cosc121: computer systems: review -...

COSC121: Computer Systems:Review

Jeremy Bolton, PhD

Assistant Teaching Professor

Constructed using materials:

- Patt and Patel Introduction to Computing Systems (2nd)

- Patterson and Hennessy Computer Organization and Design (4th)

**A special thanks to Rich Squier

Notes

• A good understanding of Digital Logic and Digital Design is a

prerequisite. Please refresh as needed.

• Review the von Neumann Design, Basic Data Path, and LC-3

Implementation

– Read Patt and Patel CHs 4-5 (PP.4-PP.5)

– Complete HW#1 + HW#2

Outline

• Von Neumann Model– Main components and bottleneck

– Instruction processing and data path

• LC-3– ISA

– Operations and codes

– Addressing Modes

– Major components of implementation

– Example program

This week … our journey takes us …

I/O systemProcessor

Compiler

Operating System(Win, Linux)

Application (Browser)

Digital Design

Circuit Design

Instruction SetArchitecture

Datapath & Control

transistors

MemoryHardware

Software Assembler

COSC 120: Computer Hardware

COSC 121: Computer

SystemsCOSC 255: Operating Systems

Drivers

PP.4Reviewing:

The Von Neumann Model and LC3 Implementation

4-6

The Stored Program Computer

• 1945: John von Neumann

– wrote a report on the stored program concept

• The basic structure proposed in the draft became known

as the “von Neumann machine” (or model).

– a memory, containing instructions and data

– a processing unit, for performing arithmetic and logical operations

– a control unit, for interpreting instructions

– Plus input and output

– All components connected by a shared bus. • The von Neumann bottleneck

4-7

Von Neumann Model

MEMORY

CONTROL UNIT

MAR MDR

IR

PROCESSING UNIT

ALU TEMP

PC

OUTPUT

Monitor

Printer

LED

Disk

INPUTKeyboard

Mouse

Scanner

Disk

4 - 8

The LC-3as a von Neumann

machine.

Lets review each component …

4-9

Memory

• 2k x m array of stored bits

• Address– unique (k-bit) identifier of location

• Contents– m-bit value stored in location

• Basic Operations:

• LOAD– read a value from a memory location

• STORE– write a value to a memory location

•••

0000000100100011010001010110

110111101111

00101101

10100010

Address SpaceAddressability

4-10

Interface to Memory

• How does processing unit get data to/from memory?

• MAR: Memory Address Register

• MDR: Memory Data Register

• To LOAD a location (A):1. Write the address (A) into the MAR.

2. Send a “read” signal to the memory.

3. Read the data from MDR.

• To STORE a value (X) to a location (A):1. Write the data (X) to the MDR.

2. Write the address (A) into the MAR.

3. Send a “write” signal to the memory.

MEMORY

MAR MDR

4-11

Processing Unit

• Functional Units– ALU = Arithmetic and Logic Unit

– could have many functional units.some of them special-purpose(multiply, square root, … more later!)

– LC-3 performs ADD, AND, NOT

• Registers– Small, temporary storage

– Operands and results of functional units

– LC-3 has eight registers (R0, …, R7), each 16 bits wide

• Word Size– number of bits normally processed by ALU in one instruction

– also width of registers

– LC-3 is 16 bits

PROCESSING UNIT

ALU TEMP

4-12

Input and Output

• Devices for getting data into and out of computer memory

• Each device has its own interface,usually a set of registers like thememory’s MAR and MDR

– LC-3 supports keyboard (input) and monitor (output)

– keyboard: data register (KBDR) and status register (KBSR)

– monitor: data register (DDR) and status register (DSR)

• Some devices provide both input and output– disk, network

• Program that controls access to a device is usually called a driver.

INPUT

Keyboard

Mouse

Scanner

D isk

OUTPUT

Monitor

Printer

LED

Disk

4-13

Control Unit• Orchestrates execution of the program

• Instruction Register (IR) contains the current instruction.

• Program Counter (PC) contains the addressof the next instruction to be executed.

• Control unit:– reads an instruction from memory

• the instruction’s address is in the PC

– interprets the instruction, generating signals that tell the other components what to do

• an instruction may take many machine cycles to complete

CONTROL UNIT

IRPC

4-14

Instruction Processing

Decode instruction

Evaluate address

Fetch operands from memory

Execute operation

Store result

Fetch instruction from memory

4-15

Instruction

• The instruction is the fundamental unit of work.

• Specifies two things:

– opcode: operation to be performed

– operands: data/locations to be used for operation

• An instruction is encoded as a sequence of bits. (Just like data!)

