z80pack on Waveshare RP2040-GEEK/RP2350-GEEK

While looking around for RP2040 based systems that can easily be used by those who don't want to tinker with the hardware, I found this:

Waveshare RP2040-GEEK product page and Waveshare RP2040-GEEK Wiki

and recently this:

Waveshare RP2350-GEEK product page and Waveshare RP2350-GEEK Wiki

Emulation

z80pack on the RP2xxx GEEK devices emulates a Intel 8080 / Zilog Z80 system from ca. 1976 with the following components:

• 8080 and Z80 CPU, switchable
• 112 (352) KB RAM, two banks with 48 KB and a common segment with 16 KB on RP2040 and seven banks with 48 KB and a common 16 KB segment on RP2350
• 256 bytes boot ROM with power on jump in upper most memory page
• three MITS Altair 88SIO Rev. 1 for serial communication with terminals, printers, modems, whatever, runs over USB and the serial UART
• an output only port for printer, runs over USB
• DMA floppy disk controller
• four standard single density 8" IBM 3740 compatible floppy disk drives
• Cromemco Dazzler graphics board with output on the LCD

Disk images, standalone programs and virtual machine configuration are saved on a MicroSD card, plugged into the GEEK. It can make the MicroSD card available as USB drive on any PC, so the MicroSD can be filled with contents, without the need to remove it and stick it into some PC.

The virtual machine can run any standalone 8080 and Z80 software, like MITS BASIC for the Altair 8800, examples are available in directory src-examples. With a bootable disk in drive 0 it can run these operating systems:

• CP/M 2.2
• CP/M 3 banked, so with all features enabled
• MP/M II banked with two terminals, this is Z80 and RP2350-GEEK only
• UCSD p-System IV
• FIG Forth 8080 using drive 1 as block device, so true operating system

All implemented operating systems other than MP/M use 8080 instructions only, so it is possible to switch CPU's anytime, even 'on the fly'. It is possible to implement MP/M on the 8080 also, but requires self modifying code, because the 8080 lacks various indirekt memory and port addressing. The DRI MP/M manuals include example code for 8080, for me it is less elegant and not very interesting, but feel free to try your self. Also MP/M will work on the RP2350-GEEK only, the VM provides 7 user memory segments and the RP2040-GEEK doesn't have enough memory for this.

The LCD can show several stati of the virtual machine, the initial shown display can be set in the configuration. Also the displays can be switched in the running system with a CP/M program:

• Z80 or 8080 registers
• frontpanel like IMSAI 8080 with the output LED's
• status of the disks subsystem
• I/O port accesses
• memory contents

And of course Cromemco Dazzler:

here playing 8080 Microchess for example.

The RP2350-GEEK even can run a MP/M multiuser system with two terminals:

History

In 2024 Intel has 50th anniversary of the 8080 CPU: https://www.intel.de/content/www/de/de/newsroom/news/50-years-ago-the-influential-intel-8080.html

The picture (CPU on Altair 8800 CPU board, courtesy Bill Lewis) shows one of the first 8080 CPU's still in ceramic pack, about the size of a modern RP2xxx-GEEK device. This project is a nice demonstration of technology advance over 50 years, on the RP2350-GEEK we cannot only emulate a 8080 CPU with about 8 times the speed of an original chip, but also a complete system like they were build back then. The 8080 was the first general purpose 8bit CPU, which allowed to build computer systems for everyone, as we have it today.

Building

To build z80pack for this device you need to have the SDK 2.1.1 for RP2040/RP2350 based devices installed and configured. The SDK manual has detailed instructions how to install on all major PC platforms, it is available here: Raspberry Pi Pico Documentation

Then clone the GitHub repositories:

1. clone z80pack: git clone https://github.com/udo-munk/z80pack.git
2. checkout dev branch: cd z80pack; git checkout dev; cd ..
3. clone this: git clone https://github.com/udo-munk/RP2xxx-GEEK-80.git

To build the application for the Waveshare RP2040-GEEK:

cd RP2xxx-GEEK-80/srcsim
mkdir build
cd build
cmake -G "Unix Makefiles" ..
make -j

For the Waveshare RP2350-GEEK use (you can also specify rp2350-riscv as the platform if you have the appropriate RISC-V toolchain installed):

cd RP2xxx-GEEK-80/srcsim
mkdir build
cd build
cmake -D PICO_PLATFORM=rp2350-arm-s -G "Unix Makefiles" ..
make -j

If you don't want to build it yourself, the directories flash-rp2040, flash-rp2350-arm-s, and flash-rp2350-riscv contain the current build, flash picosim.uf2 into the device.

The latest sources also have a bit debugging support, one can write debug messages to the DEBUG GPIO 2 pin of the device. For using this you need to connect the RX pin of a Pico Probe or some other Serial/USB converter to the DEBUG pin labeled GP2. Then add -D DEBUG80=1 to one of the above shown cmake commands to enable it.

Preparing MicroSD card

In the root directory of the card create these directories:

CONF80
CODE80
DISKS80

Into the CODE80 directory copy all the .bin files from src-examples. Into the DISKS80 directory copy the disk images from disks. CONF80 is used to save the configuration, nothing more to do there, the directory must exist though.

Optional features

I attached a battery backed RTC to the I2C port, so that I don't have to update date/time information my self anymore. This is optional, the firmware will check if such a device is available, and if found use time/date informations from it.

For experimenting with the serial UART one can connect a Pico Probe to the UART port, like this:

Or use some USB to serial adapter, like this one:

Another feature one might be missing, if just using the prebuild firmware is, that z80pack also contains a Mostek In Circuit Emulator (ICE). In the builds provided it is disabled, because we assume that those just using it don't want to mess with the sources, and just use the little machines to run some software on. For the software engineer trying to find some nasty problem on a true Von Neumann can be a challenge, and we have tools like debuggers for this. The ICE can be enabled with a commented define in srcsim/sim.h, and gives one everything needed, like software breakpoints, hardware breakpoint, single step, dump of memory and CPU registers and so on. If a program under inspection with the ICE runs away after the g command, it can be stopped anytime by sending the break signal from the terminal. The break signal causes an interrupt on the MCU and execution returns to the ICE.