Z80-MBC2

The Z80-MBC2[1] combines a Z80 and an ATMEGA32A to provide a
CP/m capable computing environment. It features a SD card
bootloader which makes running Collapse OS on it rather simple.

In this recipe, we're going to run Collapse OS on the Z80-MBC2,
interfacing through its serial port. We're going to use the
MBC's API to implement BLK on the SD card.

Gathering parts

* A Z80-MBC2 computer with its SD card module and a properly
  flashed "IOS" on the ATMega32A.
* A FTDI-to-TTL cable to connect to the serial port.

Building the binary

Running "make" in arch/z80/z80mbc2 will yield "os.bin" which is
what we want.

Running on the Z80-MBC2

Mount the SD card on your modern computer and copy "os.bin" as
"autoboot.bin", overwriting the binary that was previously
there.

We also have to copy the blkfs over. This is done by using IOS'
drive system.  Each "DSxNyy.DSK" file on the card is a drive,
each drive has 512 track of 32 sectors of 512 bytes, so one
drive is plenty for our needs. Collapse OS hardcodes drive 0.

Each drive is part of a set. IOS theoretically supports up to 10
sets, but the binary shipped by default only accepts 4. You have
to overwrite an existing set.  I used set 3. So, copy "blkfs" to
file "DS3N00.DSK". If you want, you can change the name of the
set by changing the contents of "DS3NAM.DAT".

Put back the SD card in the Z80-MBC2 and power it up by
connecting the FTDI adapter to it (red: VCC, black: GND, green:
TX, white: RX).

The FTDI adapter will show up as something like "ttyUSB0" (or
"ttyU0" on OpenBSD). Connect to it with "screen" or "cu" or
whatever you like. Baud rate of the Z80-MBC2 appears to be
hardcoded to 115200.

Then, enable IOS program selection by holding RESET and USER at
the same time, wait 2 seconds, releasing RESET, wait 2 seconds,
releasing USER. You should then be given a 1-8 choice.

You begin by selecting the proper disk set, which is through
choice 8, then you select the Autoboot binary through choice 4.

You are now in Collapse OS.

[1]: https://hackaday.io/project/159973-z80-mbc2-a-4-ics-homebrew-z80-computer