doc: add TI83+/TI84+ doc page

doc/README.md

@@ -13,3 +13,11 @@
 * [Using block devices](blockdev.md)
 * [Using the filesystem](fs.md)
 * [Assembling z80 source from the shell](zasm.md)
+
+## Hardware
+
+Some consolidated documentation about various hardware supported by Collapse OS.
+Most of that information can already be found elsewhere, but the goal is to have
+the most vital documentation in one single place.
+
+* [TI-83+/TI-84+](ti8x.md)

doc/ti8x.md

@@ -0,0 +1,38 @@
+# TI-83+/TI-84+
+
+Texas Instruments is well known for its calculators. Among those, two models
+are particularly interesting to us because they have a z80 CPU: the TI-83+ and
+TI-84+ (the "+" is important).
+
+They lack accessible I/O ports, but they have plenty of flash and RAM. Collapse
+OS runs on it (see `recipes/ti84`).
+
+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](http://wikiti.brandonw.net/index.php).
+
+## Getting software on it
+
+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 `recipes/ti84`. With the help of the
+[mktiupgrade](https://github.com/KnightOS/mktiupgrade), an upgrade file can be
+prepared and then sent through the USB port with the help of
+[tilp](http://lpg.ticalc.org/prj_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.

recipes/ti84/README.md

@@ -8,6 +8,8 @@ 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.
 
+[Further reading](../../doc/ti8x.md)
+
 ## Recipe
 
 This recipe gets the Collapse OS BASIC shell to run on the TI-84+, using its LCD