Assembly: Exact Timing Loops

Counting cycles on purpose.

Each instruction takes a known number of clock cycles, so a loop can produce a precise delay. Total time equals cycles divided by the clock frequency.

At 1 MHz one cycle is 1 microsecond. A loop that takes 3 cycles per pass and runs 100 times gives 300 microseconds, plus a few cycles of setup.

Interrupts stretch the delay, so disable them or use a hardware timer when accuracy matters.

; generic teaching syntax: assume LOAD=1 cycle, SUB=1, JNZ=2 when taken
        LOAD  R1, 100      ; 1 cycle
w:      SUB   R1, 1        ; 1 cycle
        JNZ   w            ; 2 cycles taken (1 on the last pass)
; about 3 x 100 cycles in total