Addressing Modes 1. Immediate (literal) operand part of instruction limited operand magnitude useful for storing small constants no address calculation; no memory reference <ea> = 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 <ea> = 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 <ea> = (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 <ea> = (...(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 <ea> = 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 <ea> = (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 <ea> = (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 <ea> = (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 <ea> = (SP) b) addressing is implied by the instruction for assembly languages with explicit stack instructions c) for hardware stacks, the <ea> 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 <ea> = 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 <ea> = A + (R) (b) Autoindexing automatically increment/decrement the index register before/after a data access <ea> = 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 <ea> = (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 <ea> = (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 <ea> = (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 <ea> = (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 <ea> = (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 .