Introduction to Naval Weapons Engineering

Title: Computer Basics

Reading: Computer Basics Supplement pp 1-11

I. Objectives

Understand binary numbering and the number of possible combinations associated with bits and bytes

Know the three basic components of a computer

Understand basic bus architecture

Know what limits overall processing speed

Describe the characteristics of basic memory systems

Know the characteristics of various I/O interfaces

II. Introduction

Modern combat systems rely heavily on computers for their success. In fact in some ways it can be argued that the modern weapon system is actually built around the central computer which has become the most important piece of equipment in the system.

Functions of computers:

1. Retrieve, store, process and deliver information
2. Communicate with other systems
3. Develop fire-control solutions
4. Check the condition of the system

1. How do computers work?

Internal all computers work in a binary, digital environment i.e. 0,1 on/off

So lets review what a binary system of numbers is:

binary is base 2 we are familiar with base 10

binary only has 2 numbers 0 and 1

all other numbers are represented by combinations of 0 and 1 (see example p2)

each digit is a power of 2 i.e. units are 2⁰, tens are 2¹, hundreds are 2² etc

so the number 1001101 is 1 x 2⁶ + 1 x 2³ + 1 x 2² + 1 x2⁰ = 64 + 8 + 4 + 1 = 77

Bits and Bytes

Each digit in a binary number is called a bit. Computer are designed to operate with an expected number of bits. They started as 8-bit machines and now are up to 64-bit Nintendos. Each set of 8 bits is called a byte.

In an 8 bit machine, we have 8 digits , so the maximum number of combinations available are 2⁷ + 2⁶ + 2⁵ + 2⁴ + 2³ + 2² + 2¹ + 2⁰ = 128 + 64 + 32 + 16 + 8 + 4 + 2 + 1 + all zeros = 256

In general the formula is

N = 2^b

where N = number of combinations (or numbers)

and b= number of bits the more bits, the more combinations

Two bytes together as in a 16 bit machine make up a word , 32 bit machines are 4 bytes which is a double word and 64 bit machines are 8 bytes which is a quad word.

Components

Computers vary but all will have some basic components such as a CPU, memory, interconnecting bus and an input/output (I/O) interface.

CPU:

This performs the operations.

Binary at most fundamental level.

Truth tables (see p4)

AND, NOT, +, -, ,

(if you add two numbers which together exceed the bit capability of the machine you get the classic "overflow error"

Inside the CPU are four components:

1. Control Unit - manages the overall operation of the CPU
2. Arithmetic-Logic Unit - performs basic arithmetic and logic operations
3. Internal memory - temporary storage for data during calculations and keeps the address of the program instructions
4. System Bus - connects the other three components

CPU operates on a 3 step cycle

Fetch - obtain next instruction and store

Decode - interpret instruction and collect any other necessary data

Execute - carry out the instruction and store result in internal memory

BUS:

Connects all components. Three parts:

Address Bus

Data Bus

Control Bus

BUS works on a 4 step cycle

The location of the data is placed on address bus

The address is stored in temporary memory (frees address bus)

Data is retrieved and placed on data bus

Data is sent to CPU

The timing of the bus is controlled by a clock. e.g. 200 MHz means 200,000,000 clock cycles per second. Each clock cycle is a step in the operating process. So 200M steps/sec

Each step could involve 64 bits so you could have 64 x 200M = 12.8 gigabits/sec

MEMORY

ROM - Read only memory - programmed by UV light (EPROM) or voltage (EEPROM) e.g. CD-ROM's

RAM - Random Access Memory - read and write capability

2 types of RAM

SRAM - Static RAM - stays even after power is turned off - doesn't require refreshing

DRAM - Dynamic RAM - requires constant refreshing

Access time: time it takes to retrieve a piece of data. SRAM is faster but more expensive and larger

If the access time is longer than a clock cycle then the system has to build in a pause while waiting for the memory to access.

e.g. a Pentium 233MHz system has a clock cycle of 1/233MHz = 4 ns which means it can perform an operation every 4 ns. If the memory access time is 10 ns the system has to wait 6 ns between operations in order to wait for the memory to access.

Capacity: how much data can be stored. Measured in bytes (8 bits)

Transfer Mode: how data is sent after first request (eliminates cycles)

EDO - 2 cycles after first request

Cache: high-speed (low access time) memory of limited capacity. (SRAM) It stores most frequently accessed data to save time. System checks cache first then RAM

I/O:

Modems, Monitors, Hard Disks, CD-ROM's, Mouse, Keyboard, Printer, etc

You can have a lot of compatibility problems with I/O devices because they are all built by different manufacturers, are usually analog and there is no industry standardization of the interfaces.

Serial Mode - all the bits are sent on 1 line, in order. Slow

Parallel Mode - bits are sent on separate lines (e.g. 32 bit - 32 lines) Fast

Synchronous Transfer - the I/O device and the CPU keep a common timing between them so that data can be sent at any time.

Asynchronous requires a start and stop command to let the computer know that data is coming.