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90 lines
3.1 KiB
Plaintext
90 lines
3.1 KiB
Plaintext
# Sega Master System
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The Sega Master System was a popular gaming console running on
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z80. It has a simple, solid design and, most interestingly of
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all, its even more popular successor, the Megadrive (Genesis)
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had a z80 system for compatibility!
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This makes this platform *very* scavenge-friendly and worth
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working on.
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SMS Power[1] is an awesome technical resource to develop for
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this platform and this is where most of my information comes
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from.
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This platform is tight on RAM. It has 8k of it. However, if you
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have extra RAM, you can put it on your cartridge.
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# Gathering parts
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* A Sega Master System or a MegaDrive (Genesis).
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* A Megadrive D-pad controller.
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* A way to get an arbitrary ROM to run on the SMS. Either
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through a writable ROM cartridge or an Everdrive[2].
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# Hacking up a ROM cart
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SMS Power has instructions to transform a ROM cartrige into a
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battery-backed SRAM one, which allows you to write to it through
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another device you'll have to build. This is all well and good,
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but if you happen to have an AT28 EEPROM, things are much
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simpler!
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Because AT28 EEPROM are SRAM compatible, they are an
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almost-drop-in replacement to the ROM you'll pop off your
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cartridge. AT28 are a bit expensive, but they're so handy! For
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SMS-related stuff, I recommend the 32K version instead of the 8K
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one because fitting Collapse OS with fonts in 8K is really
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tight.
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The ROM cartridge follow regular ROM pinout, which means that
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A14 are just under VCC, where WE is on the AT28. We need WE to
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be perma-disabled and A14 to be properly connected.
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1. De-solder the ROM
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2. Take a 28 pins IC socket
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3. Cut off its WE pin (the one just under VCC), leaving a tiny
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bit of metal.
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4. Hard-wire it to VCC so that WE is never enabled.
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5. Solder your socket where the ROM was.
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6. With a cutter, cut the trace leading to A14.
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7. Wire A14 to the trace just under WE (which doesn't actually
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touch WE because we've cut the IC socket's pin).
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8. Insert Collapse OS-filled EEPROM in socket.
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As simple as this! (Note that this has only been tested on a SMS
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so far. I haven't explored whether this can run on a megadrive).
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# Build the ROM
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Running "make" in /arch/z80/sms will produce a "os.sms"
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ROM that can be put as is on a SD card to the everdrive or
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flashed as is on a writable ROM cart. Then, just run the thing!
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To run Collapse OS in a SMS emulator, run "make emul".
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# Usage
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Our input is a D-Pad and our output is a TV. The screen is 32x28
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characters. A bit tight, but usable.
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D-Pad is used as follow:
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* There's always an active cursor. On boot, it shows "a".
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* Up/Down increase/decrease the value of the cursor.
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* Left/Right does the same, by increments of 5.
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* A button is backspace.
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* B button skips cursor to next "class" (number, lowcase,
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upcase, symbols).
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* C button "enters" cursor character and advance the cursor by
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one.
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* Start button is like pressing Return.
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Of course, that's not a fun way to enter text, but using the
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D-Pad is the easiest way to get started which doesn't require
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soldering. Your next step after that would be to build a PS/2
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keyboard adapter! See smsps2.txt
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[1]: http://www.smspower.org
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[2]: https://krikzz.com
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