Computers
Internal and
External
Memory
1
Characteristics of
Computer Memory
Location
Capacity
Unit of transfer
Access Method
Performance
Physical Type
Organization
2
UNIVAC Console and CPU
3
Memory Hierarchy
COST
SPEED
REGISTERS
CACHE
MAIN MEMORY
MAGNETIC DISK | DISK CACHE
MAGNETIC TAPE | OPTICAL DISK
5
Capacity Units
Bit
Byte
Kilobyte
Megabyte
Gigabyte
Terabyte
b
B = 8 b
KB = 1,000 B
MB = 1,000,000 B
GB = 1,000,000,000 B
TB = 1,000,000,000,000 B
6
Memory Access Method
Sequential - tape
Direct - floppy or hard disk
Random - internal memory
Dynamic (DRAM)
simple, small, must be refreshed
Static (SRAM)
no refresh needed
Associative - some cache
7
Random Access Memory
Chips and chip technology
8
Flip Flop Circuit Diagram
9
Memory Cells
CONTROL
read or write
SELECT
select cell
1
0/1
DATA IN / SENSE
input or output
10
Write to Memory
Read Enable
Write Enable
Row Address
Col Address
A0
.
.
.
A10
ROW
ADDRESS
BUFFER
COL
ADDRESS
BUFFER
ARRAY
MUX
REFRESH
DATA
INPUT
BUFFER
DATA
OUTPUT
BUFFER
D1
.
.
D4
11
Pin Assignments
A0 … A10: address location (multiplexed)
D1 … D4: data in or out
Vcc: power supply
Vss: ground
RAS: row address select
CAS: column address select
WE: write enable
OS: output enable
12
Cache Operation
ALU
CNTL
.....
CACHE
BUS
MAIN MEMORY
13
14
Cache
CPU
Word
CACHE
Block
Main Memory
15
Cache
Slot Tag
Number
0
Block
1
2
3
16
Random Access
Direct Access Storage Device (DASD) or
disk drives
Optical
Magnetic
17
Optical Disks (CD’s)
18
How CD’s work
19
Cylinder/Track/Block
Track
Block (Sector)
Cylinder
20
Disk
Organization
21
DASD Structure
Read-Write Heads
22
Data Storage
FAT (File Access Tables), Directories and
Catalogs
Update and Delete
Fragmentation and reorganization
Blocks, Headers and Interblock Gaps
23
DASD Access (PC)
MAIN MEMORY
CPU
BUFFER
CACHE
DASD
CONTROLLER
24
25
DASD Access (Mainframe)
MAIN MEMORY
CPU
BUFFER
CHANNEL
DASD
CONTROLLER
27
Data Structure
BLOCK
HEADER
DATA
CRC
•
Header written when disk is formatted
Data copied into block
•
Cyclical Redundancy Check calculated
•
28
Controller Operation
(DASD retrieval)
CPU passes parameters to registers in the
controller
The controller transfers data into the card
buffer
The controller checks the CRC to assure
the data was copied correctly
The controller (or CPU) transfers buffered
data to memory one word at a time
29
Time Units
Millisecond
Microsecond
Nanosecond
Picosecond
ms = 1/1000 s
µs = 1/1,000,000 s
ns = 1/1,000,000,000 s
ps = 1/1,000,000,000,000 s
32
Representative Times
Network speed = megabits per second
Disk transfer = megabytes per second
Disk access = milliseconds
Memory access = nanoseconds
Machine cycle =
microseconds/nanoseconds
33
Error Correction and
Checking
Check
Block
Body
Header
Add bits to a block to use for error
discovery
Detection only
Detection and retransmission
Detection and recovery
34
Error Detection Only
(Asynchronous Transmission)
*
Parity Bit
*
*
*
*
*
*
*
7 Data Bits
27 = 128 distinct characters
35
Error Detection &Correction
(Hamming Code: 4 bit word)
*
*
*
*
*
*
*
3 Error Checking Bits
4 Data Bits
36
Error Detection &Correction
(Hamming Code: 4 bit word)
DATA
*
*
*
1
1
1
0
1
1
1
0
37
Error Detection
PARITY (even)
1
0
0
1
1
1
0
1
1
0
1
1
0
0
38
Error Correction (4 bit word)
PARITY (even)
1
0
0
0
1
1
0
1
1
0
0
1
0
0
39
Error Correction &
Detection
Error detection takes fewer bits than error
correction
Longer packets take a smaller percent for
correction but have more types of errors
Hamming’s scheme detects all errors at a
high overhead cost; others may correct
only single bit or double bit errors with
shorter check fields
40
CRC Error Checking
The transmitted messages are divided
into predetermined blocks
The blocks are divided by a fixed divisor
The remainder is appended to the
message
41
IBM 1107 with tape drives
42
Memory
43