ID Number: Q31810
4.00
MS-DOS
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 the 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.