2. Memory
Main memory consists of a number of
storage locations, each of which is
identified by a unique address
The ability of the CPU to identify
each location is known as its
addressability
Each location stores a word i.e. the
number of bits that can be processed
by the CPU in a single operation.
Word length may be typically 16, 24,
32 or as many as 64 bits.
A large word length improves system
performance, though may be less
efficient on occasions when the full word
length is not used
3. Types of main memory
There are two types of main memory, Random Access Memory (RAM) and Read
Only Memory (ROM)
Random Access Memory (RAM)
✓holds its data as long as the computer is switched on
✓All data in RAM is lost when the computer is switched off
✓Described as being volatile
✓It is direct access as it can be both written to or read from in any order
Its purpose is to temporarily hold programs and data for processing. In
modern computers it also holds the operating system
4. Types of RAM
1. Dynamic Random Access Memory (DRAM)
• Contents are constantly refreshed 1000 times per second
• Access time 60 – 70 nanoseconds
Note: a nanosecond is one billionth of a second!
2. Synchronous Dynamic Random Access Memory (SDRAM)
• Quicker than DRAM
• Access time less than 60 nanoseconds
3. Direct Rambus Dynamic Random Access Memory (DRDRAM)
• New type of RAM architecture
• Access time 20 times faster than DRAM
• More expensive
5. 4. Static Random Access Memory (SRAM)
• Doesn’t need refreshing
• Retains contents as long as power applied to the chip
• Access time around 10 nanoseconds
• Used for cache memory
• Also for date and time settings as powered by small battery
5. Cache memory
• Small amount of memory typically 256 or 512 kilobytes
• Temporary store for often used instructions
• Level 1 cache is built within the CPU (internal)
• Level 2 cache may be on chip or nearby (external)
• Faster for CPU to access than main memory
Types of RAM
6. Types of RAM
6. Video Random Access memory
• Holds data to be displayed on computer screen
• Has two data paths allowing READ and WRITE to occur at the same time
• A system’s amount of VRAM relates to the number of colours and resolution
• A graphics card may have its own VRAM chip on board
7. Virtual memory
• Uses backing storage e.g. hard disk as a temporary location for programs
and data where insufficient RAM available
• Swaps programs and data between the hard-disk and RAM as the CPU
requires them for processing
• A cheap method of running large or many programs on a computer system
• Cost is speed: the CPU can access RAM in nanoseconds but hard-disk in
milliseconds (Note: a millisecond is a thousandth of a second)
