1. Alvaro Mauricio Peña Dariusz Niworowski Frank Rodriguez
Chapter 11
Instruction Sets:
Addressing Modes and Formats

2. INTRODUCTION:
Objective: to be able to reference a large range of locations in main memory or, for some systems, virtual memory.

3. TRADE-OFFS
Address range/addressing flexibility
Memory references in the instruction/the complexity of address calculations

4. Most Common Addressing Techniques
Immediate
Direct
Indirect
Register
Register Indirect
Displacement (Indexed)
Stack 2

5. Operand is part of instruction
Immediate Addressing
Fast
Operand is part of instruction
The effective address is the address of the byte following the instruction
Operand = address field
e.g. ADD 5
Add 5 to contents of accumulator
5 is operand
No memory reference to fetch data
Limited range 3

6. Immediate Addressing Diagram
Instruction
Opcode
Operand 4

7. Address field contains address of operand
Direct Addressing
Address field contains address of operand
Effective address (EA) = address field (A)
e.g. ADD A
Add contents of cell A to accumulator
Look in memory at address A for operand
Limited address space
No additional calculations to work out effective address
Single memory reference to access data 5

8. Direct Addressing Diagram
Instruction
Opcode
Address A
Memory
Operand 6

9. Indirect Addressing (1)
Memory cell pointed to by address field contains the address of (pointer to) the operand
EA = (A)
Look in A, find address (A) and look there for operand
ADD (A)
Add contents of cell pointed to by contents of A to accumulator 7

10. Indirect Addressing (2)
Large address space
2n where n = word length
May be nested, multilevel, cascaded
*********************************
Multiple memory accesses to find operand
Hence slower 8

11. Register Addressing (1)
Similar Direct addressing
Operand is held in register named in address filed
EA = R
Limited number of registers 10

12. Very small address field needed
Register Addressing
ADVANTAGES
Very small address field needed
Shorter instructions
Faster instruction fetch
No memory access
Very fast execution
Multiple registers helps performance
Requires good assembly programming or compiler writing
DISADVANTAGE
Very limited address space 11

13. Register Indirect Addressing
Analogous to indirect addressing
EA = (R)
Operand is in memory cell pointed to by contents of register R
Large address space (2n)
One fewer memory access than indirect addressing 13

14. Displacement Addressing
EA = A + (R)
Address field hold two values
A = base value
R = register that holds displacement
or vice versa 15

15. A version of displacement addressing R = Program counter, PC
Relative Addressing
A version of displacement addressing
R = Program counter, PC
EA = A + (PC)
i.e. get operand from A cells from current location pointed to by PC
c.f locality of reference & cache usage 17

16. Base-Register Addressing
A holds displacement
R holds pointer to base address
R may be explicit or implicit
e.g. segment registers in 80x86 18

17. Good for accessing arrays
Indexed Addressing
A = base
R = displacement
EA = A + R
Good for accessing arrays
R++ 19

18. Operand is (implicitly) on top of stack e.g.
Stack Addressing
Operand is (implicitly) on top of stack
e.g.
ADD Pop top two items from stack and add 21

19. Pentium Addressing Modes
Pentium address translation mechanism produces an address called a Virtual or effective address, that is an offset into a segment
The sum of starting address of the segment and the effective address gives linear address
This goes through page translation if paging enabled
12 addressing modes available
Immediate
Register operand
Displacement
Base
Base with displacement
Scaled index with displacement
Base with index and displacement
Base scaled index with displacement
Relative

20. PowerPC Addressing Modes
Load/store architecture
Indirect
Instruction includes 16 bit displacement to be added to base register (may be GP register)
Can replace base register content with new address
Indirect indexed
Instruction references base register and index register (both may be GP)
EA is sum of contents
Branch address
Absolute
Relative
Arithmetic
Operands in registers or part of instruction
Floating point is register only

21. Instruction Formats
Layout of bits in an instruction
Includes opcode
Includes (implicit or explicit) operand(s)
Usually more than one instruction format in an instruction set

22. Affected by and affects:
Instruction Length
Affected by and affects:
Memory size
Memory organization
Bus structure
CPU complexity
CPU speed
Trade off between powerful instruction repertoire and saving space

23. Allocation of Bits
Number of addressing modes
Number of operands
Register versus memory
Number of register sets
Address range
Address granularity

24. PDP-8 Instruction Format

25. PDP-10 Instruction Format

26. PDP-11 Instruction Format

27. VAX Instruction Examples

28. Pentium Instruction Format

29. PowerPC Instruction Formats (1)

30. PowerPC Instruction Formats (2)

31. Review Questions
Briefly define immediate addressing, direct addressing, indirect addressing, register addressing, register indirect addressing, displacement addressing, and relative addressing.
What is the advantage of auto indexing?
What is the difference between postindexing and preindexing?
What facts go into determining the use of the addressing bits of an instruction?
What are the advantages of using a variable-length instruction?