1. Part II: Addressing Modes
Chapter 10
Part II: Addressing Modes

2. LMC Addressing Mode uses direct, absolute addressing
direct address: the address of the data being referenced can be reached directly from the address contained in the instruction word
absolute address: the address given in the instruction is the actual memory location where the data is stored
too limited for real computer systems to use only this method

3. Why alternative modes?
Allow a much larger range of addressable memory locations while using reasonable number of bits in the instruction word
Facilitate implementation of certain frequently occurring program statements - such as loops
Facilitate use of registers and moving data between registers

4. Register Addressing Both operands are registers
Significantly speeds up the execution
Reduces memory accesses
RISC machines provide instruction sets that consist almost entirely of registers

5. Base Register Addressing
use a register to hold the initial address
include the offset and the R# in the instruction
address from register and offset are added to get final address

6. Base Register Example

7. IBM S/390 Load Instruction
Data value is loaded into destination register.
Data address is found via indexed base addressing.

8. Advantages Provides larger address space - usually up to several GBs
Supports relocatability of program from one memory block to another
Some systems have special base register - others permit use of general purpose registers

9. Relative Addressing - generate an address relative to the location of the current instruction-
similar to base addressing but uses Program Counter instead of base register.
Useful for branching to n locations before or after an instruction => complement arithmetic
Relative Addressing
The desired address consists of the value in the address field added to the value
in the program counter
Instruction
Program Counter (PC)
OPCODE OPERANDS
1326 42
1326 + 42
1368 Actual (absolute) address in memory

10. Immediate Addressing
Data operand is stored within the instruction itself
Useful for constants
No memory access is required

11. LMC Immediate Addressing

12. INDIRECT ADDRESSING
separates the location of the data from the instruction itself
OPCODE OPERANDS 47 48 49 50 51 52 53 52 74 72 73 74 75
DATA

13. Indirect Addressing
Indirect addressing is useful if one needs to reference a location outside the range of absolute address values
More commonly, register indirect addressing is provided

14. Register Indirect Addressing

15. Advantages Very efficient access to memory Utilized in most processors
Usually auto-incrementing or decrementing of register is supported

16. Indexed Addressing Similar to base register addressing
Index register is used as a offset for handling subscripting of arrays

17. Indexed Addressing

18. Indexed Addressing w/Base Register