1. CS149D Elements of Computer Science
Ayman Abdel-Hamid
Department of Computer Science
Old Dominion University
Lecture 6: 9/12/2002
Lecture 6: 9/12/2002
CS149D Fall 2002

2. Outline Chapter 2 Data Manipulation Central Processing Unit
Machine Instructions
Stored-Program Concept
Program Execution
Lecture 6: 9/12/2002
CS149D Fall 2002

3. Central Processing Unit1/2
CPU
Consists of 2 parts: Arithmetic Logic Unit (ALU) & Control Unit
Contains registers for temporary storage of information
General-purpose registers
Temporary holding places for data manipulated by CPU
Special-purpose registers
To perform an operation on data stored in main memory
Control unit transfers data from memory into registers
Inform ALU which registers hold data
Activate appropriate circuitry within ALU to perform the desired operation
Tell ALU which register should receive the results
Lecture 6: 9/12/2002
CS149D Fall 2002

4. Central Processing Unit2/2
Memory Facilities available
Registers (on CPU, hold operation operands)
Cache (high speed memory faster than main memory)
Often on CPU
Exploit principle of locality
Simply keep contents of most recently accessed memory locations
Main memory
Secondary memory (mass storage)
Lecture 6: 9/12/2002
CS149D Fall 2002

5. CPU/Memory Interface Main Memory CPU bus Example of adding 2 values
Read data by supplying memory cell address
Write data by supplying memory cell address
Example of adding 2 values
Get first value from memory and place in a register R1
Get second value from memory and place in another register R2
Activate addition circuitry with R1 and R2 as inputs and R3 to hold results
Store contents of R3 (result) in memory
Stop
Lecture 6: 9/12/2002
CS149D Fall 2002

6. Machine Instructions1/2
Data transfer Movement of data from one location to another
LOAD fill a register with contents of a memory cell
STORE transfer contents of a register to a memory cell
Arithmetic/Logic
Arithmetic operations
Logic operations AND, OR, XOR
SHIFT, ROTATE
Control direct execution of program
JUMP direct control unit to execute an instruction other than the next one
Unconditional Skip to step 5
Conditional If resulting value is 0, then skip to step 5
Lecture 6: 9/12/2002
CS149D Fall 2002

7. Machine Instructions2/2
Example for a conditional JUMP Division
1- LOAD a register R1 with a value from memory
2- LOAD register R2 with another value from memory
3- If contents of R2 is zero, JUMP to step 6
4- Divide contents of R1 by contents of R2, result stored in R3
5- STORE the content of R3 in memory
6- STOP
Lecture 6: 9/12/2002
CS149D Fall 2002

8. Stored-Program Concept
Program (instructions) stored in memory instead of being built into the control unit as part of the machine
A machine recognizes a bit pattern as representing a specific instruction
Instruction consists of two parts
Op-code (operation code) & operand(s) field(s)
STORE operands would be
Register containing data to be stored
Address of memory cell to receive data
Lecture 6: 9/12/2002
CS149D Fall 2002

9. A Sample Machine Architecture
CPU
ALU
register
Program
Counter
Instruction
Register 1 F
Main Memory 00 01 02 FF
256 memory cells with a capacity of 8 bits each
16 general-purpose registers
2 special-purpose registers
Bus
Program Counter address of next instruction to be executed
Instruction Register hold instruction being executed
Lecture 6: 9/12/2002
CS149D Fall 2002

10. A Sample Machine Architecture
Instruction Format
Instruction consists of 4 hex digits (2 bytes)
Op-code Operand Fields
LOAD register 3 with contents of the memory cell at address 4716
Textual representation might be “LOAD R3,47”
B25816
JUMP to instruction at address 5816 if contents of register 2 is the same as register 0
Lecture 6: 9/12/2002
CS149D Fall 2002

11. Program Execution JUMP Instruction B25816 2. Decode 1.Fetch 3. Execute
Decode bit pattern in instruction register
1.Fetch
Retrieve next instruction from memory (as per program counter) and then increment program counter
3. Execute
Perform action requested by instruction in instruction register
Machine cycle
Lecture 6: 9/12/2002
CS149D Fall 2002