Assembly之Operation Code

【Assembly之Operation Code】

　 The operation code field of an assembly language statement identifies the statement as a machine instruction, an assembler directive, or a macro defined by the programmer:

• A machine instruction is indicated by an instruction mnemonic. An assembly language statement that contains an instruction mnemonic is intended to produce a single executable machine instruction. The operation and use of each instruction is described in the manufacturer’s user manual.

• An assembler directive (or pseudo-op) performs some function during the assembly process. It doesn’t produce any executable code, but it may assign space for data in the program.

• Macros are defined with the .macro directive.

One or more spaces or tabs must separate the operation code field from the following operand field in a statement. Spaces or tabs are optional between the label and operation code fields, but they help to improve the readability of the program.