Addressing Modes

1. Immediate (literal)  operand part of instruction  limited operand magnitude  useful for storing small constants  no address calculation; no memory reference  = none e.g. ADD #3,D1 instruction MOV AX,0FFH MOVLW 3 Addressing Modes

2. Direct (absolute)  address of operand part of the instruction  limited address space, may have extension words  no address calculation  = A e.g. ADD.B 3,D1 instruction MOV AX,NUM1 MOVWF NUM1

memory Addressing Modes

3. Indirect (memory deferred)  address of operand is in the memory location whose address appears in the instruction  address retrieved from memory  = (A)

instruction e.g. ADD (PTR),R1

memory Addressing Modes

3. Indirect (memory deferred)  address space of 2**N if word length is N; however, number of effective addresses is limited to 2**K where K is the length of the address field in the machine code  some machines allow multilevel or cascaded indirect addressing  = (...(A)...)  e.g. Data General NOVA ... the first bit of the operand is an indirect bit where 0 means the operand is data and a 1 means the operand is an address Addressing Modes 4. Register Direct  operand is in a register whose number appears in the instruction; small address field in instruction  address space is very limited (i.e. # of registers)  no address calculation; no memory reference  = R e.g. ADD D0,D1 instruction MOV AX,0FFH MOVWF NUM1

registers Addressing Modes 5. Register Indirect (deferred)  address of operand is in a register whose number appears in the instruction  large address space  = (R) instruction e.g. ADD (A0),D1

LFSR 0,LIST MOVF INDF0,W memory registers Addressing Modes

5. Register Indirect - Autoincrement/Autodecrement  register indirect addressing where the address in the register is automatically incremented/decremented either before or after effective address calculations  for postincrement/postdecrement = (R) instruction (R) <-- (R) ± 1 e.g. ADD.L (A1)+,A2

memory registers Addressing Modes

5. Register Indirect - Autoincrement/Autodecrement

 for preincrement/predecrement (R) <-- (R) ± 1 = (R) instruction e.g. ADD.W -(A1),D2

memory registers Addressing Modes 6. Stack a) address of operand is in the stack pointer - variation of register indirect addressing = (SP) b) addressing is implied by the instruction for assembly languages with explicit stack instructions c) for hardware stacks, the is an address in the stack; the limited size of this stack has implications for the size and operation of the pointing mechanism Addressing Modes 7. Displacement  address of operand is the sum of a register and a displacement  variations are named with respect to the register  the basic format is instruction = disp + (R) e.g. ADD.W 10(A0),D3 ADD.W (10,A0),D3 ADD.L 8(A0,D0),D3 memory registers Addressing Modes 7. Displacement (a) Indexing  the address field references a memory address, and R is an index register containing the displacement from that address = A + (R)

(b) Autoindexing  automatically increment/decrement the index register before/after a data access = A + (R) (R) <-- (R) + 1

 the increment/decrement may be 1 or a restricted set of values Addressing Modes 7. Displacement (c) Postindexing/Preindexing (typically only one)

 combination of indirect addressing and indexing

 if indexing is performed after the indirection, it is termed postindexing = (A) + (R)  useful for accessing one of a number of blocks of data all with the same fixed format

 if indexing is performed before the indirection, it is termed preindexing = (A + (R))  useful for constructing a jump table where the table of addresses starts at location A and R indexes into the table Addressing Modes

7. Displacement (d) Relative  the register, R, is the program counter, PC  the operand is located a certain distance from the current position of the program counter = (PC) + A

 used most often in branch instructions  instruction execution done after the PC has been incremented to point to the next instruction. Therefore, relative to the next instruction. Addressing Modes 7. Displacement (e) Base-Register  the referenced register contains a memory address and the address field contains a displacement (opposite to indexing)  in the simplest case, the base register is explicitly referenced in the instruction and is local to the instruction = (R) + A  if the register is a base or segment register implied by the instruction (i.e. not explicitly specified), the primary use is implementing segmentation and/or memory protection. The program is written as if starting at location 0. The addition of the base register to all memory references is equivalent to loading the program relative to a base address. A base register is usually equal to the physical address length. A segment register is usually shorter than the physical address length. Addressing Modes 7. Displacement (f) Base-Indexed  The base register is local to the instruction and is combined with an index register and a displacement = (B) + (I) + A

e.g ADD.L 8(A0,D0),D3 ADD.L (8,A0,D0),D3

e.g. for Pentium, possible to do SEGMENT + BASE + (INDEX * SCALE) + DISPLACEMENT

 can be effectively used to index through complex data structures