inc and dec update an initialized byte variable. The one-argument forms
add or subtract one:
inc(Counter);
dec(Counter);
An optional byte expression specifies the amount:
inc(Counter, Step);
dec(Counter, Step + $01);
Updates wrap modulo 256, like other byte arithmetic. Incrementing $FF
produces $00; decrementing $00 produces $FF. The one-argument forms
generate the 6502 INC and DEC instructions directly.
The target must already be assigned because an update reads its previous
value. The target and optional amount must have type byte; incompatible uses
produce E4004.
An initialized byte value parameter may be used as the target. The update
changes only the procedure’s local parameter copy.
A for loop’s control variable cannot be changed with inc or dec inside
that loop body. See for loops.