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Convert notes into blockquotes

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James Gregory 2013-12-31 11:06:31 +11:00
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07-05.md
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@ -64,9 +64,13 @@ code snippet in Listing 7.4.
BIT_PATTERN=BIT_PATTERN SHL 1
ENDM
------------------- ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
![](images/i.jpg) *Besides illustrating the advantages of local optimization, this example also shows that it generally pays to precalculate results; this is often done at or before assembly time, but precalculated tables can also be built at run time. This is merely one aspect of a fundamental optimization rule: Move as much work as possible out of your critical code by whatever means necessary.*
------------------- ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
> ![](images/i.jpg)
> Besides illustrating the advantages of local optimization, this example
> also shows that it generally pays to precalculate results; this is often
> done at or before assembly time, but precalculated tables can also be
> built at run time. This is merely one aspect of a fundamental
> optimization rule: Move as much work as possible out of your critical
> code by whatever means necessary.
#### NOT Flips Bits—Not Flags {#Heading9}
@ -84,9 +88,11 @@ flags.) NOT is often used for tasks, such as flipping masks, where
there's no reason to test the state of the result, and in that context
it can be handy to keep the flags unmodified for later testing.
------------------- ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
![](images/i.jpg) *Besides, if you want to **NOT** an operand and set the flags in the process, you can just **XOR** it with -1. Put another way, the only functional difference between **NOT AX** and **XOR AX,0FFFFH** is that **XOR** modifies the flags and **NOT** doesn't.*
------------------- ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
> ![](images/i.jpg)
> Besides, if you want to **NOT** an operand and set the flags in the
> process, you can just **XOR** it with -1. Put another way, the only
> functional difference between **NOT AX** and **XOR AX,0FFFFH** is that
> **XOR** modifies the flags and **NOT** doesn't.
The x86 instruction set offers many ways to accomplish almost any task.
Understanding the subtle distinctions between the instructions—whether
@ -152,9 +158,11 @@ altering the arithmetic flags is a common characteristic of program
control instructions (as opposed to arithmetic and logical instructions
like **SUB** and **AND,** which do alter the flags).
------------------- ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
![](images/i.jpg) *The rule is not that the arithmetic flags change whenever the CPU performs a calculation; rather, the flags change whenever you execute an arithmetic, logical, or flag control (such as **CLC** to clear the Carry flag) instruction.*
------------------- ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
> ![](images/i.jpg)
> The rule is not that the arithmetic flags change whenever the CPU
> performs a calculation; rather, the flags change whenever you execute an
> arithmetic, logical, or flag control (such as **CLC** to clear the Carry
> flag) instruction.
Not only do **LOOP** and **JCXZ** not alter the flags, but **REP MOVS**,
which counts down CX to 0, doesn't affect the flags either.