Central Processing Unit

Teaching Plan Introduction of Unit, Machine Instruction Characteristics, Types of Operand and operations, Instruction format and Types: 2 lectures Addressing Modes, Processor Organization, Instruction Cycle: 2 lectures 8086 Processor: Features, Block Diagram, Pin Configurations: 2 lectures Max/Min Mode, Read Write Cycle: 2 lectures

CPU Function CPU must: Fetch instructions Interpret instructions Fetch data Process data Write data 22

CPU With Systems Bus

CPU Internal Structure

What is an instruction set? The complete collection of instructions that are understood by a CPU Machine Code Binary Usually represented by assembly codes 2

Elements of an Instruction Operation code (Op code) Do this Source Operand reference To this Result Operand reference Put the answer here Next Instruction Reference When you have done that, do this... 3

Where have all the Operands gone?(Areas of source and result operands) Main memory (or virtual memory or cache) CPU register I/O device 4

Instruction Cycle State Diagram

Instruction Representation In machine code each instruction has a unique bit pattern For human consumption (well, programmers anyway) a symbolic representation is used e.g. ADD, SUB, LOAD Operands can also be represented in this way ADD A,B Opcode Operand Reference Operand Reference 5

Instruction Types Ways of Classification Data processing According to function According to number of addresses Data processing Data storage (main memory) Data movement (I/O) Program flow control 6

Number of Addresses (a) Operand 1, Operand 2, Result a = b + c; May be a forth - next instruction (usually implicit) Not common Needs very long words to hold everything 7

Number of Addresses (b) One address doubles as operand and result a = a + b Reduces length of instruction Requires some extra work Temporary storage to hold some results 8

Number of Addresses (c) Implicit second address Usually a register (accumulator) Common on early machines 9

Number of Addresses (d) 0 (zero) addresses All addresses implicit Uses a stack e.g. push a push b add pop c c = a + b 10

How Many Addresses More addresses Fewer addresses More complex (powerful?) instructions More registers Inter-register operations are quicker Fewer instructions per program Fewer addresses Less complex (powerful?) instructions More instructions per program Faster fetch/execution of instructions 11

Instruction Set Design Decisions (1) Operation repertoire How many ops? What can they do? How complex are they? Data types Instruction formats Length of op code field Number of addresses 12

Instruction Set Design Decisions(2) Registers Number of CPU registers available Which operations can be performed on which registers? Addressing modes RISC v CISC 13

Types of Operand Addresses Numbers Characters Logical Data Integer/floating point Characters ASCII etc. Logical Data Bits or flags 14

Types of Operation Data Transfer Arithmetic Logical Conversion I/O System Control Transfer of Control 18

Data Transfer Specify Source Destination Amount of data May be different instructions for different movements e.g. IBM 370 Or one instruction and different addresses e.g. VAX 19

Arithmetic Add, Subtract, Multiply, Divide Signed Integer Floating point ? May include Increment (a++) Decrement (a--) Negate (-a) 20

Logical Bitwise operations AND, OR, NOT 21

Conversion E.g. Binary to Decimal 22

Input/Output May be specific instructions May be done using data movement instructions (memory mapped) May be done by a separate controller (DMA) 23

Systems Control Privileged instructions CPU needs to be in specific state Kernel mode For operating systems use 24

Transfer of Control Branch Skip Subroutine call e.g. branch to x if result is zero Skip e.g. increment and skip if zero Branch xxxx ADD A Subroutine call c.f. interrupt call 25

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

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 3

Immediate Addressing Diagram Instruction Opcode Operand 4

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 to work out effective address Limited address space 5

Direct Addressing Diagram Instruction Opcode Address A Memory Operand 6

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 e.g. ADD (A) Add contents of cell pointed to by contents of A to accumulator 7

Indirect Addressing (2) Large address space 2n where n = word length May be nested, multilevel, cascaded e.g. EA = (((A))) Draw the diagram yourself Multiple memory accesses to find operand Hence slower 8

Indirect Addressing Diagram Instruction Opcode Address A Memory Pointer to operand Operand 9

Register Addressing (1) Operand is held in register named in address filed EA = R Limited number of registers Very small address field needed Shorter instructions Faster instruction fetch 10

Register Addressing (2) No memory access Very fast execution Very limited address space Multiple registers helps performance Requires good assembly programming or compiler writing N.B. C programming register int a; c.f. Direct addressing 11

Register Addressing Diagram Instruction Opcode Register Address R Registers Operand 12

Register Indirect Addressing C.f. 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

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

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

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

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

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

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

Combinations Postindex EA = (A) + (R) Preindex EA = (A+(R)) 20

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

Instruction Cycle Fetch Indirect Interrupt Decode Execute 35

Indirect Cycle May require memory access to fetch operands Indirect addressing requires more memory accesses Can be thought of as additional instruction subcycle

Instruction Cycle with Indirect

Instruction Cycle State Diagram

Data Flow (Instruction Fetch) Depends on CPU design In general: Fetch PC contains address of next instruction Address moved to MAR Address placed on address bus Control unit requests memory read Result placed on data bus, copied to MBR, then to IR Meanwhile PC incremented by 1

Data Flow (Data Fetch) IR is examined If indirect addressing, indirect cycle is performed Right most N bits of MBR transferred to MAR Control unit requests memory read Result (address of operand) moved to MBR

Data Flow (Fetch Diagram)

Data Flow (Indirect Diagram)

Data Flow (Execute) May take many forms Depends on instruction being executed May include Memory read/write Input/Output Register transfers ALU operations

Data Flow (Interrupt) Simple Predictable Current PC saved to allow resumption after interrupt Contents of PC copied to MBR Special memory location (e.g. stack pointer) loaded to MAR MBR written to memory PC loaded with address of interrupt handling routine Next instruction (first of interrupt handler) can be fetched

Data Flow (Interrupt Diagram)

8086 Features 16-bit processor 16-bit data bus and 20-bit address bus 20-bit memory address and 16-bit I/O address Provides fourteen 16-bit registers Multiplexed lines Perform bit, byte, word and block transfer Operate in min and max mode Multiprogramming Fetch up-to six instruction bytes from memory and stores it in queue Supports different addressing modes

8086 Processor Pin

8086 Processor Pin S4 S3 Registers ES 1 SS CS or more DS BHE A0 ES 1 SS CS or more DS BHE A0 Registers Word 1 Upper byte from odd address Lower byte from even address None QS1 QS2 Status No operation (queue is idle) 1 First byte of an op-code Queue is empty Subsequent byte of an op-code S2 S1 S0 Machine Cycle Interrupt Acknowledge 1 I/O Read I/O Write Halt Instruction Fetch Memory Red Memory Write Inactive-Passive

Processor Organization / Internal Block Diagram of 8086

Minimum Mode System

Minimum Mode Read Cycle

Minimum Mode Write Cycle

Maximum Mode System

Maximum Mode Read Cycle

Maximum Mode Write Cycle

Probable Questions Explain different addressing modes. Explain instruction cycle state diagram. Explain different types of instructions. Explain the different ways of instruction classification with its types. Explain the instruction set design criteria's with example. Explain different types of operand and operations.

Probable Questions Explain the instruction cycle with block diagram of different cycles. Write short note on, Fetch Cycle, Indirect Cycle, Interrupt Cycle. Explain the processor model/block diagram of 8086. Explain the minimum mode system with read and write cycle. Explain the maximum mode system with read and write cycle