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Comment by mrlonglong | Hacker News Reader

I hate to nitpick but there's an error in the text where it's stated that the 8086 could address 4 megabytes with 20 bit addresses, but that's actually one megabyte. Otherwise, all fascinating reading. My first proper pc was an Amstrad PC1512 with a NEC V30 (clone of the 8086 with some enhanced instructions). Maybe you could take a look at the V30 at some point?

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As pointed out in another comment, it really is 4MB, since the segment register the memory is accessed through is exposed on the bus. You could include those extra 2 bits for a 22-bit total address space, although I don't think it's really practical to have such a partitioned memory model for a general purpose computer.

Thanks, I've fixed that. As for the NEC V30, the 8086 is likely to keep me busy for a long time, but it would be interesting to see how the enhanced instructions in the V30 were implemented.

Also how the V30's 8080 emulation mode was implemented. I wonder if it's "just" some clever microcode or if it required substantial extra die area. It was a great way to run most CP/M software (the exception being software for the Z80 which used LDIR etc which the 8080 lacked and the V30 therefore didn't bother emulating).

I will have to try booting CP/M 8080 on my V30 coprocessor and see how that works. Might have to do custom bdos though.

I've got a NEC V30 coprocessor plugged into my raspberry pi. It works, amazingly well enough

Is it Elijah Miller's https://www.homebrew8088.com/home/raspberry-pi-second-projec... or something else?

I ask because I'm interested (for no good reason :P) in automating 808x bus traces like http://www.jagregory.com/abrash-zen-of-asm/images/fig5.1aRT....

I have a homebrew8088 board with a V30 in it, which I've just started playing with. I've run into two issues:

(1) It runs the processor in "minimum mode", which doesn't expose as much internal/debug state on the pins. (Vs "maximum mode", where the CPU consolidates many control signals into 3 pins, which drive an Intel 8288 companion chip, which drives the control signals)

(2) To plug an intel processor in, I think I need to use a 3.3V <-> 5V shifter, and https://czh-labs.com/products/rpi-33v-to-5v-26-i-o-bidirecti... doesn't expose GPIO0/1, which the board uses for the AD5 / INTA pins.

The board is open source and I've never had a PCB made, so this might be a fun yak to shave. The Raspberry Pi doesn't have enough GPIOs to run the CPU in master mode, but I don't need to be IBM compatible, so I figure I can use an 8088 and simply steal some of the higher address pins.

Yes that's the one I have, it's sitting on my bookshelf at the moment but it does work if a bit slowly. I was going to use it to emulate an Amstrad PC1512 on my pi so I could let the good times roll ...

Nice :) It's really slow but it'd be neat to see GEM come up.

pcEM is another emulator you can use to run GEM. This will do pc1512 and an hard disk.

https://pcem-emulator.co.uk/

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