Computer Organization CS 206 T

Lec# 2: Instruction Sets

Topics

• What is an instruction set

• Elements of instruction

• Instruction Format

• Instruction types

• Types of operations

• Types of operand

• Addressing mode

• RISC & CISC

1. What is an Instruction Set? • The operation of the processor is determined by the

instruction it executes, referred to as machine

instructions or computer instructions

• The complete collection of different instructions that the processor can execute is referred to as the processor instruction set

• Machine language: binary representation of operations and (addresses of) arguments

• Assembly language: mnemonic representation for humans, e.g.,

OP A,B,C (meaning A <- OP(B,C))

2. Elements of an Instruction

• Operation code (opcode): specifies the operation to be performed (binary code)

– Do this: ADD, SUB, MPY, DIV, LOAD, STOR

• Source operand reference: the operation may involve one or more source operand as input for the operation.

– To this: (address of) argument of op, e.g. register, memory location

2. Elements of an Instruction

• Result operand reference: the operation may produce a result.

– Put the result here (as above)

• Next instruction reference (often implicit): this tells the processor where to fetch the next instruction after complete the current one (in most cases the next instruction is immediately follow the current one).

– When you have done that, do this: BR

Instruction Cycle State Diagram

3. Instruction Formats

• An instruction format defines the layout of the bits of an instruction. An instruction format must include an opcode and, implicitly or explicitly, zero or operands.

• The format must implicitly or explicitly, indicate the addressing mode for each operand.

• Usually more than one instruction format in an instruction set

A simple Instruction Format (using two addresses)

3.1 Instruction Representation

• Each instruction is represented by a

sequenced of bits.

• The instruction is divided into fields according to the constituent elements of the instruction.

3.2Instruction Length • Affected by and affects:

– Memory size

– Memory organization - addressing

– Bus structure, e.g., width

– CPU complexity

– CPU speed

• Trade off between powerful instruction repertoire and saving space

4. Instruction Types

• Data transfer: registers, main memory, stack or I/O

• Data processing: arithmetic, logical

• Control: systems control, transfer of control

• Data Movement: I/O instruction.

5. Types of Operations

• The number of different opcodes varies widely from machine to machine. However, the same general types of operations are found on all machines.

5.1 Data Transfer • Store, load, exchange, move, clear, set, push,

pop

• Specifies: source and destination (memory, register, stack), amount of data

• May be different instructions for different (size, location) movements, e.g.,

IBM S/390: L (32 bit word, R<-M), LH (halfword, R<-M), LR (word, R<-R), plus floating-point registers LER, LE, LDR, LD

Or one instruction and different addresses, e.g. VAX: MOV

5.2 Input/Output

• May be specific instructions, e.g. INPUT, OUTPUT (issue command to I/O module

• May be done using data movement instructions (memory mapped I/O)

• May be done by a separate controller (DMA): Start I/O, Test I/O

5.3 Arithmetic

• Add, Subtract, Multiply, Divide for signed integer (+ floating point and packed decimal) – may involve data movement

• May include

– Absolute (|a|)

– Increment (a++)

– Decrement (a--)

– Negate (-a)

5.4 Logical

• Bitwise operations: AND, OR, NOT, XOR, TEST, CMP, SET

• Shifting and rotating functions, e.g.

– logical right shift for unpacking: send 8-bit character from 16-bit word

– arithmetic right shift: division and truncation for odd numbers

– arithmetic left shift: multiplication without overflow

5.5 Transfer of Control

• Update program counter

• Skip, e.g., increment and skip if zero:

ISZ Reg1, cf. jumping out from loop, return, execute.

6. Types of Operand

• Addresses: immediate, direct, indirect, stack

• Numbers: integer or fixed point (binary, twos complement), floating point (sign, significand, exponent), (packed) decimal (246 = 0000 0010 0100 0110)

• Characters: ASCII (128 printable and control characters + bit for error detection)

• Logical Data: bits or flags, e.g., Boolean 0 and 1

7. Addressing Modes • Immediate

• Direct

• Indirect

• Register

• Register Indirect

• Displacement (Indexed)

