Sega Master System

The Sega Master System was a popular gaming console running on
z80. It has a simple, solid design and, most interestingly of
all, its even more popular successor, the Megadrive (Genesis)
had a z80 system for compatibility!

This makes this platform *very* scavenge-friendly and worth
working on.

SMS Power[1] is an awesome technical resource to develop for
this platform and this is where most of my information comes
from.

This platform is tight on RAM. It has 8k of it. However, if you
have extra RAM, you can put it on your cartridge.

Gathering parts

* A Sega Master System or a MegaDrive (Genesis).
* A Megadrive D-pad controller.
* A way to get an arbitrary ROM to run on the SMS. Either
  through a writable ROM cartridge or an Everdrive[2].

Hacking up a ROM cart

SMS Power has instructions to transform a ROM cartrige into a
battery-backed SRAM one, which allows you to write to it through
another device you'll have to build. This is all well and good,
but if you happen to have an AT28 EEPROM, things are much
simpler!

Because AT28 EEPROM are SRAM compatible, they are an
almost-drop-in replacement to the ROM you'll pop off your
cartridge. AT28 are a bit expensive, but they're so handy! For
SMS-related stuff, I recommend the 32K version instead of the 8K
one because fitting Collapse OS with fonts in 8K is really
tight.

The ROM cartridge follow regular ROM pinout, which means that
A14 are just under VCC, where WE is on the AT28. We need WE to
be perma-disabled and A14 to be properly connected.

1. De-solder the ROM
2. Take a 28 pins IC socket
3. Cut off its WE pin (the one just under VCC), leaving a tiny
   bit of metal.
4. Hard-wire it to VCC so that WE is never enabled.
5. Solder your socket where the ROM was.
6. With a cutter, cut the trace leading to A14.
7. Wire A14 to the trace just under WE (which doesn't actually
   touch WE because we've cut the IC socket's pin).
8. Insert Collapse OS-filled EEPROM in socket.

As simple as this! (Note that this has only been tested on a SMS
so far. I haven't explored whether this can run on a megadrive).

Build the ROM

Running "make" in /arch/z80/sms will produce a "os.sms"
ROM that can be put as is on a SD card to the everdrive or
flashed as is on a writable ROM cart. Then, just run the thing!

To run Collapse OS in a SMS emulator, run "make emul".

Usage

Our input is a D-Pad and our output is a TV. The screen is 32x28
characters. A bit tight, but usable.

D-Pad is used as follow:

* There's always an active cursor. On boot, it shows "a".
* Up/Down increase/decrease the value of the cursor.
* Left/Right does the same, by increments of 5.
* A button is backspace.
* B button skips cursor to next "class" (number, lowcase,
  upcase, symbols).
* C button "enters" cursor character and advance the cursor by
  one.
* Start button is like pressing Return.

Of course, that's not a fun way to enter text, but using the
D-Pad is the easiest way to get started which doesn't require
soldering. Your next step after that would be to build a PS/2
keyboard adapter! See smsps2.txt

[1]: http://www.smspower.org
[2]: https://krikzz.com