ti84: offset binary by 0x100 to free space for TI-specific stuff

Not yet tried on real hardware, but we should be getting pretty
close...

recipes/ti84/README.md

@@ -8,11 +8,24 @@ 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)
+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).
+
+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.
 
 ## Recipe
 
-This recipe gets the Collapse OS BASIC shell to run on the TI-84+, using its LCD
+This recipe gets the Collapse OS interpreter to run on the TI-84+, using its LCD
 screen as output and its builtin keyboard as input.
 
 ## Gathering parts
@@ -25,7 +38,7 @@ screen as output and its builtin keyboard as input.
 
 ## Build the ROM
 
-Running `make` will result in `os.rom` being created.
+Running `make` will result in `stage1.rom` being created.
 
 ## Emulate
 
@@ -38,6 +51,30 @@ 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](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.
+
+### Instructions
+
 **WARNING: the instructions below will wipe all the contents of your calculator,
 including TI-OS.**
 

recipes/ti84/xcomp.fs

@@ -4,6 +4,7 @@ RAMSTART 0x70 + CONSTANT LCD_MEM
 RAMSTART 0x72 + CONSTANT KBD_MEM
 0x01 CONSTANT KBD_PORT
 212 LOAD  ( z80 assembler )
+: ZFILL, ( u ) 0 DO 0 A, LOOP ;
 262 LOAD  ( xcomp )
 522 LOAD  ( font compiler )
 : CODE XCODE ;
@@ -12,8 +13,40 @@ RAMSTART 0x72 + CONSTANT KBD_MEM
 : CREATE XCREATE ; ( for KBD tbls )
 : : [ ' X: , ] ;
 
+( TI-84+ requires specific code at specific offsets which
+  come in conflict with Collapse OS' stable ABI. We thus
+  offset the binary by 0x100, which is our minimum possible
+  increment and fill the TI stuff with the code below. )
+
+0x100 JPnn, 0x15 ZFILL, ( 0x18 )
+0x100 JPnn, ( reboot ) 0x1d ZFILL, ( 0x38 )
+( handleInterrupt )
+DI,
+AF PUSHqq,
+    ( did we push the ON button? )
+    0x04 ( PORT_INT_TRIG ) INAn,
+    0 ( INT_TRIG_ON ) A BITbr,
+    IFNZ,
+        ( yes? acknowledge and boot )
+        0x03 ( PORT_INT_MASK ) INAn,
+        0x00 ( INT_MASK_ON ) A RESbr, ( ack interrupt )
+        0x03 ( PORT_INT_MASK ) OUTnA,
+        AF POPqq,
+        EI,
+        0x100 JPnn,
+    THEN,
+AF POPqq,
+EI,
+RETI,
+
+0x03 ZFILL, ( 0x53 )
+0x100 JPnn, ( 0x56 ) 0xff A, 0xa5 A, 0xff A,
+0xa7 ZFILL, ( 0x100 )
+( All set, carry on! )
+
 CURRENT @ XCURRENT !
 
+0x100 BIN( !
 282 LOAD  ( boot.z80 )
 393 LOAD  ( icore low )
 555 557 LOADR ( LCD low )
@@ -27,5 +60,5 @@ PC ORG @ 8 + !
 558 560 XPACKR ( LCD high )
 438 451 XPACKR ( print fmt readln )
 ," : _ LCD$ KBD$ (ok) RDLN$ ; _ "
-ORG @ 256 /MOD 2 PC! 2 PC!
+ORG @ 0x100 - 256 /MOD 2 PC! 2 PC!
 H@ 256 /MOD 2 PC! 2 PC!