Structuring large code

The purge from day 136 has grown: the display now turns red in the fault state, and a second room is coming soon. At some point every bigger chip hits the limits – 128 lines, 90 characters per line and above all 4096 characters. With German comments the character limit usually fills up first. And long before that, you no longer find your own bugs in a chip without structure.

A fixed order#

If you build like this, you know where to look for every fault: wrong reading → section 1, wrong state → section 2, device does nothing → section 3.

The register plan#

16 registers sound like a lot but are handed out quickly. Decide in the header what each one is for and stick to it:

# Registers: r0-r2 state and counter
#            r3-r7 main program
#            r8 only for the subroutine show
alias state r0
alias ticks r2
alias colour r8

The most important rule: a subroutine only uses its own registers. If it calculates with tmp while the caller still needs a value in tmp, that value is gone after j ra – without an error message, and the bug shows up somewhere else entirely.

Cutting subroutines sensibly#

A subroutine costs at least two extra lines: jal and j ra. It pays off when

• the same code is needed in several places, for example “everything off” for every fault,
• a self-contained part clutters the main loop, such as the display with number and colour.

A subroutine for a single line never pays off. And one that calls jal itself needs push ra and pop ra.

jal show               # status and colour
# ... rest of the output ...
j start
show:
seq colour state FAULT
select colour colour RED GREEN
sbn DISPLAY HASH("Status") Color colour
j ra

Saving space#

• Keep comments short. One per section and one behind every define is enough. Every character counts, spaces too.
• Write outputs only once. One sbn per device in the output instead of one in every state block.
• select instead of jumps. select next tmp FULL next needs no label and no return jump.
• Leave out empty lines. Each one counts as a line.