Daniel,

I'm a novice. Confirm this information I give you.

The 8051 instruction

JB P3.0, START

does the same as

START: ;CHECK TO SEE IF P3, PIN 0 IS ON

MOV A, P3 ;
ORL A, #00000001B
MOV P3, A
MOV A, P3
ANL A, #00000001B
;IF ON, THEN CONTINUE. IF NOT, LOOP TO START.
JZ START

(Am I right you all?)