapped video of the PC directly in 8080 mode would be a problem, since most PCs don’t have the text video memory directly adjacent to conventional memory, therefore making it impossible to set up a single 64k segment to hold both your CP/M TPA/BIOS and the 4K of text video buffer, unless NEC added some ability to do “far” memory accesses in 8080 mode without also doing a mode switch.
Edit: https://forum.vcfed.org/index.php?threads/cp-m-80-binaries-on-v20-with-pdos-86-v20.1243908/