• Stack

8.1 Immediate Addressing • Operand is part of instruction

• Operand = address field

• e.g., ADD #5 – Add 5 to contents of accumulator

– 5 is operand

• No memory reference to fetch data

• Fast

• Limited range

8.2 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

• Single memory reference to access data

• No additional calculations needed to work out effective address

• Limited address space (length of address field)

Direct Addressing Diagram

Address A Opcode

Instruction

Memory

Operand

8.3 Indirect Addressing (1) • Memory cell pointed to by address field

contains the address of the operand

• EA = (A) – Look in A, find effective address and look there for

operand

• E.g. ADD (A)

– Add content of cell pointed to by content of A to accumulator

Indirect Addressing (2) • Large address space

• 2n where n = word length

• May be nested, multilevel, cascaded – e.g. EA = (((A)))

• Multiple memory accesses to find operand

• Hence slower

Indirect Addressing Diagram

Address A Opcode

Instruction

Memory

Operand

Pointer to operand

8.4 Register Addressing (1) • Operand is held in register named in address

field

• EA = R

• Limited number of registers

• Very small address field needed – Shorter instructions

– Faster fetch

Register Addressing (2) • No memory access

• Very fast execution

• Very limited address space

• Multiple registers helps performance – Requires good assembly programming or

compiler writing – see register renaming

• cf. direct addressing

Register Addressing Diagram

Register Address R Opcode

Instruction

Registers

Operand

8.5 Register Indirect Addressing • Cf. 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

Register Indirect Addressing Diagram

Register Address R Opcode

Instruction

Memory

Operand Pointer to Operand

Registers

8.6 Displacement Addressing • EA = A + (R)

• Address field holds two values – A = base value

– R = register that holds displacement

– or vice versa

• See segmentation

Displacement Addressing Diagram

Register R Opcode

Instruction

Memory

Operand Displacement

Registers

Address A

+

Instruction Set Architecture (ISA)

–CISC is an acronym for complex instruction

set computer. E.g. x86 family

–RISC stands for reduced instruction set

computer, e.g. Pentium and MIPS

RISC • Reduced Instruction Set Computer

• Key features

– Large number of general purpose registers

(or use of compiler technology to optimize register use)

– Limited and simple instruction set

– Emphasis on optimising the instruction pipeline

RISC Characteristics

• One instruction per cycle • Register to register operations • Few, simple addressing modes • Few, simple instruction formats Also • Hardwired design (no microcode) • Fixed instruction format But • More compile time/effort

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9.2 RISC Machines

• The underlying philosophy of RISC machines is that

a system is better able to manage program

execution when the program consists of only a few

different instructions that are the same length and

require the same number of clock cycles to decode

and execute.

• RISC systems access memory only with explicit load

and store instructions.

• In CISC systems, many different kinds of instructions

access memory, making instruction length variable

and fetch-decode-execute time unpredictable.

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9.2 RISC Machines

• The difference between CISC and RISC becomes

evident through the basic computer performance

equation:

• RISC systems shorten execution time by reducing

the clock cycles per instruction.

• CISC systems improve performance by reducing the

number of instructions per program.

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9.2 RISC Machines

• It is becoming increasingly difficult to distinguish

RISC architectures from CISC architectures.

• Some RISC systems provide more extravagant

instruction sets than some CISC systems.

• Some systems combine both approaches.

• The following two slides summarize the

characteristics that traditionally typify the

differences between these two architectures.

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• RISC – Multiple register sets.

– Three operands per instruction.

– Parameter passing through register windows.

– Single-cycle instructions.

– Hardwired control. – Highly pipelined.

• CISC – Single register set.

– One or two register operands per instruction.

– Parameter passing through memory.

– Multiple cycle instructions.

– Microprogrammed control.

– Less pipelined.

9.2 RISC Machines

Continued....

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• RISC – Simple instructions,

few in number. – Fixed length

instructions. – Complexity in

compiler. – Only LOAD/STORE

instructions access memory.

– Few addressing modes.

• CISC – Many complex

instructions. – Variable length

instructions. – Complexity in

microcode. – Many instructions can

access memory. – Many addressing

modes.

9.2 RISC Machines

Chapter 10: Instruction Sets: Characteristics and Functions Addressing Modes.

Chapter 11: Instruction Sets: Addressing Modes and Format. Chapter 13: Reduced Instruction Set Computers (RISC).

End of Slides