CPU Lecture from the November 1, 1996 main meeting

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Computer Basics - The C.P.U.

Bruce Arnold

COMPUTER:

• A device capable of performing a series of arithmetic or logical operations and controlled by a written program.

The four major physical components:

• Central Processing Unit (CPU)
• Main storage - RAM
• Auxiliary storage - ROM, CDROM, DISK, and TAPE
• Input/Output devices - Keyboard, Screen and Printer

CPU

• Central Processing Unit
• Contains the CIRCUITS for arithmetic and logical operations and for controlling other parts of the system.

RAM

• Random Access Memory
• Millions of flip-flops (devices that are either ON or OFF.)
• Data is usually lost whenever the computer is turned off.
• Also called "Read & Write Memory".

ROM & CDROM

• Read Only Memory
• Compact Disk Read Only Memory
• Memory is fixed and cannot change.

DISK & TAPE

• Magnetic disk drives, floppies, and tape drives.
• Data remains permanent even if power is removed.
• Data may be written any number of times.

Keyboard

• An input device consisting of many switches.

Screen & Printer

• Output devices
• Logic and hardware
• Capable of making alpha-numeric characters and graphics appear on a video screen or on paper copy.

Understanding the CPU

• Basic parts
• Let's make one!
• A sample program

Basic parts

• BUS UNIT - Reads program and data.
• INSTRUCTION UNIT - Decodes instructions.
• EXECUTION UNIT - Acts on instructions using Registers, Control Logic, and A.L.U.
• A.L.U. - Arithmetic Logic Unit

Let's make one! - A sample CPU

• Five (5) Registers -
• AX, BX, CX, F, IP
• Seven (7) Instructions -
• ADD, MUL, MOV, DEC, JZ, JNZ, PRT
• Data - 16 bit information
• Numeric Range - 0 to 65535

Instruction Set

• ADD reg, reg ..........; Add 2 registers
• MUL reg, reg ..........; Multiply
• MOV reg, reg ..........; Move
• MOV reg, immediate ....; Assign value
• DEC reg ...............; Decrement by 1
• JZ step ...............; Jump if zero
• JNZ step ..............; Jump if not zero
• PRT reg ...............; Mystery Print

A sample program - "Factorial 6"

• MOV AX,1
• MOV CX,6
• MUL AX,CX
• PRT AX
• DEC CX
• JNZ -4
• PRT CX

Machine Language - Factorial 6

• :0100 B80100
• :0103 B90600
• :0106 F7E0
• :0108 40
• :0109 49
• :010A 75F3
• :010C 41

Quick Basic - Factorial 6

• LET AX = 1
• FOR CX = 6 TO 1 STEP -1
• AX = AX * CX
• PRINT AX
• NEXT CX
• PRINT CX
• END

C Language - Factorial 6

void main() 

{ 

unsigned int AX = 1, CX;

for ( CX=6; CX > 0; CX-- ) 

{ 

AX = AX * CX;  printf ("%d \n", AX); 

} 

printf ("%d \n", CX); 

} 

The C++ CPU Emulator Program

• An IBM compatible software program which will "run" the "Factorial 6" program code.
• Based on the Instruction Pointer (IP), the emulator steps through each line of the program, decodes the instruction and executes it.
• CPU.cpp (to be discussed at the November CSIG meeting, 3rd Tuesday.)

CPU Emulator Program - FLOW

• READ the instruction where the IP points.
• DECODE or INTERPRET the instruction.
• EXECUTE the instruction.
• MODIFY registers (IP, etc.) as required.
• Repeat forever

More information

• Intel Books and Manuals
• Other Vendors Manuals
• General Computer Books
• ACGNJ SIG's
• C/C++ Users Group