Thuc notes - CPU Register addressing modes

• Specifies how to calculate the effective memory address of an operand by using information held in registers and/or constants contained within a machine instruction or elsewhere

Immediate mode

• The operand is an immediate value is stored explicitly in the instruction
• The “data” is embedded in the instruction itself
• E.g. loads the immediate value of “3” (data) into register $11
• E.g. move immediate value “200” (data) in register R0

Register mode

• Data is stored in register. Or the register contains the value of the operand
• E.g. register $11 contains value 5

Indirect mode

• The effective address of the operand is the contents of a register or main memory location, location whose address appears in the instruction
• e.g. (A) is the operand address = pointer variable
• e,.g. Store value at location whose address is givin by content of register $1

Base pointer addressing mode

• Same Indirect mode but support an offset to add to address before lookup

Index mode

• Instruction contains memory address to access.
• The address of the operand is obtained by adding to the contents of the general register (called index register) a constant value
• E.g. Load value of register with address 2000 + (value in index register R5)

Absolute (Direct) Mode

• The address of the operand is embedded in the instruction code.