NEC V20 and 8080 Emulation

smb...@gmail.com

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Oct 15, 2023, 3:05:29 AM10/15/23

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Every so often the topic comes up, whether we can build a V20 board and patch the monitor to put it into 8080 mode and then go on our merry way. I started to look into the idea as it sounds fun. But I think there's probably quite a few complications. The first one is interrupts.

Attached is a state diagram of the V20 CPU. If I'm reading this right (and the text of the document seems to back this up), then any time an interrupt occurs the CPU will switch back to native (8088) mode. It will be in native mode until the interrupt handler completes and issues the RETI instruction, at which point it would reenter emulation (8080) mode. This suggests to me that we would have to rewrite all interrupt handlers.

Maybe this isn't that hard -- we could write an interrupt handler stub that executes BRKEM (to put us back into emulation), jump to the 8080 interrupt handler, return, call RETEM to go back to native mode, and then do our RETI.

The interrupt circuitry would have to be redesigned somewhat as well. Rather than sticking an RST instruction on the bus, we have to stick a "type vector" on the bus. This might not that hard. Or we could just us an 8259 like most other 8088/8086 based systems.

Has anyone thought about this in more detail?

Scott

v20-modes.jpg

norberto.collado koyado.com

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Oct 15, 2023, 3:56:10 AM10/15/23

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This project uses the ATMEGA32A-PU CPU to handle the interrupts. Perhaps it can provide a simpler solution to such problem.

https://hackaday.io/project/170924-v20-mbc-a-v20-8088-8080-cpu-homebrew-computer

Schematics: https://cdn.hackaday.io/files/1709247312812416/A250220%20-%20SCH.pdf

Norberto

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norberto.collado koyado.com

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Oct 15, 2023, 3:01:52 PM10/15/23

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On seconds thoughts, why not use the V20 just to run CP/M-86 as we have the 8080A, Z80, Z180, 8085A CPU boards to do CP/M-80/HDOS?

I think this is where you can add value…

Norberto

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Joseph Travis

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Oct 15, 2023, 3:09:20 PM10/15/23

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I looked at that project a couple years ago as a possible quick way to get the V20 into an H8. As configured, it doesn't necessarily lend itself to be shoehorned into the H8. In the case of the H8, I think you'd be better off staying with the 8284 for clock generation and the 8259 for interrupts.

I like the 8284 because it allows you to have two different clocks (one XTAL, one oscillator) and interfaces with the CPU in such a way to allow for seamless switching between the two. It may be useful to be able to switch down to a lower (2 MHz) speed in 8080 emulation mode for certain I/O operations, etc. The high speed oscillator can be as fast as the CPU allows.

I think using the 8259 (somewhat) simplifies how interrupts are handled in normal or emulation mode. The 8-bit side will be running in a different memory segment than the 16-bit side which can allow for some versatility although in all honesty, I haven't delved deeply into this... It might be interesting to see how they handled interrupts in the H/Z-100 as all interrupts went through the 8088 and the 8 bit stuff would have been passed to the 8085.

Norberto just responded as I was typing. The advantages of using a V20 (for 8080 emulation) are: (1) faster CPU, (2) the 16-bit side could be used as a MMU for the 8-bit side for CP/M 3, (3) the challenge.

BR,

Joe

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smb...@gmail.com

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Oct 15, 2023, 3:17:30 PM10/15/23

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Yeah, I think what makes the V20 interesting is the 8080 emulation, with the prospect of being able to run the original software (with modifications as necessary).

A pure x86 project aimed at CP/M-86 might as well just use the 8088, 80188, 8086, 80186, or any number of other viable ICs. I have actually thought about that, including designing a DOS-compatible board. It's not like I haven't written enough front-panel monitors lately (for the Z-8000 and 8008 boards).

Hybrid projects could also get interesting -- boot MS-DOS and execute HDOS or CP/M-80 software inside of it.

Scott

norberto.collado koyado.com

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Oct 15, 2023, 6:17:01 PM10/15/23

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Isn’t V20 pin compatible with the 8088?

Also found this link: https://www.homebrew8088.com/home/raspberry-pi-second-project

Norberto

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smb...@gmail.com

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Oct 15, 2023, 7:17:57 PM10/15/23

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Yes, that's why I suggested if the goal is CP/M-86, might as well call it an 8088. The V20 can be an upgrade like it was 'back in the day.

I think I installed a V20 in my Tandy 1000. Also a switchable clock for it to increase the CPU speed.

I'd rather just jump to 8086 though. There's no need to hobble the memory bus. Put the RAM/ROM on the CPU board like we do with almost everything else. Then the V30 can be the upgrade. :)

Scott

norberto.collado koyado.com

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Oct 15, 2023, 7:54:56 PM10/15/23

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Using the 8086 is a better solution.

Thank you,

Norberto

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Joseph Travis

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Oct 15, 2023, 8:25:03 PM10/15/23

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Yes, it is pin compatible. I bought a similar board with a V20 a couple of years ago. As I recall, the developer had a version of CP/M running on it. I bought it (cheap) to use the V20 in my H/Z-100 and never used it with my RPi.

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Tim McCaffrey

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Oct 19, 2023, 2:17:00 PM10/19/23

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You could also use a V30, and get the ~40% speed up from that as well.

This might require that the memory needs to be on the CPU board as well.

From what I recall about the Z-100, yes the 8088 BIOS handled handing off the interrupts.

(Wasn't there a version of HDOS for the Z-100?)

- TIm

glenn.f...@gmail.com

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Oct 19, 2023, 2:21:27 PM10/19/23

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I believe the z100 started up in 8088 mode. Most of the CP/M 2.2 BIOS was in 8088 code as I recall, as such it didn’t take up space in the 64K memory space of the 8085. I think most of the BIOS calls just did a quick pass over to the 8088 side where the work was done.

