Category:8086 Assembly: Difference between revisions

Another frequently used looping construct is <code>REP</code>. <code>REP</code> can be combined with certain instructions to repeat that instruction until <code>CX</code> equals zero. However, unlike <code>LOOP</code>, which can be used to repeat a block of instructions, <code>REP</code> can only repeat one. It doesn't work on all instructions, only the "string" instructions which operate on a block of memory. Typically these include <code>MOVSB</code>, <code>LODSB</code>, <code>STOSB</code>, <code>CMPSB</code>, and <code>SCASB</code> (each has a variant that ends in W instead of B, for 16-bit data.) There are also <code>REPZ</code> and <code>REPNZ</code>, which stand for "Repeat untilif Zero" and "Repeat untilif Nonzero" respectively. These two only work properly with <code>CMPSB</code> and <code>SCASB</code>, as <code>MOVSB</code>, <code>LODSB</code>, <code>STOSB</code> do not affect the flags. (The CPU doesn't detect whether <code>CX</code> equals zero using the flags, as the flags never reflect this equality to zero like you would expect.)