An Introduction to

1
Chapter 2

• An Introduction to
• Computers
• Microcomputers
• Microprocessors

2
Types of Computers

• Mainframes
• largest and most powerful
• massive amounts of memory
• use large data words64 bits or greater
• mostly used for military defense and large
  business data processing
• examples IBM 4381, Honeywell DPS8
• Minicomputers
• scaled-down mainframes
• smaller data words32 bits typically
• used for business data processing, industrial
  control, and scientific research
• examples DEC VAX 6360, Data General MV/800II,
  Sun Sparcstation

3

• Microcomputers
• range from small controllers that work with 4 bit
  words to the PCs we are familiar with that work
  with 32 bit words
• modern microcomputers are becoming
  indistinguishable from early minicomputersfunctio
  nally speaking
• large variety of uses from specialized controls
  like a printer to personal publishing
• the CPU is usually 1 Integrated Circuit (IC)
  called a microprocessor
• examples Intel 8051 controller chip, IBM PC,
  Apple Macintosh
• Supercomputers
• fastest and most powerful mainframes
• contains multiple central processors
• used for scientific applications, and number
  crunching
• now have teraflop performance

4
Timesharing Systems

• A computer, usually a mainframe or mini, can
  serve many users by dividing its time among the
  users
• based on the fact that the rate at which a user
  enters data is extremely slower than the rate at
  which the computer can process that data
• this is usually transparent to the user

5
Multitasking Systems

• A computer can appear to be doing many tasks at
  the same time
• becausethe computer is much faster than the
  machines connected to it, and the processes
  controlling those machines

6
A Bottleneck

• The mainframe or minicomputer becomes
  saturated/overloaded
• computer response time increases as the number of
  users and the amount of work increases
• What if something happens to the computer?
• Work stoppage
• Potentially lots of lost data in case of a disk
  crash
• What about maintenance?

7
Relieve the Bottleneck

• Distribute the work!
• This is a distributed processing system
• Each user has a microcomputer (PC) connected over
  a LAN (Local Area Network) or even a WAN (Wide
  Area Network)
• Now they can do some computing locally, and use
  the central computer as needed

8
A Distributed Environment
9
Microcomputer Structure

• Central Processing Unit (CPU)
• Memory
• Input/Output (I/O) circuitry
• Buses
• Address bus
• Data bus
• Control bus

10
Memory

• A mixture of RAM and ROMmay also include
  magnetic hard disks and optical disks
• 2 purposes of memory
• store the binary codes for the sequence of
  instructions specified by programs
• store binary data that the computer needs to
  execute instructions

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I/O

• the way the computer communicates with the
  outside world
• peripherals are connected to the I/O ports
• printers, modems, keyboard, mouse, scanner
• Universal Serial Bus (USB)
• ports
• physical devices needed to interface with the
  computers internal buses
• actually a set of D flip-flops connected in
  parallel
• how do we distinguish between an input port and
  an output port?

12
CPU

• the brains of the computer
• its job is to fetch instructions, decode them,
  and then execute them
• contains
• an Instruction Pointer register which contains
  the address of the next instruction
• general purpose registers for temporary storage
• circuitry to generate signals to the control bus

13
Address bus

• consists of 16,20, 24, or 32 parallel signal
  lines (wires)
• these lines contain the address of the memory
  location to read or written
• just how many unique addresses can an address bus
  specify?

14
Data bus

• consists of 8,16, or 32 parallel signal lines
• these are bi-directional meaning that data can
  be read from/written to either memory or a port
• only one device at a time can have its outputs
  enabled, even though many will have their outputs
  connected to the same data bus
• this requires the devices to have three-state
  output

15
Control bus

• consists of 4 to 10 parallel signal lines
• CPU sends signals along these lines to memory and
  to I/O ports
• examples Memory Read, Memory Write, I/O Read,
  I/O Write

16
A Typical CPU
17
Typical CPU Behavior

• Goal
• Read a word of data from a memory location
• Process
• CPU first sends out the address along the address
  bus to the memory device
• CPU then sends the Memory Read signal along the
  control bus
• the output from the memory device travels back to
  the CPU along the data bus

18
Executing a 3 Instruction Program

• 1. Input a value from a keyboard connected to
  the port at address 05H.
• 2. Add 7 to the value read in.
• 3. Output the result to a display connected to
  the port at address 02H.

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Executing a 3 Instruction Program (cont.)
20
Microprocessor Evolution

• microcomputers are commonly categorized by the
  number of bits that their ALU can work with at a
  time
• regardless of the number of address lines or data
  lines
• the first commercial microprocessor was the Intel
  4004a 4-bit device combined with other devices
  to make a calculator
• next came the Intel 8008an 8-bit device, but it
  required many additional devices to be a
  functional CPU
• Intel 8080another 8-bit device, but it only
  required 2 additional devices
• it also used different transistors making it much
  faster, and started the 2nd generation of
  microprocessors
• then Motorola entered the market with the MC6800

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Directions taken by Microprocessors

• Embedded controllers
• used to control smart machines
• printers, auto braking systems
• also called microcontrollers
• Bit-slice processors
• custom-designed hardware and custom-designed
  instruction set made by connecting deviceseach
  part becomes a slice needed for a specific
  application
• these were created because general-purpose CPUs
  were not fast enough or did not have a rich
  enough instruction set

