Describe the operation of each of the following immediate-data MOV instructions of the 8051. ```asm MOV A, #25

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Q. Describe the operation of each of the following immediate-data MOV instructions of the 8051. MOV A, #25H ; load 25H into A MOV R4, #62 ; load the decimal value 62 into R4 MOV DPTR, #4562H ; DPTR = 4562H MOV DPTR, #1235H ; put the immediate 16 bit number 1235H in register DPTR MOV R7, #06H ; put the immediate 8 bit number 06 in register R7

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Concept understanding — 8051 Addressing Modes

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.

3. Immediate addressing carries the data inside the instruction, marked with a # sign: MOV A,#22H loads the constant 22H, and MOV DPTR,#1246H loads a 16-bit constant.

4. Indirect addressing uses R0 or R1 as a pointer to the data, marked with @: MOV A,@R1 copies the RAM byte whose address is held in R1.

5. Base-register-plus-index addressing adds an index to a base register (as in MOVC A,@A+DPTR) to read look-up tables in code memory. Choosing the right mode is what makes 8051 programs both compact and fast.

[!TLDR]

The # sign marks immediate data, so each MOV simply copies the constant written after # directly into the register named as its destination.

Concept - immediate addressing. In the 8051 the source of the data can be made part of the opcode itself; this is called immediate addressing. The constant is always preceded by the pound (hash) sign #, which tells the assembler that the following number is a literal value, not an address. Immediate data can be loaded into A, into any working register R0-R7 of the current bank, or (as a 16-bit constant) into the data pointer DPTR.

Working through each instruction:

MOV A, #25H - the 8-bit constant 25H is copied into the accumulator, so A = 25H.
MOV R4, #62 - 62 has no H suffix, so it is a decimal constant; R4 receives 62 decimal, which is 3EH.
MOV DPTR, #4562H - DPTR is a 16-bit register, so the full 16-bit constant 4562H is loaded (DPH = 45H, DPL = 62H).
MOV DPTR, #1235H - likewise loads 1235H into DPTR (DPH = 12H, DPL = 35H).
MOV R7, #06H - the 8-bit constant 06H is copied into R7.

When an immediate move executes, the program counter is automatically advanced past the opcode byte(s) that hold the constant, so the next instruction is fetched correctly.

[!ANSWER]

A = 25H; R4 = 62 decimal (3EH); DPTR = 4562H, then re-loaded to 1235H; R7 = 06H. Each value is the literal that follows the # sign.

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