The two-instruction sequence is: movb tty+3(r1),0f / put clist id in sleep argument jsr r0,sleep; 0:.. The "0:" on the second line is a label and it is referenced by the "0f" in the first line. The first line is putting a value into the argument being passed to the sleep subroutine. Self-modifying code. The ".." assembles to a 0. With the incorrect code, "0" assembled to a 0 and ".." assembled to a 0, so there was one extra word of zeroes, and the return from the sleep would have executed it (halt) instead of the "br 1b" on the next line. James Markevitch