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General Purpose CPUs

• Intel released the 8086, a 16-bit microprocessor,
  in 1978
• Motorola followed with the MC68000 as their
  16-bit processor
• the 16-bit processor works with 16 bit words,
  rather than 8 bit words
• instructions are executed faster
• provide single instructions for more complex
  instructionsmultiply and divide

23
32 bit Processors

• 16 bit processors evolved into 32 bit processors
• now able to work with gigabytes (109 bytes) and
  terabytes (1012 bytes)
• Intel released the 80386
• Motorola released the MC68020

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The 8086 Microprocessor Family

• Characteristics
• 16 bit microprocessor
• 16 bit data bus
• it can read from or write to memory and I/O ports
  either 8 or 16 bits at a time
• 20 bit address bus
• it can address 220 memory locations
• each location is 1 byte (8 bits) wide, thus 16
  bit words will require consecutive memory
  locations

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Members of this family

• 8088
• same as 8086 but has an 8 bit data bus
• 80186/80188
• enhanced instruction setbut still backwards
  compatible
• 80286
• designed for a multi-user, multi-tasking
  microcomputer
• users a virtual address mode to prevent collision
  of users programs
• 80386/80486
• first Intel 32 bit processor
• can directly address up to 4 GB of memory

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Internal Architecture

• Bus Interface Unit (BIU)
• handles all transfers of data and addresses on
  the buses for the execution unit
• Execution Unit (EU)
• where stuff really happens
• tells BIU where to fetch an instruction, decodes
  it, and then executes it

27
Execution Unit (EU)

• contains control circuitry for internal
  operations
• a decoder for translating instructions
• a 16-bit ALU
• registers

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The Flag Register

• a 16 bit register containing nine active flags
• 6 conditional flags carry, parity, aux carry,
  zero, sign, and overflow
• 3 control flags trap, interrupt, and direction
• a flag is a flip-flop which indicates some
  condition produced by executing an instruction,
  or controls certain operations

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General Purpose Registers

• there are 8 8-bit general purpose registers
• AH, AL
• Accumulator
• BH, BL
• Base
• CH, CL
• Count
• DH, DL
• Data
• can pair these together to store 16-bit
  words...AX, BX, CX, DX

30
The BIU Queue

• if the BIU is not busy...the EU is decoding or
  executing without the need of any buses...it can
  pre-fetch up to 6 instruction bytes
• these instructions are stored in a FIFO register
  set called a queue
• an example of pipelining
• doesnt provide an advantage when the instruction
  being executed is JMP or CALL

31
Segment Registers

• the 220 bytes of addresses are divided in to 4
  64KB segments
• code segment - CS register stores upper 16 bits
  of its starting address
• where instructions are coming from
• stack segment - SS register stores upper 16 bits
  of its starting address
• stores addresses and data while a subprogram is
  executed
• data segment - DS register stores upper 16 bits
  of its starting address
• holds data
• extra segment - ES register stores upper 16 bits
  of its starting address
• holds more data
• the lower 4 bits are always zero! why?
• example if, the starting address of the code
  segment is 348A0, then CS contains the value
  348AH...this is called the segment base
• the segments can overlap in the case of small
  program

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Instruction Pointer Register (IP)

• contains the 16-bit address of the next code byte
  in the code segment
• this is called an offsetit must be added to the
  address contained in the CS register, which is
  called the base
• example
• CS contains the value 348AH
• IP contains the value 4214H
• whats the physical address of the next code
  byte?
• you can represent an address in segment
  baseoffset form348A4214

33
Stack Pointer Register (SP)

• Holds the 16-bit offset from the start of the
  stack segment where a word was most recently
  stored
• the top of the stack
• add this offset to the contents of the SS
  register to get the physical addresssound
  familiar?
• you can use SSSP notation to represent physical
  address

34
Types of Programming Languages

• Machine code
• a series of 0s and 1s representing instructions
  to be executed
• machine specific
• Assembly language
• not quite so cryptic
• typically use a 4 field representation label
  field, opcode field, operand field, comment field
• example NEXT ADD AL, 07H Add correction
  factor\
• translated into machine code
• often used when working very close to the
  hardware
• High level language
• english-like, I.e. BASIC, Pascal, C, C
• easier and often quicker to write
• requires a compiler (translator) to translate
  into machine code
• can embed assembly instructions in some higher
  level languages

35
Accessing Immediate and Register Data

• addressing modes
• specify the different ways the processor can
  access data
• immediate addressing mode
• MOV CX, 437BH
• puts the specified value, 437BH directly into CX
• can you do this in 2 instructions?
• register addressing mode
• MOV CX, AX
• a register is the source of the operand
• MOV CX, BL ...legal?
• direct addressing mode
• MOV CX, 437AH
• MOV 437AH, CX
• an effective address (offset) is the source of
  the operand

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Why Use Segmented Model?

• Easier to manipulate 16 bit numbers than 20 bit
  numbersone less byte to deal with
• Easier to timeshare and multitask
• each users process can get its own logical
  segments for its code and data
• these values are stored in the Process Control
  Block (PCB) for a process when it is swapped out.