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 [ 3 posts ] 
 V20-MBC: a V20 (8088 + 8080) CPU homebrew computer 
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Joined: Mon May 04, 2020 3:29 pm
Posts: 7
Hi all,
first post here!

This is my last "toy":


The V20-MBC is an easy to build V20HL (full static CMOS version) CPU SBC (Single Board Computer). It follows the same "concept" of the Z80-MBC2 (, with a SD as "disk emulator" and up to 1024KB RAM.

It has an optional on board 16x GPIO expander, and uses common cheap add-on modules for the SD and the RTC options.

It has an "Arduino heart" using an Atmega32A as EEPROM and "universal" I/O emulator (so a "legacy" EPROM programmer is not needed) programmed with Arduino IDE.

It is possible to use an 80C88 (CMOS version).

Here running the IMSAI Basic (8080 mode):


Currently I'm working at the porting of CP/M 2.2 in 8080 mode.

V20-MBC link:

Mon May 04, 2020 3:46 pm

Joined: Wed Jan 09, 2013 6:54 pm
Posts: 1666
welcome! That looks rather good. Nice big old-style heatsinks for the power regulators.

The V20 is pretty interesting, but it doesn't seem to be much used.

Mon May 04, 2020 4:46 pm

Joined: Sun Jul 05, 2020 9:00 pm
Posts: 15
Yeah, but the V20 was a drop-in replacement for the 8088. I like how it also has the 186 instruction set and a hardware multiplier.

Too bad they didn't create a V23A (there is a V33A) which eliminates the multiplexing of address and data signals. Maybe someone could design one on an FPGA if they wanted to and then modify the original XT (or any clone with discrete logic and not the Siemens glue chip) to where the address and data lines can be wired to the motherboard directly past the demultiplexing, thus eliminating a bottleneck and allowing the motherboard to be clocked faster.

If cloning an 8088/V20 in FPGA, then one could forego some of the cycle correctness, though modifying the ROM may be required for that (or the ability to enable cycle correctness until POST completes) so the machine will boot. Another idea would be to put all the conventional memory in BRAM.

And if one is modifying the original IBM ROM, it would be good to fix inefficient code and known bugs too. There is a malformed loop in the original XT ROM. The purpose of the loop is to reduce snow when writing to video memory. However, it has no effect. They likely never really tested that since using an 8088 and IBM video cards never produced snow. For other CPU and card combinations, the loop doesn't prevent snow.

PS., The vertical things are not heatsinks. They are circuit boards. I think one is an SD chip reader and the other is likely the RTC/temperature sensor that loads with the machine as is showing in the boot pic. The big black thing on the one card is a battery holder to keep the RTC chip powered between boots.

Sun Jul 05, 2020 9:41 pm
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