The primary storage section
2 fundamental functions : • Storing
• Retrieving
Characteristics : • Capacity (Mbytes Æ Gbytes)
Trigger
• Functional speed (10 ns Æ 1 ns)
• Working mode
new address
data
definition Trigger
access
acces
time
t
Memory cycle
Effective cycle
First Year University Studies in Science. ULB . Computer Principles. Chapter 5
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The solid state memories
Dynamic Random Access Memory : DRAM
• Information stored in capacitors driven by a transistor
• Destroyed when read
• Need to be refreshed
Data (1 bit)
Address lines
A
n lines for a 2n bits
d
capacity
d
r
bit 1 Æ charged
e
capacitor
Chip select (trigger signal) s
s
CS
Control
WR
Write/read signal
First Year University Studies in Science. ULB . Computer Principles. Chapter 5
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Internal organisation
Rows
000
001
010
64 (26) bits memory : multiplexing
reduces the
number of lines
by 2 (3 lines for
64 bits)
011
100
101
Refresh every
millisec.
110
Row/
Column
Address
Selection
111
RAS
CAS
WR
Refresh and control circuitry
Columns
111 110 101 100 011 010 001 000
First Year University Studies in Science. ULB . Computer Principles. Chapter 5
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Organisation in bytes
bit
7
R0
R1
R2
R3
R4
R5
R6
R7
R8
6
5
4
3
2
1
0
WR
RAS
CAS
C4C3 C2C1 C0
First Year University Studies in Science. ULB . Computer Principles. Chapter 5
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Static RAM (SRAM)
Bit line
0V
Vcc(5V)
Bit line
5V
Address
lines
A
d
d
r
e
s
s
CS
CS
D
a
t
a
Data lines
(bytes)
WR
• Flip-flop always in stable state
• No refresh needed (static !)
• Information lost when power cut
• Higher number of components
First Year University Studies in Science. ULB . Computer Principles. Chapter 5
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Other types of memory
• Read Only Memory (ROM) :
• Switches burned in (permanent masks)
• Information kept when power cut
• Ex.: Bootstrapping
• Programmable ROM (PROM)
• User defined data (instructions)
• Slow write operation (~ 1 µsec)
• Can be written once
• Erasable PROM (EPROM)
• Many writes
• Erased with UV light
• Electrically Erasable PROM (EEPROM)
• Erased with a special electrical signal (slow !!)
First Year University Studies in Science. ULB . Computer Principles. Chapter 5
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Outside the primary store …
• The registers :
• Inside the ALU : The accumulator
• Inside the Control unit :
o The Program Counter (PC)
o The Instruction Register (IR)
o The Status Register (SR)
o The working registers (PowerPC : R01 Æ R31; FR01 Æ FR31)
• The stack :
word
• Stack Pointer (SP)
SP
new data
address of last entered data.
SP
last data
• The cache memory :
data
• Very fast memory storing blocks
data
of intructions of the executing task
data
• Pseudo (RAM) disks :
bottom
• Slow memory with very large capacity
Memory
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Evolution …
The Moore Law :
capacity x 4 every 3 years
capacity (Kbytes)
10,000,000
Future :
• GaAs : speed x 7
but stability
• Biochips : ADN mimics
• 3D
1G
1,000,000
100M
100,000
7.5M
10,000
1M
1,000
64K
100
10
6K
1
1970 1975 1980 1985 1990 1995 2000 2005 2010 2015
year
First Year University Studies in Science. ULB . Computer Principles. Chapter 5
D. Bertrand 8
Memory locations and addresses
Each memory position is identified
by an address :
Address
n bits
The standard capacity unit is the byte :
2 schemes to go from the byte to the word address :
byte
byte
word
word
0
1
2
i
bn-1
b2 b1 b0
2k-1
Memory
0
0
1
2
3
0
3
2 1
0
4
4 5
6
7
4
7
6
4
8
2k-4
5
8
2k-4 2k-3 2k-2 2k-1
2k-4
2k-1 2k-2 2k-3 2k-4
Big-endian assignment Little-endian assignment
First Year University Studies in Science. ULB . Computer Principles. Chapter 5
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Addressing modes …
• Absolute addressing :
Physical location
address Æ
• Relative addressing :
Offset from a physical location
• Indirect addressing :
Address from another location
start address Æ
+
n
final address Æ
start address Æ
final address Æ
address
• Implicit addressing :
Addresses of registers or other
special memories
• The stack :
Stack Pointer (SP)
• The information strings :
≡ relative addressing (offset from 1st character)
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