Overview of Addressing Modes in Computer Architecture

1. William Stallings Computer Organization and Architecture8th Edition Chapter 11 Instruction Sets: Addressing Modes and Formats Gabriel Baron Sydney Chow

2. Addressing Modes • Immediate • Direct • Indirect • Register • Register Indirect • Displacement (Indexed) • Stack

3. Immediate Addressing • Simplest form of addressing • Operand value is present in instruction • Operand = A • e.g. ADD 5 • Add 5 to contents of accumulator • 5 is operand • Advantages • No memory reference to fetch data • Fast • Disadvantages • Size restriction to the size of address field

4. Immediate Addressing Diagram Instruction Opcode Operand Example: LDAA Opcode = 86 Operand = 05

5. Direct Addressing • Simple form of 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 • Single memory reference to access data • No additional calculations to work out effective address • Disadvantage • Limited address space

6. Direct Addressing Diagram Instruction Opcode Address A Memory Operand Example: LDAA Opcode = 96 A= E5 Address A = 00 E5

7. Indirect Addressing • Memory cell pointed to by address field that contains the address of (pointer to) the operand • EA = (A) [parentheses means contents of] • Look in A, find address (A) and look there for operand • e.g. ADD (A) • Add contents of cell pointed to by contents of A to accumulator • Advantages • For word length N, space 2n now available • Large address space

8. Indirect Addressing (continued…) • Disadvantages • Requires two memory references to fetch operand • One to get its address • Second to get its value • Slower • Multiple memory accesses to find operand

9. Indirect Addressing Diagram Instruction Opcode Address A Memory Pointer to operand Operand Example: Opcode = XX A = 11C5 11C5 = Operand

10. Register Addressing • Address field refers to a register rather than a main memory address • EA = R • Advantages • Very small address field needed • Shorter instructions • Faster instruction fetch • No memory reference required • Very fast execution • Multiple registers helps performance • Requires good assembly programming or compiler writing

11. Register Addressing (continued…) • Disadvantage • Very limited address space

12. Register Addressing Diagram Instruction Opcode Register Address R Registers Operand Example: Opcode = XX Register R = Operand

13. Register Indirect Addressing • EA = (R) • Operand is in memory cell pointed to by contents of register R • Advantages • For word length N, space 2n now available • Large address space • One fewer memory access than indirect addressing • Disadvantages • Requires two memory references to fetch operand • Multiple memory accesses to find operand • Slower

14. Register Indirect Addressing Diagram Instruction Opcode Register Address R Memory Registers Operand Pointer to Operand

15. Displacement Addressing • Combines the capabilities of direct addressing and register indirect addressing • EA = A + (R) • Advantages • Flexibility • Disadvantages • Complexity • Requirements • Address field hold two values • A = base value • R = register that holds displacement • or vice versa

16. Displacement Addressing Diagram Instruction Address A Opcode Register R Memory Registers Pointer to Operand Operand +

17. Stack Addressing • Linear array of locations (last-in-first-out) • Operand is (implicitly) on top of stack • EA = top of stack • Advantages • No memory reference • Disadvantage • Limited applicability • e.g. • ADD Pop top two items from stack and add

18. Stack Addressing Diagram

19. X86 Segment Registers • The memory is divided into portions that maybe addressed by a single index register without changing a 16-bit segment selector. • A segment is always 64 KB in size using a 16-bit offset. • The X86 Addressing modes are as follows: • Register • Memory • Displacement Only • Indirect • Indexed • Based Indexed

20. 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

21. Instruction Length • The most basic design issue to be faced • Affected by and affects: • Memory size • Memory organization • Bus structure • CPU complexity • CPU speed • Trade off between powerful instruction repertoire and saving space

22. Allocation of Bits • Number of addressing modes • Sometimes an addressing mode can be indicated implicitly • i.e. certain opcodes might always call for indexing, so the addressing modes must be explicit and one or more mode bits will be needed • Number of operands • Fewer addresses can make for longer, more awkward programs • Register versus memory • The more that registers can be used or operand references, the fewer bits are needed

23. Allocation of Bits (continued…) • Number of register sets • These registers can be used to store data and can be used to store addresses for displacement addressing • Address range • For addresses that reference memory, the range of addresses that can be referenced is related to the number of address bits • Address granularity • In a system with 16- or 32- bit words, an address can reference a word or byte at the designer’s choice

24. PDP-8 • One of the simplest instruction designs for a general purpose • Each memory reference consist of 7 bits plus two 1-bit modifiers • The memory is divided into fixed-length pages of 27 = 128 words • Supports indirect addressing, displacement addressing and indexing

25. PDP-8 Instruction Format

26. PDP-10 • Designed to be a large-scale time-shared system, with an emphasis on making the system program easy • Some design principals • Orthogonality • Two variables are independent of each other • Completeness • Each arithmetic data type should have a complete and identical set of operations • Direct addressing • Base plus displacement addressing, usually avoided in favor of direct addressing

27. PDP-10 Instruction Format

28. PDP-11 • Provides a powerful and flexible instruction set within the constraints of a 16-bit microcomputer • Employs a set of eight 16-bit general-purpose registers • Increases hardware cost and programming complexity because of addressing capability • More efficient or compact programs can be deeloped

29. PDP-11 Instruction Format

30. VAX • Begins with a 1-byte opcode • On hexadecimal codes • Actual opcode being specified in the second byte • Remainder of instructions consist of up to six operand specifiers • Minimum 1-byte format which leftmost 4 bits are the address mode specifier • Provides for a wide variety of operations and addressing modes

31. VAX Instruction Examples

32. Summary • Types of addressing modes are: • Immediate, Direct, Indirect, Register, Register-Indirect, Displacement , Stack … • Some of the Key Design issues for instruction formats are: • Instruction length, allocation of bits, PDP-8, PDP-10, PDP-11, VAX.

33. Review Questionsand Answers • What are four addressing modes? Ans: Immediate, Direct, Indirect, Stack. • What is simplest addressing mode? Ans: Immediate Addressing Mode. • What is the disadvantage of the direct addressing mode? Ans: Limited Address Space. • What is indirect addressing? Ans: The operand is stored in an address, which is stored in an memory cell. • Which addressing mode refers to a register instead of a memory address? Ans: Register Addressing Mode

34. Review Questionsand Answers Part II • How does the register indirect addressing mode access the operand? Ans: The operand is store in an address that is pointed to by a memory cell. • What two values does the address field hold in the Displacement Addressing mode? Ans: Base value, Register value • What order is data sorted in the stack addressing mode? Ans: Last in First Out • What are four X86 Addressing Modes? Ans: Register, memory, displacement, indirect.

35. Review Questionsand Answers Part III • Which instruction design is the simplest for general purpose? Ans: PDP-8

36. References • X86 Addressing Modes • http://www.arl.wustl.edu/~lockwood/class/cs306/books/artofasm/Chapter_4/CH04-2.html#HEADING2-1 • http://www.cs.cmu.edu/~410/doc/segments/segments.html