TI-84+

The TI-84+ is a machine with many advantages, one being that
it's very popular.  It also has a lot of flash memory and RAM.

Its builtin keyboard and screen, however, are hard to use,
especially the screen. With a tiny font, the best we can get is
a 24x10 console.

There is, however, a built-in USB controller that might prove
very handy.

I haven't opened one up yet, but apparently, they have limited
scavenging value because its z80 CPU is packaged in a
TI-specific chip. Due to its sturdy design, and its ample RAM
and flash, we could imagine it becoming a valuable piece of
equipment if found intact.

The best pre-collapse ressource about it is WikiTI[1].

As it is now, with its tiny screen and cumbersome keyboard,
Collapse OS is not really usable on the TI-84+. One could
imagine a scenario where one has a terminal and uses the TI-84+
through the link for its large amount of flash and RAM. But
using it standalone? Nah, not even post-collapse.

Therefore, this recipe is more of a "look, I run!" demo.

Gathering parts

* A TI-84+
* A USB cable
* tilp[2]
* mktiupgrade[3]

Build the ROM

Running "make" in /arc/z80/ti84 will result in "os.rom" being
created.

Emulate

Collapse OS has a builtin TI-84+ emulator using XCB for display
in emul/hw/ti. You can invoke it with "make emul".

You will start with a blank screen, it's normal, you haven't
pressed the "ON" key yet. This key is mapped to tilde (~) in the
emulator. Once you press it, the Collapse OS prompt will appear.
See emul/hw/ti/README.md for details.

Upload to the calculator

Background notes

Getting software to run on it is a bit tricky because it needs
to be signed with TI-issued private keys. Those keys have long
been found and are included in keys/. With the help of the
mktiupgrade, an upgrade file can be prepared and then sent
through the USB port with the help of tilp.

That, however, requires a modern computing environment. As of
now, there is no way of installing Collapse OS on a TI-8X+
calculator from another Collapse OS system.

Because it is not on the roadmap to implement complex
cryptography in Collapse OS, the plan is to build a series of
pre-signed bootloader images. The bootloader would then receive
data through either the Link jack or the USB port and write that
to flash (I haven't verified that yet, but I hope that data
written to flash this way isn't verified cryptographically by
the calculator).

As modern computing fades away, those pre-signed binaries would
become opaque, but at least, would allow bootstrapping from
post-modern computers.

Instructions

WARNING: the instructions below will wipe all the contents of
your calculator, including TI-OS.

To send your ROM to the calculator, you'll need two more tools:
mktiupgrade and tilp.

Once you have them, you need to place your calculator in
"bootloader mode", that is, in a mode where it's ready to
receive a new binary from its USB cable.  To do that you need
to:

1. Shut down the calculator by removing one of the battery.
2. Hold the DEL key
3. But the battery back.
4. A "Waiting... Please install operating system now" message\
   will appear.

Once this is done, you can plug the USB cable in your computer
and run "make send". This will create an "upgrade file" with
mktiupgrade and then push that upgrade file with tilp. tilp will
prompt you at some point.  Press "1" to continue.

When this is done, you can press the ON button to see Collapse
OS' prompt!

Validation errors

Sometimes, when uploading an upgrade file to your calculator,
you'll get a validation error. You can always try again, but in
my own experience, some specific binaries will simply always be
refused by the calculator. Adding random "nop" or reordering
lines (when it makes sense, of course) should fix the problem.

I'm not sure whether it's a bug with the calculator or with
mktiupgrade.

Usage

The shell works like a normal BASIC shell, but with very tight
screen space.

When pressing a "normal" key, it spits the symbol associated to
it depending on the current mode. In normal mode, it spits the
digit/symbol. In Alpha mode, it spits the letter. In Alpha+2nd,
it spits the uppercase letter.

Special keys are Alpha and 2nd. Pressing them toggles the
associated mode.  Alpha and 2nd mode don't persist for more than
one character. After the character is spit, mode reset to
normal.

Pressing 2nd then Alpha will toggle the A-Lock mode, which is a
persistent mode.  The A-Lock mode makes Alpha enabled all the
time. While A-Lock mode is enabled, you have to enable Alpha to
spit a digit/symbol.

Simultaneous keypresses have undefined behavior. One of the keys
will be registered as pressed. Mode key don't work by
simultaneously pressing them with a "normal" key. The presses
must be sequential.

Keys that aren't a digit, a letter, a symbol that is part of
7-bit ASCII or one of the two mode key have no effect.

[1]: http://wikiti.brandonw.net/index.php
[2]: http://lpg.ticalc.org/prj_tilp/
[3]: https://github.com/KnightOS/mktiupgrade