ADDRESSING MODES

Addressing modes:

The different ways in which the location of an operand is specified in an instruction are referred to as addressing modes. The term "addressing mode" can be interpreted in two ways: either "effective address calculation mode" or "operand accessing mode". Effective address (EA) is the address where the operand is actually located. Addressing modes have direct impact on the instruction count and average clock cycles per instruction (CPI), so the choice of addressing mode is an important design issue.

Addressing Mode	Description	Assembler Syntax	Addressing Function	Example	Diagram	Used For	Remarks
Immediate	Operand is given explicitly in the instruction	#Value	EA= Nil	Move #30,R1	Instruction	constants	No bus cycle needed to fetch operands
Register	Register given in the instruction  contains the operand	Ri	EA=Ri	ADD R1,R2	Instruction	Frequently accessed variables	No bus cycle needed to fetch operands. Limited registers
Direct (or) absolute	Memory location given in the instruction contains the operand	LOC	EA=LOC	ADD A, B	Instruction	Variables	Requires one memory reference
Register Indirect	Register contains the address of the operand	(Ri)	EA=[Ri]	ADD (R1), R0	Instruction	Pointers	Requires one memory reference
Memory Indirect	Memory location given in the instruction contains the address of the operand	(LOC)	EA=[LOC]	ADD (A), B	Instruction	Pointers	Requires two memory references

Displacement Addressing Mode Add displacement offset to the base/index register	Index	X(Ri)	EA = [Ri] + X	ADD 20(R1), R2	Instruction	Arrays and Lists	More flexible
	Base with index	(Ri, Rj)	EA= [Ri] +[Rj]; Ri- base of array Rj – Index amount	ADD (R1,R2), R3
	Base with index and offset/displacement	X(Ri,Rj)	EA= [Ri] + [Rj] + X	ADD 20(R1,R2), R3
Relative	Add offset to PC	X(PC)	EA=[PC]+X	Branch>0 LOOP	If LOOP is 16 bytes prior to the next instruction, then EA=[PC]-16 (since -16(PC))	Used in branching	Makes branching instructions flexible
Autoincrement	Access data items  in successive memory locations	(Ri)+	EA=[Ri] Increment Ri	Add (R1)+, R2	R2 <- [[R1]]+[R2] Increment R1	Used in stacks (push/pop) and stepping through arrays	Additional addressing modes
Autodecrement		-(Ri)	Decrement Ri EA=[Ri]	Add –(R1), R2	Decrement R1 R2 <- [[R1]]-[R2]