• Virtual memory is much slower than RAM
7. Read only memory (ROM)
✓ROM holds programs and data permanently even when computer is switched off
✓Data can be read by the CPU in any order so ROM is also direct access
✓The contents of ROM are fixed at the time of manufacture
✓Stores a program called the bootstrap loader that helps start up the computer
✓Access time of between 10 and 50 nanoseconds
8. Types of ROM
1. Programmable Read Only Memory (PROM)
• Empty of data when manufactured
• May be permanently programmed by the user
2. Erasable Programmable Read Only Memory (EPROM)
• Can be programmed, erased and reprogrammed
• The EPROM chip has a small window on top allowing it to be erased by
shining ultra-violet light on it
• After reprogramming the window is covered to prevent new contents being
erased
• Access time is around 45 – 90 nanoseconds
9. Types of ROM
3. Electrically Erasable Programmable Read Only Memory (EEPROM)
• Reprogrammed electrically without using ultraviolet light
• Must be removed from the computer and placed in a special machine to do this
• Access times between 45 and 200 nanoseconds
4. Flash ROM
• Similar to EEPROM
• However, can be reprogrammed while still in the computer
• Easier to upgrade programs stored in Flash ROM
• Used to store programs in devices e.g. modems
• Access time is around 45 – 90 nanoseconds
10. DYNAMIC RAM
Bits stored as charge in capacitors
Charges leak
Need refreshing even when powered
Simpler construction
Smaller per bit
Less expensive
Need refresh circuits
Slower
Main memory
Essentially analogue
Level of charge determines value
11. STATIC RAM
Bits stored as on/off switches
No charges to leak
No refreshing needed when powered
More complex construction
Larger per bit
More expensive
Does not need refresh circuits
Faster
Cache
Digital
Uses flip-flops
12. SRAM V DRAM
Both volatile
Power needed to preserve data
Dynamic cell
Simpler to build, smaller
More dense
Less expensive
Needs refresh
Larger memory units
Static
Faster
Cache
13. MEMORY HIERARCHY
Memory technology Typical access time $ per GB in 2004
SRAM 0.5–5 ns $4000–$10,000
DRAM 50–70 ns $100–$200
Magnetic disk 5,000,000–20,000,000 ns $0.50–$2
CPU
Level n
Level 2
Level 1
Levels in the
memory hierarchy
Increasing distance
from the CPU in
access time
Size of the memory at each level
Processor
Data are transferred
16. MEMORY HIERARCHY: HOW DOES IT WORK?
Temporal Locality (Locality in Time):
=> Keep most recently accessed data items closer to the
processor
Spatial Locality (Locality in Space):
=> Move blocks consists of contiguous words to the upper
levels
Lower Level
Memory
Upper Level
Memory
To Processor
From Processor
Blk X
Blk Y
17. MEMORY HIERARCHY: TERMINOLOGY
Hit: data appears in some block in the upper level (example: Block
X)
Hit Rate: the fraction of memory access found in the upper
level
Hit Time: Time to access the upper level which consists of
RAM access time + Time to determine hit/miss
Miss: data needs to be retrieve from a block in the lower level
(Block Y)
Miss Rate = 1 - (Hit Rate)
Miss Penalty: Time to replace a block in the upper level +
Time to deliver the block the processor
Hit Time << Miss Penalty
Lower Level
Memory
Upper Level
Memory
To Processor
From Processor
Blk X
Blk Y
18. CONT..
How does the memory hierarchy work? Well it is rather
simple, at least in principle.
In order to take advantage of the temporal locality, that is
the locality in time, the memory hierarchy will keep those
more recently accessed data items closer to the processor
because chances are (points to the principle), the processor
will access them again soon.
In order to take advantage of the spatial locality, not ONLY
do we move the item that has just been accessed to the
upper level, but we ALSO move the data items that are
adjacent to it.
19. MEMORY HIERARCHY OF A MODERN COMPUTER SYSTEM
By taking advantage of the principle of locality:
Present the user with as much memory as is available in the cheapest
technology.
Provide access at the speed offered by the fastest technology.
Control
Datapath
Secondary
Storage
(Disk)
Processor
Registers
Main
Memory
(DRAM)
Second
Level
Cache
(SRAM)
On-Chip
Cache
Tertiary
Storage
(Tape)
Ts
20. THE OPERATION OF CACHE MEMORY
1. Cache fetches data
from next to current
addresses in main
memory
2. CPU checks to see
whether the next
instruction it requires is in
cache
3. If it is, then the
instruction is fetched from
the cache – a very fast
position
4. If not, the CPU has to
fetch next instruction
from main memory - a
much slower process
Main
Memory
(DRAM)
CPU
Cache
Memory
(SRAM)
= Bus connections
21. 21
Problem: Main Memory is slow compared to CPU.
Solution: Store the most commonly used data in a smaller,
faster memory. Good trade off between $$ and performance.
CACHE AND MAIN MEMORY
22. CACHE OPERATION - OVERVIEW
CPU requests contents of memory location
Check cache for this data
If present, get from cache (fast)
If not present, read required block from main
memory to cache
Then deliver from cache to CPU
24. QUESTIONS
What is the difference between DRAM and SRAM in terms
of applications?
What is the difference between DRAM and SRAM in terms
of characteristics such as speed, size and cost?
What is the distinction between spatial and temporal
locality?
What are the strategies for exploring spatial and temporal
locality?
