Microsoft KB Archive/31810

= Writing Faster Macro Assembler Programs =

Article ID: 31810

Article Last Modified on 11/19/2003

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APPLIES TO


 * Microsoft Macro Assembler 4.0
 * Microsoft Macro Assembler 5.0
 * Microsoft Macro Assembler 5.1 Standard Edition
 * Microsoft Macro Assembler 6.0 Standard Edition

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This article was previously published under Q31810



SUMMARY
The Microsoft Macro Assembler is useful for writing fast programs. For example, the following is a fast method to take the absolute value 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 positive notneg:        ; jump to here if positive



MORE INFORMATION
This method achieves part of its speed by avoiding the use of a jump instruction to keep the 8086's pre-fetch queue full.

To save time while a program is running, the 8086 tries to fetch the next instruction from memory while it is processing the current instruction. However, a jump instruction moves the location of the next instruction to fetch, making invalid the instruction that the 8086 just fetched into its pre-fetch queue.

This process forces the 8086 to spend time fetching the correct instruction from memory after the jump. Whenever possible, avoid jumps to increase the execution speed of Macro Assembler programs.

Additional query words: 4.00 5.00 5.10 6.00

Keywords: KB31810

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