Computer Architecture
CS 110
Fall 2005
Review


Humans devise algorithms to solve
problems
Humans implement an algorithm in a
high-level programming language
• Program
• Software


Compiler translates program into machine
code
Machine executes the machine code
(version of the algorithm)
Review

Recall the “Deal a Hand of Poker”
algorithm
• For each of five cards

For each person sitting at the table
• Hand that person a card face down

There’s memory involved
• How many cards have I handed out?
• Which person gets the next card?
• What card is next on the deck?
Algorithms and Memory

Computers are simple machines
(conceptually)
• Algorithms (steps of actions)
• Memory (the subjects and objects of the
actions)

Conveniently, algorithms are stored
in memory (as programs)
• Von Neumann architecture
Algorithms and Memory

Your web browser is algorithm and
memory
• Accepts input from you (URLs)

Stores in memory
• Sends requests for html on Internet

Uses memory to remember that request is pending
• Receives html from web servers

Collects packets, reorganizes them, and stores whole
• Reads the html to identify text, pictures,
formatting instructions

Access memory
• Displays the desired information on the screen

Stores formatted web page for display
Computer Architecture

Computer (the physical device)
emphasizes two elements
• CPU

for running algorithms
• RAM, disk, flash drives, CDs

for storing memory
• Bus

A means of moving data from memory to
CPU
Computer Architecture
CPU
Bus
Memory
Computer Architecture
CPU
Instructions:
Read memory
Write memory
Bus
Memory
Computer Architecture
CPU
Example:
Read (104)
Bus
Memory
Computer Architecture
CPU
Example:
Read (104)
Bus
100
104
Memory
108
112
116
“a”
Computer Architecture
CPU
Example:
“a”
Bus
100
104
Memory
108
112
116
“a”
Memory

Conceptually
• Data that is stored as “words” and organized
with numerical “addresses”

Physically
• Data is stored on disks, chips, CDs, etc.

Abstraction
• The physical location of the memory is
irrelevant to the CPU
Combining CPU / Memory

Fetch / Execute cycle
• Basic repetitive process going on inside
computer for all tasks


CPU “fetches” next step (instruction)
of algorithm
CPU executes next step of algorithm
• A legal step is to do nothing
Combining CPU / Memory

Three basic “steps” of each
fetch/execute cycle
• Arithmetic or logical

Add, Multiply, NOT
• Memory operations

Load, store
• Control flow

If a, then b
Fetch / Execute

Fetch / Execute cycle serves to
synchronize all computer tasks
•
•
•
•
Reading and writing to memory
Displaying to screen
Acquiring user input from keyboard
Sending music to speakers
A 1 hertz (Hz) CPU executes 1 fetch/execute
cycle per second
A 1 gigahertz (GHz) CPU executes 1 Billion
fetch/execute cycles per second
Architecture