Begin repeat loop.
.REPEAT
stmtlist
.UNTILexpression
The stmtlist is executed repeatedly until expression is true. When expression becomes true, advance to the next instruction following .UNTIL.
The stmtlist is executed at least once, even if expression is true upon entry to the .REPEAT loop.
.REPEAT
...UNTIL x:(r1)+ <EQ> #0 ; loop until zero is found