This article was previously published under Q31810
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The Microsoft Macro Assembler is useful for writing fast programs.For example, the following is a fast method to take the absolutevalue of a number held in the AX register:
cwd ; replicate the high bit into DX xor ax, dx ; take 1's complement if negative; no change if positive sub ax, dx ; AX is 2's complement if it was negative The standard : absolute value method works on any register but is much ; slower: or bx, bx ; see if number is negative jge notneg ; if it is negative... neg bx ; ...make it positivenotneg: ; jump to here if positive
This method achieves part of its speed by avoiding the use of ajump instruction to keep the 8086's pre-fetch queue full.
To save time while a program is running, the 8086 tries to fetch thenext instruction from memory while it is processing the currentinstruction. However, a jump instruction moves the location of thenext instruction to fetch, making invalid the instruction that the8086 just fetched into its pre-fetch queue.
This process forces the 8086 to spend time fetching the correctinstruction from memory after the jump. Whenever possible, avoidjumps to increase the execution speed of Macro Assembler programs.