CPU BASICS, THE BUS, CLOCKS, I/O SUBSYSTEM

Philip Chan

CPU Basics

We know data must be binary-coded. We know memory is used to store data

and instructions. CPU

Fetches instructions Decodes instructions Performs sequence of operations on data

CPU Continued

All CPUs have 2 pieces: Datapath

Network of storage units (registers) and arithmetic logic units

Connected by buses Timing of data is controlled by clocks

Control unit Responsible for sequencing data at the right place at

the right time Imagine a Water Slide, and how it is managed.

Order of people, clearance to go, landing

The Registers

A Register is a place used to store a wide variety of data Addresses Program Counters Data for Program Execution

Located on Processor to be accessed quickly Since data processing is usually done on

fixed-size binary words, most computers have certain sized registers. For this reason, 16, 32, and 64bit processors

exist.

Registers Continued

Info is written, read, and transferred from register to register.

Not addressed the same way memory is addressed. Manipulated by control unit itself.

Special registers for special tasks Store Info Shift Values Compare Values Counts

No “scratchpad registers”, that control: Looping Stack pointers to info Temp Values

The ALU

Arithmetic Logic Unit Carries out all logic and arithmetic

operations Typically 2 data inputs, 1 data output All operations in ALU affect status

register bits. Told by Control Unit

Control Unit

Policeman/Traffic Manager of CPU Monitors execution of:

Transfer of all information Program Instructions

Process: Extracts from memory Decodes instructions Makes sure data is in right place at right time Tells which registers to use, and any interrupts Manipulates correct circuitry for operation

Uses a program counter to keep track and find execution, overflows, etc.

The BUS

Set of wires that acts as datapath to connect multiple subsystems in system.

Only 1 device may use a bus at a time This sharing bottlenecks instruction speed

Divided into 2 Categories; Master – Initiates commands Slave – Responds to requests of Master

Can be: Point to Point

2 Specific component conversation Common pathway

Aka Multipoint, shared across many devices

BUS Continued

Because of this method of sharing, Bus Protocols are important.

Different lines of data include: Data Lines – Actual Information To Be Moved Control Lines – Permissions for bus to be used

Interrupts, clock sync, requests Address Lines – Location data should go Power Lines – Provided electricity

Each happens within bus cycle, 2 ticks of bus clock

And Yet More BUS

Buses also divided into different types Processor-Memory

Short, high speed, closely matched to memory to maximize bandwidth

I/O Longer and allow many types of devices with

varying bandwidth Backplane

Built into chassis, connects processor, I/O Devices, Memory

And Yet More BUS

PCs have own terminology in terms of BUSes Internal

Connects CPU, Memory, Others Expansion

Connects External Devices, Peripherals, Expansions Slots, I/O Ports

Slow, but allow for generic connectivity Local

High-Speed for limited number of similar devices

And Yet More BUS Info

Physically little more than bunches of wires, have standards for connectors ,timing, and signaling.

Synchronous Only at Tick, Clocked, synchronized at clock’s rate Length of the bus imposes restrictions on rate and

time. Asynchronous

Control lines control operations Complex protocol required to enforce instruction

timing Scale better with technology and support more

devices

BUS Arbitration

Required when there is more than 1 master device Determines which Bus is used at a point in time for

what reason Daisy Chain Arbitration

Uses Grant Bus control line that is passed down from bus of highest priority to lowest.

Centralized parallel arbitration Each devices has request control line to bus, and

centralized arbiter selects who gets the bus Distributed Arbitration using Self-Selection

Similar to Centralized but instead of central authority, devices themselves choose who has highest priority

Distributed arbitration using Collision Detection Each devices allowed to make request, any collisions

mean another request is needed

Clocks

Every computer contains Clock to regulate instructions to be executed.

Without clocks speed of digital gates would be unpredictable, and regulation of programs would be impossible

Measured in Clock Cycles This is simply frequency’s reciprocal

Besides Individual Clock Cycles, There are Bus Clocks for other uses

Overclocking Pushing bounds of components to make more

efficient process execution times

INPUT/OUTPUT SUBSYSTEM

I/O Allows us to communicate with the computer

Transfer of data with peripherals and memory An Interface handles all the transfers of data 2 Types

Memory-Mapped I/O Registers in interface, no difference between

computer’s memory allocations and current I/O Devices Instruction-Based I/O

Specialized instructions for I/O No memory required, but requires specific instructions

Interrupts are essential in I/O as they notify the CPU more efficiently than atypical data calls