Vending machine brain

A snack machine sells one thing for 75 cents. It takes nickels (5), dimes (10), quarters (25), loonies (100) and toonies (200), and hands anything else straight back. Pulling the return lever, input 0, gives back whatever credit is in the machine.

The starter's main reads the inputs one at a time, calls coin(value) for each, and prints a summary at the end. Write coin. The machine's memory, the credit so far and the number of snacks sold, lives in the .bss words credit and sold, not in registers, so it lasts from one call to the next: coin loads what it needs, updates it, and stores it back.

Each input prints exactly one line:

• a value it does not take: rejected v, and the credit is unchanged
• the return lever: returned c, and the credit goes back to 0
• a coin that leaves the credit under 75: credit c
• a coin that brings the credit to 75 or more: vend! change c, where c is the credit minus 75; the credit goes back to 0 and sold goes up by 1

The starter's input prints:

credit 25
credit 35
credit 60
credit 70
vend! change 0
sold 1, holding 0

coin calls printf, so it needs a frame, and any of x19 to x28 it uses must be saved and restored.

What is checked

• the lines above
• the program exits with status 0
• the same checks on other inputs the checker keeps hidden: exactly 75 cents, a toonie, rejected values (a negative one too), the lever on an empty machine, and no input at all
• the state is written back to memory (strStore to memory. Same addressing modes as LDR.)