Q.What is an addressing mode?

An addressing mode is the way an instruction tells the CPU where to find the data it must act on. Because an operand can live in a register, in memory or be supplied as a constant, the 8051 provides five addressing modes; the first four are the ones used most often. In every instruction the destination is written first, then the source, and a MOV only copies — it never alters the source.

1. Register addressing uses the CPU registers A and R0-R7 directly, e.g. MOV A,R0 copies R0 into A and MOV R5,A saves A into R5. The register is selected by the lowest three bits of the opcode.

2. Direct addressing names an 8-bit memory address. The lower 128 bytes of RAM occupy 00H-7FH and the Special Function Registers occupy 80H-FFH, so MOV A,45H reaches location 45H. Register banks share these addresses — R5 of bank 2, for example, is direct address 15H. …