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cpy a d ; set a = d = a+2538, call this p
cpy 9 c ; LOOP 9 times, i.e. set d = d+2538, b = c = 0
cpy 282 b ; set d = d + 282, b = 0
inc d ; LOOP set d = d + 282, b = 0
dec b
jnz b -2 ; END LOOP
dec c
jnz c -5 ; END LOOP; note that d never changes hereafter
cpy d a ; LOOP: set a = d; note: nothing below branches above this line, plus this is an infinite loop
jnz 0 0 ; LOOP; each iteration uses a to set a and b
cpy a b ; set b = a
cpy 0 a ; set a = 0
cpy 2 c ; LOOP; c is 2 on entry; purpose is to set a and b
jnz b 2 ; X:
jnz 1 6
dec b
dec c
jnz c -4 ; if c!=0 GOTO X
inc a ; a++
jnz 1 -7 ; END LOOP; on exiting this loop, c is is either 1 or 2 (it starts the loop at 2, and gets decremented inside the loop, and if it gets decremented to 0 the loop will iterate again and start by resetting it to 2; so the question is, under what conditions will c be 1, and when will it be 2?
cpy 2 b ; set b = 2 - c; I'm guessing b will always be 0 or 1, which would mean c is 1 or 2
jnz c 2 ; LOOP; set b = b - c, c = 0
jnz 1 4
dec b
dec c
jnz 1 -4 ; END LOOP
jnz 0 0 ; nop
out b ; output b, the last thing we do at the bottom of this infinite loop
jnz a -19 ; END LOOP; if a has become 0, start the loop by resetting it to d (which, recall, is original a plus 2538)
jnz 1 -21 ; END LOOP; this is an infinite loop