– Often, but not always, instructions have a fixed length,such as 16 or 32 bits.

– Control unit interprets instruction:generates sequence of control signals to carry out operation.

• A computer’s instructions and their formats is known as itsInstruction Set Architecture (ISA).

4-16

Example: LC-3 ADD Instruction

• LC-3 has 16-bit instructions.

– Each instruction has a four-bit opcode, bits [15:12].

• LC-3 has eight registers (R0-R7) for temporary storage.

– Sources and destination of ADD are registers.

“Add the contents of R2 to the contents of R6,and store the result in R6.”

4-17

Example: LC-3 LDR Instruction

• Load instruction -- reads data from memory

• Base + offset mode:– add offset to base register -- result is memory address

– load from memory address into destination register

“Add the value 6 to the contents of R3 to form amemory address. Load the contents of that memory location to R2.”

4-18

Instruction Processing: FETCH

• Load next instruction (at address stored in PC) from memoryinto Instruction Register (IR).– Copy contents of PC into MAR.

– Send “read” signal to memory.

– Copy contents of MDR into IR.

• Then increment PC, so that it points to the next instruction in sequence.– PC becomes PC+1.

EA

OP

EX

S

F

D

4-19

Instruction Processing: DECODE

• First identify the opcode.– In LC-3, this is always the first four bits of instruction.

– A 4-to-16 decoder asserts a control line correspondingto the desired opcode.

• Depending on opcode, identify other operands from the remaining bits.– Example:

• for LDR, last six bits is offset

• for ADD, last three bits is source operand #2

EA

OP

EX

S

F

D

4-20

Instruction Processing: EVALUATE ADDRESS

• For instructions that require memory access,

compute address used for access.

• Examples:

– add offset to base register (as in LDR)

– add offset to PC

– add offset to zero

EA

OP

EX

S

F

D

4-21

Instruction Processing: FETCH OPERANDS

• Obtain source operands needed to

perform operation.

• Examples:

– load data from memory (LDR)

– read data from register file (ADD)

EA

OP

EX

S

F

D

4-22

Instruction Processing: EXECUTE

• Perform the operation,

using the source operands.

• Examples:

– send operands to ALU and assert ADD signal

– do nothing (e.g., for loads and stores)

EA

OP

EX

S

F

D

4-23

Instruction Processing: STORE RESULT

• Write results to destination.

(register or memory)

• Examples:

– result of ADD is placed in destination register

– result of memory load is placed in destination register

– for store instruction, data is stored to memory

• write address to MAR, data to MDR

• assert WRITE signal to memory

EA

OP

EX

S

F

D

4-24

Changing the Sequence of Instructions

• In the FETCH phase,we increment the Program Counter by 1.

• What if we don’t want to always execute the instructionthat follows this one?– examples: loop, if-then, function call

• Need special instructions that change the contents of the PC.

• These are called control instructions.– jumps are unconditional -- they always change the PC

– branches are conditional -- they change the PC only ifsome condition is true (e.g., the result of an ADD is zero)

4-25

Example: LC-3 JMP Instruction

• Set the PC to the value contained in a register. This becomes the

address of the next instruction to fetch.

“Load the contents of R3 into the PC.”

4-26

Instruction Processing Summary

• Instructions look just like data -- it’s all interpretation.

• Three basic kinds of instructions:– computational instructions (ADD, AND, …)

– data movement instructions (LD, ST, …)

– control instructions (JMP, BRnz, …)

• Six basic phases of instruction processing:

• F D EA OP EX S

– not all phases are needed by every instruction

– phases may take variable number of machine cycles

4-27

Control Unit State Diagram

• The control unit is a state machine. Here is part of a

simplified state diagram for the LC-3:

A more complete state diagram is in Appendix C.It will be more understandable after Chapter 5.

4-28

The off-switch?: Stopping the Clock

• Control unit will repeat instruction processing sequenceas long as clock is running. – If not processing instructions from your application,

then it is processing instructions from the Operating System (OS).

– The OS is a special program that manages processorand other resources.

• To stop the computer:– AND the clock generator signal with ZERO

– When control unit stops seeing the CLOCK signal, it stops processing.

– HALT instruction

• Does LC3 have an instruction to re-start the computer?

Chapter 5The LC-3

5-30

Instruction Set Architecture

• ISA = All of the programmer-visible

components and operations of the computer– memory organization

• address space -- how may locations can be addressed?

• addressibility -- how many bits per location?

– register set

• how many? what size? how are they used?

