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Currently I am doing this to test for a negative number in x86 assembly (r/m32,imm8):
83F800 CMP EAX, 0
This can be followed by JL. This is 3 bytes and supposedly clocks at "1/2". I could use TEST EAX, or CMP EAX,imm32 (encoding 3D), both of which clock at "1", but take 5 bytes. In general, if I am trying to minimize code size, is the way I am doing it correct? Once again, this is to test if a number is less than zero.
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Negative numbers are represented in Two's Complement in assembly. In order to obtain Two's Complement of a number you have two options: to complement all it's bits and add one. to complement all it's bits until the last 1.
The lea instruction places the address specified by its first operand into the register specified by its second operand. Note, the contents of the memory location are not loaded, only the effective address is computed and placed into the register.
In the x86 assembly language, the TEST instruction performs a bitwise AND on two operands. The flags SF , ZF , PF are modified while the result of the AND is discarded. The OF and CF flags are set to 0 , while AF flag is undefined.
add eax, eax is only two bytes (01 C0 or 03 C0), but destructive. (check for carry afterwards)
test eax, eax is also only two bytes (85 C0). (check for sign afterwards)
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You can use:
or eax, eax
it is one byte shorter (only two bytes op code) instruction!
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