marie:an introduction to a simple computer michael dougherty 4.1 - 4.5 september 17, 2009

MARIE:An Introduction to a Simple Computer

Michael Dougherty

4.1 - 4.5

September 17, 2009

Central Processing Unit

CPUFetches Program InstructionsDecode Each InstructionPerform Operation

Divided into two partsDatapathControl Unit

Datapath

Registers

ALUs

Buses

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Registers

D Flip-Flops can be used as registersA clock pulse controls access to the data

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Registers

Registers have a fixed “word” size

Number of registers are usually a power of 2Special Purpose

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ALU

Performs the operations such as comparisons, adding, and multiplying

Generally has two data inputs and one data output

These operations often affect bits in the status register

A Simple ALU circuit

Control Unit

Sequences operations

Ensures data is at the correct place at the correct time

The BUS

The wires that connect the CPU to other components

Only one item at a time can use the bus

Speed is affected by length and number of devices sharing it.

Point-to-point bus

Point-to-point bus connects two specific devices

Common pathway

Common pathway connects a number of devices

“Multipoint” bus

Bus Protocol

Data bus

Control lines

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Address lines

Power lines

Data Bus

Data bus - the actual information to move from one location to another

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Control lines

Control lines - tell which device has permission to use the bus and for what.

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Address lines

Address lines - the location in memory that the data should be read from or written to.

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Power lines

Power lines - provides the power to the circuitry.

Bus Types

Processor-memory Buses I/O Buses Backplane Bus Personal Computer Buses

Internal Bus (System Bus) External Bus Local Bus

Just Some Wires?

Synchronous buses

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Asynchronous buses

Handshaking ProtocolReqREADReadyDATAACK

Bus arbitration

Daisy Chain Arbitration

Centralized Parallel Arbitration

Distributed Arbitration using self-selection

Distributed arbitration using collision detection

Clocks

Speed

Clock cycles

Clock Frequencies (MHz or GHz)

Clock Cycle TimeBus clocks

Overclocking

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Speed vs. Performance

Same clock speed ≠ same performance Intel 286

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Overclocking

Pushes manufacturer’s time constraints

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The I/O Subsystem

Most familiar items of computer

Not connected directly to the CPUMemory-mapped I/O

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Instruction-based I/O

No memory use

Limited CPUs can use devices