Q.Write mnemonic for the operation “save content of A in RAM location 45H”.

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. …