汇编语言学习笔记（十九） -- Logical Operations

stanleylei

Boolean logic
When working with binary numbers, it is handy to have the standard Boolean logic functions available. The following Boolean logic operations are provided:

• AND
• NOT
• OR
• XOR

The AND, OR, and XOR instructions use the same format:
and source, destination
where source can be an 8-, 16-, or 32-bit immediate value, register, or value in memory, and destination can be an 8-, 16-, or 32-bit register or value in memory (but as usual, you cannot use memory values for both the source and destination). The NOT instruction uses a single operand, which is both the source value and location of the destination result.
The Boolean logic functions perform bit-wise operations on the source and destination. That is, each bit of the data elements is compared individually in order, using the logic function specified.
The most efficient way to clear out a register is to exclusive OR the register with itself using the XOR instruction. Each bit that was set to 1 when XOR’d with itself becomes 0, and each bit that was set to 0 when XOR’d with itself also becomes 0.
This ensures that all of the bits in the register will be set to 0, faster than what it would take to load the immediate value of 0 using the MOV instruction.
Bit testing
The TEST instruction performs a bit-wise logical AND between two 8-, 16-, or 32-bit values, and sets the sign, zero, and parity flags accordingly, without modifying the destination value.
The format of the TEST instruction is the same as for the AND instruction. Even though no data is written to the destination location, you still must specify any immediate values as the source value. This is similar to how the CMP instruction works like the SUB instruction, but it does not store the result anywhere.
                               
               
               
               
               

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