Chapter 11Instruction Sets:

Addressing Modes and Formats

Gabriel BaronSydney Chow

Immediate Direct Indirect Register Register Indirect Displacement (Indexed) Stack

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

OperandOpcode

Instruction

Example: LDAAOpcode = 86Operand = 05

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

Address AOpcode

Instruction

Memory

OperandExample: LDAAOpcode = 96A= E5Address A = 00 E5

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

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

Address AOpcode

Instruction

Memory

Operand

Pointer to operand

Example:Opcode = XXA = 11C511C5 = Operand

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

Disadvantage Very limited address space

Register Address ROpcode

Instruction

Registers

Operand

Example:Opcode = XXRegister R = Operand

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

Register Address ROpcode

Instruction

Memory

OperandPointer to Operand

Registers

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

Register ROpcode

Instruction

Memory

OperandPointer to Operand

Registers

Address A

+

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 stackand add

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

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

in an instruction set

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

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

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

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

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

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

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

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.

1. What are four addressing modes?Ans: Immediate, Direct, Indirect, Stack.

2. What is simplest addressing mode?Ans: Immediate Addressing Mode.

3. What is the disadvantage of the direct addressing mode?

Ans: Limited Address Space.

4. What is indirect addressing?Ans: The operand is stored in an address, which is stored

in an memory cell.

5. Which addressing mode refers to a register instead of a memory address?

Ans: Register Addressing Mode

6. 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.

7. What two values does the address field hold in the Displacement Addressing mode?

Ans: Base value, Register value

8. What order is data sorted in the stack addressing mode?

Ans: Last in First Out

9. What are four X86 Addressing Modes?Ans: Register, memory, displacement, indirect.

10. Which instruction design is the simplest for general purpose?

Ans: PDP-8

X86 Addressing Modes http://www.arl.wustl.edu/~lockwood/class/cs306/books/artofasm/Chapter_4/CH

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