– instruction set

• opcodes

• data types

• addressing modes

• ISA provides all information needed for someone that wants to

write a program in machine language

(or translate from a high-level language to machine language).

5-31

LC-3 Overview: Memory and Registers

• Memory– address space: 216 locations (16-bit addresses)

– addressability: 16 bits

• Registers– temporary storage, accessed in a single machine cycle

• accessing memory generally takes longer than a single cycle

– eight general-purpose registers: R0 - R7• each 16 bits wide

• how many bits to uniquely identify a register?

– other registers• not directly addressable, but used by (and affected by)

instructions

• PC (program counter), condition codes

5-32

LC-3 Overview: Instruction Set

• Opcodes

– 15 opcodes

– Operate instructions: ADD, AND, NOT

– Data movement instructions: LD, LDI, LDR, LEA, ST, STR, STI

– Control instructions: BR, JSR/JSRR, JMP, RTI, TRAP

– some opcodes set/clear condition codes, based on result:

• N = negative, Z = zero, P = positive (> 0)

• Data Types

– 16-bit 2’s complement integer

• Addressing Modes

– How is the location of an operand specified?

– non-memory addresses: immediate, register

– memory addresses: PC-relative, indirect, base+offset

5-33

Operate Instructions

• Only three operations: ADD, AND, NOT

• Source and destination operands are registers– These instructions do not reference memory.

– ADD and AND can use “immediate” mode,where one operand is hard-wired into the instruction.

• In the following we will show a dataflow diagram with each instruction.– illustrates when and where data moves

to accomplish the desired operation

5-34

NOT (Register)

Note: Src and Dstcould be the same register.

5-35

ADD/AND (Register)this zero means “register mode”

5-36

ADD/AND (Immediate)

Note: Immediate field issign-extended.

this one means “immediate mode”

5-37

Using Operate Instructions

• Things to ponder …

LC3 is quite limited … with only ADD, AND, NOT…

– How do we subtract?

– How do we OR?

– How do we copy from one register to another?

– How do we initialize a register to zero?

5-38

Data Movement Instructions

• Load -- read data from memory to register– LD: PC-relative mode

– LDR: base+offset mode

– LDI: indirect mode

• Store -- write data from register to memory– ST: PC-relative mode

– STR: base+offset mode

– STI: indirect mode

• Load effective address -- compute address, save in register– LEA: immediate mode

– does not access memory

5-39

PC-Relative Addressing Mode

• Want to specify address directly in the instruction– But an address is 16 bits, and so is an instruction!

– After subtracting 4 bits for opcodeand 3 bits for register, we have 9 bits available for address.

• Solution:– Use the 9 bits as a signed offset from the current PC.

• 9 bits:

• Can form any address X, such that:

• Remember that PC is incremented as part of the FETCH phase;

• This is done before the EVALUATE ADDRESS stage.

255offset256

255PCX256PC

5-40

LD (PC-Relative)

5-41

ST (PC-Relative)

5-42

Indirect Addressing Mode

• With PC-relative mode, can only address data within 256 words of the instruction.– What about the rest of memory?

• Solution #1: – Read address from memory location,

then load/store to that address.

• First address is generated from PC and IR(just like PC-relative addressing), thencontent of that address is used as target for load/store.

5-43

LDI (Indirect)

5-44

STI (Indirect)

5-45

Base + Offset Addressing Mode

• With PC-relative mode, can only address data within 256 words of the instruction.– What about the rest of memory?

• Solution #2:– Use a register to generate a full 16-bit address.

• 4 bits for opcode, 3 for src/dest register,3 bits for base register -- remaining 6 bits are usedas a signed offset.

– Offset is sign-extended before adding to base register.

5-46

LDR (Base+Offset)

5-47

STR (Base+Offset)

5-48

Load Effective Address

• Computes address like PC-relative (PC plus signed offset) and

stores the result into a register.

• Note: The address is stored in the register,

not the contents of the memory location.

5-49

LEA (Immediate)

5-50

Example: Different addressing methods

Address Instruction Comments

x30F6 1 1 1 0 0 0 1 1 1 1 1 1 1 1 0 1 R1 PC – 3 = x30F4

x30F7 0 0 0 1 0 1 0 0 0 1 1 0 1 1 1 0 R2 R1 + 14 = x3102

x30F8 0 0 1 1 0 1 0 1 1 1 1 1 1 0 1 1M[PC - 5] R2

M[x30F4] x3102

x30F9 0 1 0 1 0 1 0 0 1 0 1 0 0 0 0 0 R2 0

x30FA 0 0 0 1 0 1 0 0 1 0 1 0 0 1 0 1 R2 R2 + 5 = 5

x30FB 0 1 1 1 0 1 0 0 0 1 0 0 1 1 1 0M[R1+14] R2

M[x3102] 5

x30FC 1 0 1 0 0 1 1 1 1 1 1 1 0 1 1 1

R3 M[M[x30F4]]

