ISA PicoMEM Extension board (For 8086/8088 PC)

Introduction

The PicoMEM is designed as a way to run Emulated ISA boards on a real PC.
The PicoMEM Board currently connect the full 8Bit Memory and I/O Bus plus an IRQ to a Raspberry Pi Pico, through some multiplexor/Level shifter chip.
The Pi Pico also has a 8Mbyte PSRAM connected in SPI and a MicroSD Slot.

The PicoMEM Board can be seen as both a working PC extention board as well as a Development platform.

This GitHub Repository does not contains the Firmware for the moment.
However, PMMOUSE, PMEMM and PM2000 Source are available.

To follow the PicoMEM news, see it in action on multiple machines you can follow me on X: FreddyV on X

To see my other projects, here is my Youtube Channel: FreddyV Retro Zone

How to get a PicoMEM ?

The PicoMEM is available at Texelec ! (Mainly for the USA) Texelec PicoMEM link

NEW: The PicoMEM at Serdahop (Mainly for the Europe) Serdashop PicoMEM

NEW: The PicoMEM at Flamelily Shop (For the UK) Flamelily PicoMEM link

You can have one from me (Mainly for Europe, with wait time), on my form: FreddyV PicoMEM Form

Gerber and full sources are not available for the moment. The source code will be released soon.

How to use the PicoMEM ?

Please go to the Wiki page : https://github.com/FreddyVRetro/ISA-PicoMEM/wiki

Board description

The PicoMEM exist in 3 Releases : 1.0, 1.1 and 1.11

Hardware :

• 2 Layers ISA Board connecting a Raspberry Pi Pico through some buffers/Multiplexor and inverter.
• The Pi Pico is connected to the full Memory and I/O Address space, and one IRQ. (No DMA)
• The Pi Pico is connected to a PSRAM (8Mb, 100/130MHz, PIO) and a MicroSD (12/24MHz, SPI) slot in SPI.
• The Pi Pico is overclocked at 270MHz.
• The MicroSD connector share the SPI BUS of the PSRAM, adding some limitations. (We need to stop the PC IRQ during Disk Access)
• One IRQ Line that can be connected to IRQ 3 or 5 for Rev 1.0, 2,5 or 7 for Rev 1.1.
• QwiiC Connector (SPI) (Added to V1.1)
• Rev 1.11 Changes : Allow for programming with the Board disconnected and USB Powser Jumper can be always On.

Software :

• a Full BIOS with a "Phoenix BIOS Like" text interface in assembly.
• C/C++ Code, PIO and ARM assembly on the Pi Pico.
• Multiple other projects library used. (List below)
• The PC can send multiple command to ask the Pico to perform tasks.
• In the reverse, the Pico can also send commands to the Pico.

Current Functionality

- Memory emulation with 16Kb Address granularity:

128Kb of RAM can be emulated from the Pi Pico internal RAM with No Wait State. We can emulate the whole 1Mb of RAM address space from the PSRAM. (With 6 Wait Stated minimum added) EMS Emulation of Up to 6/7 Mb. (Only 4Mb for the moment as using the LoTech EMS Driver) Memory emulation is used to add 4Kb of "Private" memory for the PicoMEM BIOS Usage. 4Kb of RAM is also added for disk access (Or other) 512b only is used for the moment.

• ROM Emulation for its internal BIOS and custom ROM loaded from the MicroSD. (Custom ROM not implemented yet)
  The Board has its own BIOS, used to automatically detect/Extend/Configure the RAM emulation and select Floppy/Disk images.
• Floppy and Disk "emulation" from .img files stored in uSD through FasFs and DosBOX int13h emulation code.
  Emulate 2 Floppy and 4 Disk (80h to 83h), Disk up to 4Gb (More later)
• USB Mouse support through a USB OTC Adapter. (Micro USB to USB A or USB Hub)
• POST Code on Rev 1.1 and 1.11 (Port 80 Display in Hexa) through this device : https://www.sparkfun.com/products/16916
• ne2000 network card emulation via Wifi is working on boards with a PicoW.
• USB Joystick for PS4 and Xinput controllers.
• NEW: Adlib using a PCM5102 I2S module.

Future Functionality

• There is already a mechanism implemented, so that the Pi Pico can send command to the PC, we can have the Pi Pico taking "Virtually" the control of the PC.

  This can be used to perform ROM/RAM dump, Disk/Floppy DUMP/Write, display/kb redirection....

• More USB Host to be added (Keyboard, MIDI...)
• Bluetooth support for device like Gamepad may be added.
• Use of the Qwiic connector for more information display (OLED), maybe RTC and other.

Compatibility/Limitations

The Board can't be used for Video emulation, as it require a way for the Pico to actually display something, and only 3 pins are "Free". The Pi Pico is limmited in its speed, this is excellent and bad at the same time:

• Multiple complex function can't be emulated at the same time, choices need to be done.
• We can still program the Pico and Feel like doing coding on a "Retro" Machine, so we don't have the effect "Look, it is easy, he put a processor in the PC that can emulate the full PC"
• There are still problems on various machines.

