added the TI-74 BASIC port and the mu808asm'MU74 export support

README.md

@@ -303,7 +303,10 @@ Reference implementations
 * [Python 3](mu808asm.py): supports assembling MU8A source files into both
   plain text (MU8) and binary (MU8B) machine code formats. Also supports
   disassembling MU8 and MU8B files into their MU8A sources and converting
-  machine code between MU8 and MU8B formats.
+  machine code between MU8 and MU8B formats. Additionally, this assembler
+  implementation provides the `t74` mode to convert plaintext MU8 machine code
+  into the format suitable for entering into TI-74 BASIC for running on the
+  TI-74 port of mu808 (see below).
 
 These lists are going to be expanded as soon as new reference implementations
 appear.
@@ -331,6 +334,59 @@ keys of the standard phone keypad, the following convention is recommended:
 * the `#` (pound) key actually confirms entering a single number field, and
   all instructions are required to contain exactly five numbers.
 
+Retro/minimal platform implementations
+--------------------------------------
+Here's the list of known/existing mu808 implementations that adhere to the
+specification to most extent and run compatible machine code but cannot 
+provide full operating environment functionality due to the limits of the
+platform itself.
+
+### TI-74 BASIC
+
+The [mu808.b74](mu808.b74) file contains a BASIC port of mu808 for the Texas
+Instruments TI-74 portable computer (tested on the TI-74S variant). Due to
+the resource constraints, the following limitations apply:
+
+* only 149 program steps and 256 data cells supported (adjustable at line 110),
+* no interactive mode (the `RUN` command directly executes the predefined
+  program in the VM),
+* the program itself is entered into the DATA statements in the so-called
+  MU74 format (see below),
+* no support for I/O port 2, tracing or intruction dumping,
+* no boundary checks for the addresses inside the program.
+
+Also, since the VM runs on top of a BASIC interpreter, program execution is
+extremely slow most of the time. Keep in mind, this is more of a proof of
+concept than a viable solution, and using the "native" TI BASIC is preferred
+for any serious computing on that machine.
+
+In order to store your programs on the device to run via this mu808 VM version,
+you need to enter them in DATA statements in two-number pairs starting from
+`DATA 0,0` at the BASIC line 5000. The last `DATA` line of the program must be
+`DATA -1,0`. In between these two, every program instruction `lno cmd a1 a2 a3`
+must be compressed into the form:
+```
+[5000+lno] DATA [cmd*10000 + a1], [a2 * 10000 + a3]
+```
+This storage format is called MU74. For example, the compound interest
+calculator from the official mu808 examples will transform into:
+```
+5000 DATA 0,0
+5001 DATA 50100,1
+5002 DATA 40002,30000
+5003 DATA 90002,10004
+5004 DATA 50001,5
+5005 DATA 70005,40005
+5006 DATA 130001,50005
+5007 DATA 70000,30005
+5008 DATA 130000,50005
+5009 DATA 30005,50000
+5010 DATA -1,0
+```
+To make the conversion easier, the official mu808 assembler, `mu808asm.py`, now
+supports the `t74` mode that accepts a plain MU8 machine code file and outputs
+the same program in the MU74 format.
+
 FAQ
 ---