Memory
Key component of a computer system is its
memory system to store programs and data.
ITCS 3181 Logic and Computer Systems 2014 B. Wilkinson Slides12.ppt
Modification date: Nov 11, 2014
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Requirements
1. Able to store accessible binary words (patterns of 0’s and 1’s) by
electrical, magnetic or other means.
2. For storing programs currently being executed and storing
associated data, need to be able to access any storage location in
the memory at will in any order with high speed, i.e. memory
must be random access memory (RAM).
3. For data, need capability of both reading and writing information.
4. For program execution, the capability of reading sufficient.
5. Usually some of the memory at least must be non-volatile, i.e. the
information is not lost when the power is removed.
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Memory Hierarchy
Historically, memory organized in levels of decreasing speed but
decreasing cost/bit:
• Cache memory relatively small semiconductor memory operating
at a speed that matches the processor
• Main memory – semiconductor random access memory
• Disk memory – not random access but not volatile.
• CD drives, tape drives, … – not random access but not volatile.
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Semiconductor RAM
Memory bus
Memory chips
These are very old memories – new ones are similar in concept.
Above actually shows the two organizations, see next.
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Semiconductor Memory Organizations
x 1 organization
Each chip organized to hold one bit of each word.
Memory chip
Memory cell
(St ores one bit)
n
n to 2
dec oder
Ro w
select
lin es
Me mory
add res s
lines
Co lumn
select
lin es
Data
(in/out)
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Semiconductor Memory Organizations
x n organization
Each chip organized to hold n bits of each word, where n is usually 4 or
8.
Memory chip
Memory
address
lines
Row
select
lines
Column
select
lines
Data
(n data in/out lines)
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Types of Semiconductor (RAM) Memory Cells
Two basic types devised:
• Static RAM (SRAM)
• Dynamic RAM (DRAM)
Static RAM faster operation than dynamic RAM (around 10 times
faster) but requires more internal components (see next).
Hence DRAM has more memory cells in chip (around 4-8 times
capacity).
Usually main memory is dynamic and cache memory usually static.
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Static RAM
Each cell - an S-R flip-flop formed with pair of cross-coupled gates.
Column j
Column j
Row i
S-R flip-flop
Q
Memory cell
Transistor
Q
Transistor
operates as
a switch
Two complementary column lines used also to read stored value
(Q/ Q) and write new values (Q/Q operating as S/R).
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Static RAM Memory Cell Circuit
Column j
Column j
Row i
Memory cell
Transistor
Transistor
operates as
a switch
Design requires 4 or 6 transistors and two column select lines per column.
Do not worry about its operation. Key point is number of components.
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Dynamic RAM cell circuit
Each memory cell uses a capacitor to store charge. Two states,
charged and not charged represent the two binary values 0 and 1.
Less components, higher memory density. Usual design:
Column j
Row i
One column line
This capacitor is
extremely small
(< 0.04 pF)
C
Memory
cell
Because charge will decay, memory cell must be refreshed periodically, typically
every 2-4 ms. Refresh consists of reading data and rewriting it. Usually done
automatically within chip.
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Asynchronous dynamic random access
memory
Original (1970’s) DRAM were “Asynchronous” - emory
operation not synchronized to processor clock - processor
makes request for a memory location and waits whatever
time it takes to access location.
Asynchronous dynamic random access memory (DRAM)
operates with access times of 50-70 ns. (originally 100-150
ns).
Access time - time to select memory location and obtain
stored information or write new information.
1ns = 10-9seconds
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Synchronous DRAM (SDRAM)
Introduced in the last few years replacing original
asynchronous DRAM but still using same basic dynamic
memory cell design. Access time of individual memory cell
essentially the same.
Speed improvement obtained by synchronizing memory
operation with processor and a burst mode in which a group
of sequential locations accessed one after the other at high
speed after an initial read/write latency.
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Other Types of Semiconductor Memory
Read-Only Memory (ROM)
Memory cells store binary values which can only be read
during normal operation. Binary values established either
during manufacture of device or subsequently depending
upon type of ROM. This memory is still random access.
Key feature - memory is non-volatile, that is, information
not lost when power removed and can be obtained after
power returned.
Very useful for bootstrap programs - small necessary
programs that are executed when the computer is turned
on to start the computer.
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General Arrangement of ROMs
Column lines
Connection* made
or not made
(say corresponds
to 1 or 0)
Row
lines
Memory cell
Various ways connection
can be made depending
upon type of ROM
Column
select
Data in/out
* Connection has to be such to allow
current to flow in one direction only.
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Types Read-Only Memory
Several types:
1. Fixed Read-Only Memory (ROM)
Memory cells store fixed binary values permanently - as defined
during manufacture.
Column line
Row line
Direct connection*
made during
manufacture if a 1 is
required, no
connection if a 0.
(Could be other way
round.)
Memory cell
* has to be such to allow current to flow in one direction only - typical transistor used.
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2. Erasable Programmable Read-Only Memory
(EPROM)
Memory cells stores fixed binary values but can be altered by user.
Loosely based upon previous dynamic cell design using very small
capacitors that are charged to represent one binary state and
unchanged for the other binary state, but the semiconductor
construction allows the charge to remain for many years. In early
designs, charge remains for maybe 10 years if not released.
Can be re-programmed with different data. Normally not altered
while used as normal memory of the computer.
First need to erase information and then re-write required
information.
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UV EPROM
Original EPROM (1970’s) -- Erasure done by shining ultra violet
light thro window in chip which releases charge in entire memory.
UV light
Memory package
Then, writing done electrically.
Electrically Erasable Programmable Read-Only Memory (EEPROM)
More recent EPROM in which the information can be removed by
electrical means as well as writing. Still this is not done in normal
computer operation.
Flash memory - a version of EEPROM in which the entire memory is
erased electrically rather than individual bytes.
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Questions
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