[HN Gopher] PicoZ80 - Drop-In Z80 Replacement
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PicoZ80 - Drop-In Z80 Replacement
Author : rickcarlino
Score : 94 points
Date : 2026-04-09 18:53 UTC (4 hours ago)
(HTM) web link (eaw.app)
(TXT) w3m dump (eaw.app)
| unixhero wrote:
| Zilog?
| monocasa wrote:
| Yes.
| Lerc wrote:
| I had been pondering about doing more or less the same thing for
| 6502 (6510).
|
| It was always the dilemma of whether to pull the CPU out of a C64
| and replace it like this, do it as a bus mastering cartridge, or
| replace the RAM.
|
| I have been leaning towards the cartridge plan to avoid the
| requirement of doing machine surgery. If you get the RP2350 to
| pretend to be the RAM then the video hardware could read directly
| out of it which makes all sorts of shenanigans possible (every
| line a BADLINE).
|
| At some point it would look like just plugging A VIC-II and a SID
| into a board with the RP2350 though, The cartridge approach means
| you have to do transfers across into the computer's RAM, but you
| could also write to hardware registers every CPU cycle, which
| would enable some potentially new modes that would not be
| entirely dissimilar to every line a BADLINE.
|
| Right now I'm mucking around with getting the RP2350 to output
| video constructed a scanline at a time, using as little CPU as
| possible. I got three layers of tiles and two layers of sprites
| each with different pixel formats working yesterday. Quite
| pleased with that. The CPU calculates a handful of values per
| scanline, but fetching tilemap data, then tile data, then
| conversion to pixel values, transparency and palette lookup are
| all DMA and PIO. Does 1,2,4, and 8 bits per pixel, each
| tile/sprite/imagebuffer layer with independent 24 bit palettes.
| topspin wrote:
| You have such ponderings in common with engineers@work:
|
| https://eaw.app/pico6502/
|
| "and palette lookup are all DMA and PIO"
|
| PIO is a revelation.
| teaearlgraycold wrote:
| I do love the PIO. I want to show it to a computer engineer
| from the 80s.
| zeckalpha wrote:
| Not all that different from the MCS-96 HSIO.
| Lerc wrote:
| It's great, but I think the critique from the other day was
| also pretty valid. and offered an alternative.
|
| https://www.bunniestudios.com/blog/2026/bio-the-bao-i-o-
| copr...
|
| I think, for my use, just having the ability to write to DMA
| registers would have been a big advantage. It feels wasteful
| to have A DMA waiting on a FIFO just to write what it gets to
| DMA registers to do the transfer you actually wanted.
|
| Looking at the Architecture diagram It seems like it could
| have allowed that and stayed on the same side of the AHB5
| splitter.
| YasuoTanaka wrote:
| This is less of a "CPU replacement" and more of a bus-level
| participant.
|
| Once you control the bus cycle-accurately, the CPU abstraction
| kind of disappears. You're effectively redefining the whole
| machine behavior from the outside.
| rahimnathwani wrote:
| How do the personas work? Does the board direct the Z80 to an
| alternate ROM stored in the flash?
| jgrahamc wrote:
| Oh wow. Enhancements for the Sharp MZ line! Wonderful. I spent a
| lot of time with those machines in the 1980s and own a few. Being
| able to emulate the Sharp MZ-80K's
| (https://blog.jgc.org/2009/08/in-which-i-switch-on-30-year-ol...)
| MZ80FD would be cool.
| uticus wrote:
| how is "tranZPuter" not "transputer"
| (https://en.wikipedia.org/wiki/Transputer)
| uticus wrote:
| and elsewhere on the page, ZPU (FPGA-based microprocessor)
| sounds a lot like "ZipCPU" FPGA-based microprocessor
| https://zipcpu.com/
| JVIDEL wrote:
| So is this just for hobbyists or it has any, say, industrial
| applications? are there any machines still running on a Z80?
| brudgers wrote:
| Approximately every old digital technology is still in use
| industrially.
| boznz wrote:
| Hot tip: Ignore the RP2350 design sheet and use a standard 1.2V
| LDO in to provide the internal vCore - you save having to use
| that weird inductor and can clock it at a 300Mhz much more
| reliably at 1.2V.
| Lerc wrote:
| What was the reasoning behind that? Were there specific
| features of that inductor that led them to choose it, or did
| they choose it and then found some of their design relied on
| atypical generic inductor behaviour.
|
| The problem with going off design sheet is you don't know what
| might change. There's usually a good chance that you are not
| depending on the difference, but it's the not knowing that gets
| to you.
| jhallenworld wrote:
| I want to make something like this as a classic CPU ICE, with
| trace memory, disassembly, etc. (note that you need a crystal
| oscillator circuit for many CPUs- 6802, 8085, etc.)
|
| It would be useful for debugging classic computers like Altair
| 8800, etc. What you do is get a boot trace (record the first
| 100,000 instructions) of a working machine and diff it with the
| one from your broken machine. This finds the problem in like 5
| seconds.
| Dwedit wrote:
| The one thing that makes a modern computer faster than an 80s
| computer is _cache_. Without cache, your computer has to go to
| the memory bus to fetch every instruction and memory read or
| write, and your system will wait to get the bytes back before it
| takes any action. You end up at the performance level of an 80s
| computer.
|
| So you replace the CPU with a faster one with built-in cache. CPU
| ends up with its own private copy of the RAM and ROM sitting in
| its cache. But that's not the end.
|
| Computers have a memory map, memory bank switching, memory-mapped
| IO, and other things to consider. The CPU with its cache has to
| be kept in sync with the actual memory map of the system. Both
| the CPU and any memory mapping hardware need to be kept in sync
| with each other. Memory-mapped IO reads and writes need to go to
| the actual memory bus at native bus speed.
|
| Then you're left with the issue of other devices that need to
| access the RAM. This requires cache flushing for writes, and
| cache invalidation for reads.
| bilekas wrote:
| I am a newbie to the 'homebrew' hw, have many years in plc
| systems in various environments with different requirements.
|
| I love for reasons maybe unexplainable my Teensy2.. I want this
| one too. I feel this hobby is going to become very expensive. I
| have an old family Tandon and I'm getting ideas.
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