R3 M[x3102]

R3 5

opcode

Load Immediate

Store PC - relative

Store - register

Load indirect

5-51

Control Instructions

• Used to alter the sequence of instructions(by changing the Program Counter)

• Conditional Branch– branch is taken if a specified condition is true

• signed offset is added to PC to yield new PC

– else, the branch is not taken

• PC is not changed, points to the next sequential instruction

• Unconditional Branch (or Jump)– always changes the PC

• TRAP– changes PC to the address of an OS “service routine”

– routine will return control to the next instruction (after TRAP)

5-52

Condition Codes

• LC-3 has three condition code registers:N -- negativeZ -- zeroP -- positive (greater than zero)

• Set by any instruction that writes a value to a register(ADD, AND, NOT, LD, LDR, LDI, LEA)

• Exactly one will be set at all times– Based on the last instruction that altered a register

5-53

Branch Instruction

• Branch specifies one or more condition codes.

• If the set bit is specified, the branch is taken.

– PC-relative addressing:target address is made by adding signed offset (IR[8:0])to current PC.

– Note: PC has already been incremented by FETCH stage.

– Note: Target must be within 256 words of BR instruction.

• If the branch is not taken,the next sequential instruction is executed.

5-54

BR (PC-Relative)

What happens if bits [11:9] are all zero? All one?

5-55

Using Branch Instructions

• Compute sum of 12 integers.Numbers start at location x3100. Program starts at location x3000.

R1 x3100

R3 0

R2 12

R2=0?

R4 M[R1]

R3 R3+R4

R1 R1+1

R2 R2-1

NO

YES

5-56

Sample Program


x3000 1 1 1 0 0 0 1 0 1 1 1 1 1 1 1 1 R1 x3100 (PC+0xFF)

x3001 0 1 0 1 0 1 1 0 1 1 1 0 0 0 0 0 R3 0

x3002 0 1 0 1 0 1 0 0 1 0 1 0 0 0 0 0 R2 0

x3003 0 0 0 1 0 1 0 0 1 0 1 0 1 1 0 0 R2 12

x3004 0 0 0 0 0 1 0 0 0 0 0 0 0 1 0 1 If Z, goto x300A (PC+5)

x3005 0 1 1 0 1 0 0 0 0 1 0 0 0 0 0 0 Load next value to R4

x3006 0 0 0 1 0 1 1 0 1 1 0 0 0 0 0 1 Add to R3

x3007 0 0 0 1 0 0 1 0 0 1 1 0 0 0 0 1 Increment R1 (pointer)

X3008 0 0 0 1 0 1 0 0 1 0 1 1 1 1 1 1 Decrement R2 (counter)

x3009 0 0 0 0 1 1 1 1 1 1 1 1 1 0 1 0 Goto x3004 (PC-6)

5-57

JMP (Register)

• Jump is an unconditional branch -- always taken.

– Target address is the contents of a register.

– Allows any target address.

5-58

TRAP

• Calls a service routine, identified by 8-bit “trap vector.”

• When routine is done, PC is set to the instruction following TRAP.

• (We’ll talk about how this works later.)

vector routine

x23 input a character from the keyboard

x21 output a character to the monitor

x25 halt the program

5-59

LC-3 Data PathRevisited

Filled arrow

= info to be processed.

Unfilled arrow

= control signal.

Summary: LC-3 Implementation

5-60

Summary: Data Path Components

• Global bus– special set of wires that carry a 16-bit signal

to many components

– inputs to the bus are “tri-state devices,”that only place a signal on the bus when they are enabled

– only one (16-bit) signal should be enabled at any time• control unit decides which signal “drives” the bus

– any number of components can read the bus

• register only captures bus data if it is write-enabled by the control unit

• Memory– Control and data registers for memory and I/O devices

– memory: MAR, MDR (also control signal for read/write)

5-61


• ALU– Accepts inputs from register file

and from sign-extended bits from IR (immediate field).

