MicroTrainer

ARM Trainer

This is a minimal monitor and and hardware stack for a small microcontroller trainer board, that was used to teach a Computer Organization class to students from ages 11-14.

Microprocessor trainers were a commonplace tool used to learn computing in the 1970s and early 1980s. They replicated the console found on minicomputers, and allowed engineers to gain experience writing programs in machine code.

The Computer Organization class that I taught was a "how computers work" class with minimal use of conventional computers in the classroom. We focused initially on gates, building from there to understand simple combinatorial and sequential circuits, and eventually ALUs, register files, and microprocessor subsystems. This culminated in a study of ARM-Thumb machine code.

ARM-Thumb machine code was a fairly ideal architecture for students to learn, because:

• It is a real architecture, in real use.
• It has a reasonable number of general purpose registers.
• A fixed-length, relatively simple encoding avoids unnecessary complexity.

Repository Layout

• src/ and inc/ contain the trainer firmware.
• hw/ contains the KiCad hardware designs and manufacturing outputs.
• arm-manual/ contains the student manual sources.
• examples/ contains programs captured from trainers during class.
• tools/ contains host-side conversion, disassembly, capture, and printing tools.

Run make tools to build the host-side utilities in build/tools/. The classroom scripts should be run from any directory; they locate the repository and examples automatically.

Example Program

By a student, Robin K:

20000000	DF20		(clear screen)
20000002	2001		(mov r0, #1)
20000004	2102		(mov r1, #1)
20000006	180A		(add r2, r0, r1)
20000008	DF21		(print contents of register 0)
2000000A	4608		(mov r0, r1)
2000000C	4611		(mov r1, r2)
2000000E	E7FA		(b 20000006)

TODO

• Students have complained that some invalid instructions-- BBAD, EF78-- seem to lock up the machine monitor. I think I removed some safeguard code that I didn't think was necessary, but I need to inspect that again.
• Input calls do not allow the user to enter the number "0".
• The manual should be updated with some of the system calls that have not been documented
  • Branch calculation formulae are wrong in the manual.
  • In particular, the input calls.
  • There may be a typo in the subtract register instruction.
  • The display icon and screen coordinate system could be more clearly explained
  • Need to update manual for new key shortcuts
• Overall curriculum, etc, should get posted here.