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James Gregory 2013-12-31 11:06:31 +11:00
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07-01.md
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@ -68,9 +68,11 @@ algorithm selection and good design are fundamental to performance. The
extra horsepower a superb assembly language implementation gives a
program is worth bothering with only in the context of a good design.
------------------- ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
![](images/i.jpg) *Assembly language optimization is a small but crucial corner of the PC programming world. Use it sparingly and only within the framework of a good design—but ignore it and you may find various portions of your anatomy out in the cold.*
------------------- ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
> ![](images/i.jpg)
> Assembly language optimization is a small but crucial corner of the PC
> programming world. Use it sparingly and only within the framework of a
> good design—but ignore it and you may find various portions of your
> anatomy out in the cold.
So, drawing fortitude from the knowledge that our quest is a pure and
worthy one, let's resume our exploration of assembly language
@ -96,9 +98,11 @@ instinctively stuffs the loop count in CX and reaches for **LOOP** when
setting up a loop. That's fine—**LOOP** does, of course, work as
advertised—but there is one problem:
------------------- ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
![](images/i.jpg) *On half of the processors in the x86 family, **LOOP** is slower than **DEC CX** followed by **JNZ**. (Granted, **DEC CX/JNZ** isn't precisely equivalent to **LOOP,** because **DEC** alters the flags and LOOP doesn't, but in most situations they're comparable.)*
------------------- ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
> ![](images/i.jpg)
> On half of the processors in the x86 family, **LOOP** is slower than
> **DEC CX** followed by **JNZ**. (Granted, **DEC CX/JNZ** isn't precisely
> equivalent to **LOOP,** because **DEC** alters the flags and LOOP
> doesn't, but in most situations they're comparable.)
How can this be? Don't ask me, ask Intel. On the 8088 and 80286,
**LOOP** is indeed faster than **DEC CX/JNZ** by a cycle, and **LOOP**
@ -116,6 +120,8 @@ Pentium as well. (By the way, all this is not just theory; I've timed
the relative performances of **LOOP** and **DEC CX/JNZ** on a cached
386, and LOOP really is slower.)
------------------- --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
![](images/i.jpg) *Things are stranger still for **LOOP**'s relative **JCXZ,** which branches if and only if CX is zero. **JCXZ** is faster than **AND CX,CX/JZ** on the 8088 and 80286, and equivalent on the 80386—but is about twice as slow on the 486!*
------------------- --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
> ![](images/i.jpg)
> Things are stranger still for **LOOP**'s relative **JCXZ,** which
> branches if and only if CX is zero. **JCXZ** is faster than **AND
> CX,CX/JZ** on the 8088 and 80286, and equivalent on the 80386—but is
> about twice as slow on the 486!