Pat Swayne did an HDOS emulator for the Z100.

And all of this is from the dusty cobwebs of my ancient memory so I could be way off…

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Lee Hart

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Jan 10, 2024, 5:01:11 PM1/10/24

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I still have the schematic (and even PCB tapes) for the H-1000 CPU board for the H89 (Z80 + 8086). It ran essentially all H89 software without modifications, and could boot and run DOS. It took ~100 chips back then, but half of them were memory.

The tricky part was how to marry the Z80's 8-bit bus to the 8086's 16 bit bus so they both shared memory and I/O. That part could be copied to make a dual-CPU H8 board.

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norberto.collado koyado.com

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Jan 10, 2024, 5:32:28 PM1/10/24

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So we can use your tapes to Order some boards?

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Lee Hart

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Jan 10, 2024, 8:35:38 PM1/10/24

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Sure; we could! But they're tape on film; not electronic Gerbers. I'm not sure if a modern PC fab house would know what to do with them.

Also, I would make a number of changes. It used a special stacking socket for the dynamic RAMs (two 256Kx1 chips in each socket). Today, I'd use bytewide static RAMs. That would save a lot of room.

It used several 256x8 bipolar PROMs. Today, I'd use CMOS GALs, which could also reduce the number of chips.

Finally, it was a 4-layer board. Expensive, but necessary back then due to FCC EMI requirements. The reduced chip count and higher density achievable with modern PCBs could probably make it 2-sided.

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norberto.collado koyado.com

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Jan 10, 2024, 10:43:43 PM1/10/24

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As you have the schematics and board top view, it should not be that bad to duplicate. The schematics can be easily entered onto KiCad with the new mods.

The man question; do you still have the monitor ROM and of the bipolar PROMs code?

Norby

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rw...@robhome.com

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Jan 10, 2024, 11:02:34 PM1/10/24

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I think this would be an awesome project to pursue!

Rob Waggoner
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From: se...@googlegroups.com <se...@googlegroups.com> on behalf of norberto.collado koyado.com <norberto...@koyado.com>
Sent: Wednesday, January 10, 2024 9:43:38 PM
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Subject: RE: [sebhc] NEC V20 and 8080 Emulation

As you have the schematics and board top view, it should not be that bad to duplicate. The schematics can be easily entered onto KiCad with the new mods.

The man question; do you still have the monitor ROM and of the bipolar PROMs code?

Norby

From: se...@googlegroups.com <se...@googlegroups.com> On Behalf Of Lee Hart
Sent: Wednesday, January 10, 2024 5:35 PM
To: se...@googlegroups.com
Subject: Re: [sebhc] NEC V20 and 8080 Emulation

Sure; we could! But they're tape on film; not electronic Gerbers. I'm not sure if a modern PC fab house would know what to do with them.

Also, I would make a number of changes. It used a special stacking socket for the dynamic RAMs (two 256Kx1 chips in each socket). Today, I'd use bytewide static RAMs. That would save a lot of room.

It used several 256x8 bipolar PROMs. Today, I'd use CMOS GALs, which could also reduce the number of chips.

Finally, it was a 4-layer board. Expensive, but necessary back then due to FCC EMI requirements. The reduced chip count and higher density achievable with modern PCBs could probably make it 2-sided.

On Wed, Jan 10, 2024 at 4:32 PM norberto.collado koyado.com<http://koyado.com> <norberto...@koyado.com<mailto:norberto...@koyado.com>> wrote:

So we can use your tapes to Order some boards?

Get Outlook for iOS<https://aka.ms/o0ukef>

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From: se...@googlegroups.com<mailto:se...@googlegroups.com> <se...@googlegroups.com<mailto:se...@googlegroups.com>> on behalf of Lee Hart <electr...@gmail.com<mailto:electr...@gmail.com>>
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To: se...@googlegroups.com<mailto:se...@googlegroups.com> <se...@googlegroups.com<mailto:se...@googlegroups.com>>
Subject: Re: [sebhc] NEC V20 and 8080 Emulation

I still have the schematic (and even PCB tapes) for the H-1000 CPU board for the H89 (Z80 + 8086). It ran essentially all H89 software without modifications, and could boot and run DOS. It took ~100 chips back then, but half of them were memory.

The tricky part was how to marry the Z80's 8-bit bus to the 8086's 16 bit bus so they both shared memory and I/O. That part could be copied to make a dual-CPU H8 board.

On Thu, Oct 19, 2023 at 1:21 PM <glenn.f...@gmail.com<mailto:glenn.f...@gmail.com>> wrote:

I believe the z100 started up in 8088 mode. Most of the CP/M 2.2 BIOS was in 8088 code as I recall, as such it didn’t take up space in the 64K memory space of the 8085. I think most of the BIOS calls just did a quick pass over to the 8088 side where the work was done.

Pat Swayne did an HDOS emulator for the Z100.

And all of this is from the dusty cobwebs of my ancient memory so I could be way off…

From: 'Tim McCaffrey' via SEBHC <se...@googlegroups.com<mailto:se...@googlegroups.com>>
Sent: Thursday, October 19, 2023 2:17 PM
To: SEBHC <se...@googlegroups.com<mailto:se...@googlegroups.com>>
Subject: Re: [sebhc] NEC V20 and 8080 Emulation

You could also use a V30, and get the ~40% speed up from that as well.

This might require that the memory needs to be on the CPU board as well.

From what I recall about the Z-100, yes the 8088 BIOS handled handing off the interrupts.

(Wasn't there a version of HDOS for the Z-100?)

- TIm

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