Main Memory
• Main memory –
– a collection of storage locations,
– each with a unique identifier called the address.
• Word– Data are transferred to and from memory in groups
of bits called words.
– The number of bits that can be stored in one CPU
register in a computer.
Figure 5-3
Main memory
Address Space
• Although programmers use a name to identify a word,
at the hardware level, each word is identified by an
address.
• Address space – The total number of uniquely identifiable locations in memory.
– For example:
a memory with 64KB and a word size of 1 byte has an
address space that range from 0 to 65535.
Table 5.1 Memory units
Unit
-----------KB-kilobyte
MB-megabyte
GB-gigabyte
TB-terabyte
PB-petabyte
EB-exabyte
Exact Number of bytes
Approximation
-----------------------210 bytes
220 bytes
230 bytes
240 bytes
250 bytes
260 bytes
-----------103 bytes
106 bytes
109 bytes
1012 bytes
1015 bytes
1018 bytes
Address as Bit Pattern
• Because computers operate by storing
numbers as bit patterns, the address itself is
also represented as a bit pattern.
• If a computer has N words of memory, you
need an unsigned integer of size log2N bits to
refer to each memory location.
Note:
Memory addresses are defined using
unsigned binary integers.
Example 1
A computer has 32 MB (megabytes) of memory. How
many bits are needed to address any single byte in
memory?
Solution
The memory address space is 32 MB, or 225 (25 x 220).
This means you need
log2 225 or 25 bits, to address each byte.
Example 2
A computer has 128 MB of memory. Each word in
this computer is 8 bytes. How many bits are
needed to address any single word in memory?
Solution
The memory address space is 128 MB, which
means 227. However, each word is 8 (23) bytes,
which means that you have 224 words. This
means you need log2 224 or 24 bits, to address
each word.
Memory Types
• RAM (Random Access Memory)
– Volatile
– R/W by user
– Two categories:
• DRAM(Dynamic RAM)
– Refresh
– used in most PCs
• SRAM(Static RAM)
– No refresh
– faster 、more reliable
– more expensive
– often used only as a memory cache
Memory Types
• ROM (Read Only Memory)
– Nonvolatile
– Written by manufacture
– Hold the booting program
– Categories:
• ROM
• PROM(Programmable ROM)- Write once by
user
• EPROM(Erasable PROM)- physical removal
and reinstallation by special device
• EEPROM(Electronically EPROM)- without
being removed from computer
Figure 5-4
Memory hierarchy
Speed
Secondary Memory
Cost Space
Cache Memory
• Cache memory at any time contains a copy of a
portion of main memory.
1. CPU checks the cache
2. If exist, copy the word,
otherwise
1) access main memory and copy a block of memory
starting with the desired word.
2) CPU accesses cache and copies the word.
Figure 5-5
Cache
• It is very probable that the CPU, in next cycle,
needs to access the words following the first word.
• The existence of the cache speeds processing.
• 80-20 rule
Most computers typically spend 80% of the time
accessing only 20% of the data.
• Same data are accessed over and over again.
Input/Output Subsystem
• I/O subsystem allows a computer to
– Communicate with the outside world
– Store programs and data
even when the power is off.
Nonstorage devices
• allows the CPU/memory to
communicate with the outside world
• Keyboard – provides input
• Monitor
– display output
– Echoes the input typed on the keyboard
• Printer
Storage devices
• can store large amount of information to be
retrieved at a later time.
• Cheaper than main memory
• Nonvolatile
• Auxiliary storage device (Secondary memory)
• Categories:
– Magnetic
– optical
Magnetic Storage devices
• Use magnetization to store bits of data
• If a spot is magnetized  1
• If a spot is not magnetized  0
• Magnetic Disk
– Random access device
• Magnetic Tape
– Sequential access device
Physical layout of a magnetic disk
• Performance depends on several factors
 Rotational speed
 Seek time – the time to move the R/W head
to desired track
Transfer time – the time to move data from
the disk to the CPU/Memory
Figure 5-6
Surface organization of a disk
• Each surface is divided into tracks
• Each track is divided into sectors
• Sectorthe smallest storage area that can be accessed
at one time.
• Blockcan be stored in one or more sectors and
retrieved together.
Figure 5-7
Figure 5-8
Mechanical configuration of a tape
•
•
•
•
Surface organization of a tape
Sequential access
Slower
Cheaper
Backup large amount of data
• 9 vertical spots
 8 bits of information
 1 bit for error detection
Figure 5-9
Optical Storage devices
• Use light (laser) to store and retrieve data.
• CD-ROM(Compact disc read-only memory)
– Same technology as CD
• CD-R (Compact disc recordable)
– WORM(Write Once, Read Many)
• CD-RW (Compact disc rewritable)
– Also called Erasable optical disc
• DVD(Digital Versatile Disc)
– Higher capacity
Memory hierarchy
Speed
Secondary Memory
• Hard disk
• Floppy disk (Diskette)
•Flash memory
• CD-R/CD-RW/DVD
• Magnetic tape
Cost Space
Connecting CPU and memory using three buses
• Data bus# of wires depends on the size of the word.
• Address bus# of wires depends on the address space of memory.
• Control bus# of wires depends on the total number of control
commands a computer needs.
Figure 5-14
Connecting I/O devices
• The CPU and memory are electronic devices.
• The I/O devices are eletromechanical, magnetic,
or optical devices.
– Much slower speed
– Need for an intermediary- I/O controller (or interface)
• Serial controlleronly one wire connection to the device
• Parallel controllerseveral connections to the device
Figure 5-15
Connecting I/O devices to the buses
SCSI controller
• SCSI(Small Computer System Interface)
Parallel (8/16/32 bits)
Daisy chained connection
Unique ID for each device
Figure 5-16
FireWire controller
• IEEE 1394 (iLink)
 serial
 High speed up to 50MB/sec (400Mbps)
 Daisy-chain/Tree connection
 1394b (800Mbps)
Figure 5-17
USB controller
• USB(Universal Serial Bus)
serial
1.0 (12 Mbps)
2.0 (480 Mbps)
Figure 5-18
Addressing I/O devices
• The CPU use the same bus to R/W memory
and I/O devices.
The only difference is the instruction.
• Two methods to handle the addressing of
I/O devices
– Isolated I/O
– Memory-Mapped I/O
Isolated I/O addressing
• Different instructions
• The I/O addresses can overlap with memory
addresses without any ambiguity because
the instruction itself is different.
Figure 5-19
Memory-mapped I/O addressing
• Same instructions
• The CPU treats each register in the I/O controller as
a word in memory.
• Adv. – smaller # of instructions
• Disadv. – part of the address space allocated for I/O
Figure 5-20
Program Execution
•
•
•
•
General–purpose computers use a set of
instructions called a program to process data.
Both the program and the data are stored in
memory.
The CPU uses repeating machine cycles to
execute instructions in the program, one by one.
A simplified cycle consists of:
1. Fetch
2. Decode
3. Execute
Figure 5-21
Steps of a cycle