L9 Memory - Computer Science 230H Memory Prepared by...

Computer Science 230H
Memory
Prepared by Michael Jack
Fall 2007
Slides set 1

Memory 2
References
• The information and figures for the following slides were prepared
from the following source:
– Patterson, D., and Hennessy J., Computer Organization and
Design, 3rd edition, 2005
– Stallings, W., Computer Organization and Architecture: Designing
for Performance 7
th
edition, 2006
– Hircock B., Computer Science 230 Course Notes

Memory 3
Introduction

Memory 4
• Processor processing speeds are much faster than memory access time.
Basic Problem
Year
Performance
1
10
100
1,000
10,000
100,000
CPU
Memory
• Design challenge: dealing with this growing disparity.

Memory 5
• Historically, the limiting factor in a computer’s performance has been
memory access time
– Memory speed has been slow compared to the speed of the processor.
– A process could be bottlenecked by the memory system’s inability to “keep
up” with the processor
– Why?
Basic Problem (cont)

Memory 6
Location Performance
Processor Access time
Internal (main) Cycle time
External (secondary) Transfer rate
Capacity Physical Type
Word size Semiconductor
Number of words Magnetic
Unit of Transfer
Optical
Word Magneto-Optical
Block
Physical Characteristics
Access Method
Volatile/nonvolatile
Sequential Erasable/nonerasable
Direct
Organization
Random
Associative
Characteristics of Computer Memory Systems

Memory 7
• Word
– The “natural” unit of memory organization.
– The size of the world is typically equal to the number of bits used to represent
an integer and to the instruction length.
• Addressable unit
– The fundamental data element size that can be addressed in the memory.
– Might either be a word or a byte.
– The relationship between the length in bits A of an address and the number N
of addressable units is 2 A = N .
• Unit of transfer
– The number of data elements transferred at a time.
– Usually words between CPU and cache, blocks between cache and memory,
pages between memory and secondary storage.
Characteristics of Computer Memory Systems

Memory 8
• Sequential access
– Memory is organized into units of data, called records.
– Access is made in a specific linear sequence.
• Direct access
– Data items have unique addresses.
– Access is done using a combination of moving to a general memory “area”
followed by a sequential access to reach the desired data item.
– Example: disk drives.
• Random access
– Each addressable location in memory has a unique physical address.
– Locations can be accessed in any order and all access times are the same.
– Example: main memory.
Characteristics of Computer Memory Systems

Memory 9
• Associative access
– A variation of random access memory.
– Data items are accessed based on their contents rather than their actual
location.