– Output goes to bus.• used by condition code logic, register file, memory

• Register File– Two read addresses (SR1, SR2), one write address (DR)

– Input from bus• result of ALU operation or memory read

– Two 16-bit outputs• used by ALU, PC, memory address

• data for store instructions passes through ALU

5-62


• PC and PCMUX– Three inputs to PC, controlled by PCMUX

1. PC+1 – FETCH stage

2. Address adder – BR, JMP

3. bus – TRAP (discussed later)

MAR and MARMUX– Two inputs to MAR, controlled by MARMUX

1. Address adder – LD/ST, LDR/STR

2. Zero-extended IR[7:0] -- TRAP (discussed later)

5-63


• Condition Code Logic– Looks at value on bus and generates N, Z, P signals

– Registers set only when control unit enables them (LD.CC)

• only certain instructions set the codes(ADD, AND, NOT, LD, LDI, LDR, LEA)

• Control Unit – Finite State Machine– On each machine cycle, changes control signals for next

phaseof instruction processing

• who drives the bus? (GatePC, GateALU, …)

• which registers are write enabled? (LD.IR, LD.REG, …)

• which operation should ALU perform? (ALUK)

• …

– Logic includes decoder for opcode, etc.

Appendix

Jeremy Bolton, PhD

Assistant Teaching Professor

Constructed using materials:

- Patt and Patel Introduction to Computing Systems (2nd)

- Patterson and Hennessy Computer Organization and Design (4th)

**A special thanks to Rich Squier

5-65

Prgramming Example

• Count the occurrences of a character in a file– Program begins at location x3000

– Read character from keyboard

– Load each character from a “file”• File is a sequence of memory locations

• Starting address of file is stored in the memory locationimmediately after the program

– If file character equals input character, increment counter

– End of file is indicated by a special ASCII value: EOT (x04)

– At the end, print the number of characters and halt(assume there will be less than 10 occurrences of the character)

• A special character used to indicate the end of a sequenceis often called a sentinel.

– Useful when you don’t know ahead of time how many timesto execute a loop.

5-66

Flow Chart

Count = 0(R2 = 0)

Ptr = 1st file character(R3 = M[x3012])

Input char

from keybd(TRAP x23)

Done?(R1 ?= EOT)

Load char from file(R1 = M[R3])

Match?(R1 ?= R0)

Incr Count(R2 = R2 + 1)

Load next char from file(R3 = R3 + 1, R1 = M[R3])

Convert count to

ASCII character(R0 = x30, R0 = R2 + R0)

Print count(TRAP x21)

HALT(TRAP x25)

NO

NO

YES

YES

5-67

Program (1 of 2)


x3000 0 1 0 1 0 1 0 0 1 0 1 0 0 0 0 0 R2 0 (counter)

x3001 0 0 1 0 0 1 1 0 0 0 0 1 0 0 0 0 R3 M[x3102] (ptr)

x3002 1 1 1 1 0 0 0 0 0 0 1 0 0 0 1 1 Input to R0 (TRAP x23)

x3003 0 1 1 0 0 0 1 0 1 1 0 0 0 0 0 0 R1 M[R3]

x3004 0 0 0 1 1 0 0 0 0 1 1 1 1 1 0 0 R4 R1 – 4 (EOT)

x3005 0 0 0 0 0 1 0 0 0 0 0 0 1 0 0 0 If Z, goto x300E

x3006 1 0 0 1 0 0 1 0 0 1 1 1 1 1 1 1 R1 NOT R1

x3007 0 0 0 1 0 0 1 0 0 1 1 0 0 0 0 1 R1 R1 + 1

X3008 0 0 0 1 0 0 1 0 0 1 0 0 0 0 0 0 R1 R1 + R0

x3009 0 0 0 0 1 0 1 0 0 0 0 0 0 0 0 1 If N or P, goto x300B

5-68

Program (2 of 2)


x300A 0 0 0 1 0 1 0 0 1 0 1 0 0 0 0 1 R2 R2 + 1

x300B 0 0 0 1 0 1 1 0 1 1 1 0 0 0 0 1 R3 R3 + 1

x300C 0 1 1 0 0 0 1 0 1 1 0 0 0 0 0 0 R1 M[R3]

x300D 0 0 0 0 1 1 1 1 1 1 1 1 0 1 1 0 Goto x3004

x300E 0 0 1 0 0 0 0 0 0 0 0 0 0 1 0 0 R0 M[x3013]

x300F 0 0 0 1 0 0 0 0 0 0 0 0 0 0 1 0 R0 R0 + R2

x3010 1 1 1 1 0 0 0 0 0 0 1 0 0 0 0 1 Print R0 (TRAP x21)

x3011 1 1 1 1 0 0 0 0 0 0 1 0 0 1 0 1 HALT (TRAP x25)

X3012 Starting Address of File

x3013 0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 0 ASCII x30 (‘0’)

cosc121: computer systems: review -...

Documents