Memory emulation

As the PicoMEM is an 8Bit ISA Card, the RAM Emulation may be slower than the PC own RAM. The PicoMEM is then more suitable to extend a 512Kb PC to 640K, Add some UMB (For Driver) than extend an IBM PC With 128Kb of RAM. (RAM Card for 5150/5160 is still recommended)

Memory emulation capabilities :

• The PicoMEM BIOS auto detect (At Hardwsare level) the 1st Mb of RAM and Display the RAM MAP with a "Checkit" like Display.
• You can select to add RAM from the Internal SRAM (128Kb with No Wait State)
• Emulation from the PSRAM (External RAM) can add RAM to any Address Space.
• Then, EMS Emulation emulate a LOTECH Board with Up to 4Mb.
• Disk access on PM RAM emulated memory are done at the Pico uSD speed. (3Mb/s)

Memory emulation limitations :

• Memory emulation with PSRAM is quite slow for the moment, but multiple mechanism like a 32bit cache will improve this. (And Maybe DMA)
• The emulated Memory does not support DMA, Add support for it may be done in one or two months (May/June 2024) Anyway:
  • As the real floppy use DMA, you should disable temporarily the RAM emulation if the real Disk access are not working.
  • For SoundCard, if the PC has 512Kb of base RAM, it is really unlikely that the DMA Buffer will be placed in emulated RAM, it may work 90% of the time.

Disk "emulation"

The PicoMEM can add 4 disks to the BIOS, Disk up to 4Gb.
It can also mount Floppy image as A: or B:

The PicoMEM does not emulate disk, it send the BIOS Disk access commands to the Pi Pico.
Then, it is more a "Disk BIOS" emulator than a Disk emulator.

Another particularity is that it use Memory to perform the Data transfer, this allow for the maximum possible transfer speed, even on 8088 CPU.
Anyway, single sector read is slower than multiple sector reas as the PicoMEM need to read the sector from the SD, then send the data to the PC Memory.
With multiple sector Read, the Pico read the next sector while the data is copied to the PC Memory.

It is highly recommended to use reasonable disk image size, below 500Mb.
100 or 200Mb are ideal size, as it will be recognized and usable by DOS 6 and DOS 3.31
As you can add 4 Disk with 4 partitions, you can create diferent disk images for different usage, like a partition with games, another with music ...

Warning : You may have compatibility problem with some uSD, even if the compatibility has been greatly improved in the November 2023 firmware.

ne2000 emulation via Wifi

The PicoMEM can emulate a ne2000 network card via Wifi.
The wifi code is preliminary, the PicoMEM can't see if the access point is connected or not, code improvement in progress.

How to use it :

• Create a wifi.txt file with the SSID in the first line and the Password in the 2nd line
• use ne2000.com 8Bit (command line : ne2000 0x60 0x3 0x300) or pm2000 (command line : pm2000 0x60)
• The PicoMEM can't tell if the connection fail and does not try to re connect.
• The Wifi Access point need to be relatively close to increase the chance of connection success. The IRQ Can be changed in the BIOS Menu (Default is IRQ 3)
  

Tested machines :

• IBM 5150, 5160, 5170
• IBM PS/2 30 286 (Warning : As its HDD use DMA, does not boot if emulated RAM is added)
• Compaq Portable 2 (286): Ok
• Amstrad PC1512, PC1640, PPC1640 (Address D000), Sinclair PC200 (It is my DEV Machine)
• Schneider Euro PC2, Olivetti M21, Sega TeraDrive.
• Worked on Various 486, 386, 286 (Has more chance to work with lower ISA Clocks)
• Tested with some Pentium, Pentium MMX, Pentium 2, AMD K6 ...
  
• Amiga 2000 with a A2286/A2386 SX Board. (Use the BIOS in C800)
• Book8088 (At 4.77MHz and 8MHzwith the latest firmware), The Power need to be connected at 8MHz.

Failing Machines :

• Failed on a 386 with 12MHz ISA Clock
• Fail on some fast 286 (16MHz)
• Commodore PC10 / PC20 (Timing issue, fix in progress)
• Schneider Euro PC 1.
• NuXT / EMM8088 homebrew computers seems to have problem.
• Amiga 2000 with a A2088.
• Tandy 1000 : Does not work yet due to the specific memory map. (SX, EX, HX, TL ...)
  

License

Hardware and Software are currently closed.

As the PicoMEM contains various public source, I Will publish it as well as Public.
Anyway, I consider it not ready to be opened yet, as I am not sure about the License to use and how to manage it.
If you want to participate, You can ask me access to the source on demand.

Contributors / External Libraries

• Ian Scott: Idea to use the Pi Pico instead of the not available Pi 2/4 and Zero plus help on the Hardware design.
• PSRAM Code by Ian Scott : PSRAM PIO Code.
• FatFS by Chan : ExFS Library for microcontrollers.
• FatFS by Carl J Kugler III : FatFS for SD Access in SPI with a Pi Pico
• DOSBox (www.dosbox.com): DosBOX BIOS Int13h Code, heavily modified for ExFS support and PicoMEM Interface, with bug correction.
• ne2000 Emulation code, adapted by yyzkevin.
• XInput library for the USB Joystick.