Legacy apps in Windows/OS X was Re: Old MS-DOS & Win Software

[Jim Leonard]

woodelf wrote:
> I thought the pre-fetch queue was longer than that. Still most 
> instructions  I bet are 3 to 4 bytes long when you

4 measly bytes: Hey, better to have 4 bytes than none at all.  As for 3-4 bytes 
long, remember, the Bus Interface Unit can be fetching at the same time an 
instruction is executing, so that's where you go break out the Zen timer. 
There are gray areas, such as:

Which is faster on 8088?  A or B?

A: SHL AX,1       B: MOV CL,3
    SHL AX,1          SHL AX,CL
    SHL AX,1

A is faster.  Now add another shift:

A: SHL AX,1       B: MOV CL,4
    SHL AX,1          SHL AX,CL
    SHL AX,1
    SHL AX,1

B (and all subsequent increases in operand) is faster.  So that's the 10% I was 
mentioning where you just have to time it to find out whether or not the BIU or 
EU won the race.  Same goes for multiple INCs vs. ADD reg,immediate vs. ADD 
reg,reg vs. ADD accum.,reg (ADDing with AX/AL is smaller/faster than with other 
registers)
-- 
Jim Leonard (trixter at oldskool.org)                    http://www.oldskool.org/
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