1. Internal and External Memory
Computers
Internal and External Memory

2. Characteristics of Computer Memory
Location
Capacity
Unit of transfer
Access Method
Performance
Physical Type
Organization

3. UNIVAC Console and CPU

4. CPU’s

5. Memory Hierarchy COST SPEED REGISTERS CACHE MAIN MEMORY
MAGNETIC DISK | DISK CACHE
MAGNETIC TAPE | OPTICAL DISK

6. 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

7. 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

8. Random Access Memory
Chips and chip technology

9. Flip Flop Circuit Diagram

10. Memory Cells 1 0/1 CONTROL CONTROL read or write read or write
DATA IN / SENSE
input or output
SELECT
select cell
SELECT
select cell 1
0/1

11. Write to Memory Read Enable Write Enable Write Enable Row Address
ARRAY
Col Address
Col Address A0 .
A10
ROW
ADDRESS
BUFFER
ROW
ADDRESS
BUFFER
DATA
INPUT
BUFFER
DATA
INPUT
BUFFER
MUX
MUX D1 . D4
COL
ADDRESS
BUFFER
COL
ADDRESS
BUFFER
DATA
OUTPUT
BUFFER
REFRESH

12. 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

13. Cache Operation
ALU
CNTL
.....
CACHE
BUS
MAIN MEMORY

15. Cache
CPU
Word
CACHE
Main Memory
Block

16. Cache

17. Random Access Direct Access Storage Device (DASD) or disk drives
Optical
Magnetic

18. Optical Disks (CD’s)
                                                                        

19. How CD’s work

20. Cylinder/Track/Block
Block (Sector)
Cylinder

21. Disk Organization

22. DASD Structure
Read-Write Heads

23. Data Storage FAT (File Access Tables), Directories and Catalogs
Update and Delete
Fragmentation and reorganization
Blocks, Headers and Interblock Gaps

24. DASD Access (PC)
MAIN MEMORY
CPU
BUFFER
CACHE
DASD
CONTROLLER

26. DASD Access (PC)
MAIN MEMORY
CPU
BUFFER
DASD
CONTROLLER

27. DASD Access (Mainframe)
MAIN MEMORY
CPU
BUFFER
CHANNEL
DASD
CONTROLLER

28. Data Structure BLOCK HEADER HEADER DATA CRC
Header written when disk is formatted
Data copied into block
Cyclical Redundancy Check calculated

29. 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

30. Access Time: time from call until first data available
Disk Access Time =
Rotational Delay +
Seek Time +
Transfer Time
RAM Access Time =
Refresh Time +

31. Access Time Example Disk Drive has
7200 rpm = 120 rps
8 ms average seek time
80 MBps transfer rate
Rotational delay = 1/2*1/120 = 1/240 s = .0042
Seek Time = 8/1,000 s = .0080
Transfer Time = 512/80,000 s =

32. Time Units Millisecond Microsecond Nanosecond Picosecond ms = 1/1000 s
ns = 1/1,000,000,000 s
ps = 1/1,000,000,000,000 s

33. Representative Times Network speed = megabits per second
Disk transfer = megabytes per second
Disk access = milliseconds
Memory access = nanoseconds
Machine cycle = microseconds/nanoseconds

34. Error Correction and Checking
Block
Check
Body
Header
Add bits to a block to use for error discovery
Detection only
Detection and retransmission
Detection and recovery

35. Error Detection Only (Asynchronous Transmission)*
Parity Bit *
7 Data Bits
27 = 128 distinct characters

36. Error Detection &Correction (Hamming Code: 4 bit word)*
3 Error Checking Bits *
4 Data Bits

37. Error Detection &Correction (Hamming Code: 4 bit word)
DATA * 1 1 1 1

38. Error Detection
PARITY (even)
100 1 1 1 1 1

39. Error Correction (4 bit word)
PARITY (even)
100 1 1 1 1

40. 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

41. 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

42. IBM 1107 with tape drives

43. Memory