The document discusses the fundamentals of computer memory, focusing on main memory characteristics, types (SRAM and DRAM), and performance measures such as latency and bandwidth. It examines memory optimization techniques, including cache strategies and reducing hit times, miss penalties, and miss rates. Additionally, it addresses the memory hierarchy in modern systems and recent advancements in DRAM technologies like DDR standards.

1. Presented by:
Trupti Diwan
Shweta Ghate
Sapana Vasave

2.  Basics of memory
 Memory Technology
 Memory optimization

3.  Main memory is RAM.(The words Memory,
Buffer, Cache are all refers Ram). Which is
Nearly 11,000 times faster than secondary
memory (Hard Disk) in Random Access.)
 Characteristics of Main Memory is as vital as
the processor chip to a computer system.
Fast systems have both a fast processor and a
large memory

4.  Here is a list of some characteristics of
computer memory.
 closely connected to the processor.
 Holds programs and data that the processor is
actively working with.
 Used for long term storage.
 The processor interacts with it millions of times
per second.
 The contents is easily changed.
 Usually its contents are organized into files.

5.  Main memory is the short term memory of a
computer. It retains data only for the period
that a program is running, and that's it.
 Memory is used for the purpose of running
programs

6.  Main memory satisfies the demands of caches
and serves as the I/O interface.
 Performance measures of main memory
emphasize both latency and bandwidth
(Memory bandwidth is the number of bytes
read or written per unit time)
 Memory latency is the time delay required to
obtain a specific item of data
 Memory Bandwidth is the rate at which data can
be accessed (e.g. bits per second)
 Bandwidth unit is normally 1/cycle time
 This rate can be improved by concurrent access

7.  The main memory affects the cache miss
penalty,
 the primary concern of the cache.
 Main memory bandwidth
 the primary concern of I/O and multiprocessors.

8.  Although caches are interested in low
latency memory, it is generally easier to
improve memory bandwidth with new
organizations than it is to reduce latency
 cache designers increase block size to take
advantage of the high memory bandwidth.

9.  Memory latency is traditionally quoted using
two measures
 Access time
 Access time is the time between when a read is
requested and when the desired word arrives and
 Cycle time
 Cycle time is the minimum time between requests to
memory.
 Cycle time is greater than access time
because the memory needs the address lines
to be stable between accesses.

10. 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
1s 10,000,000s
(10s ms)
Speed (ns): 10s 100s
100s Gs
Size (bytes): Ks Ms
Tertiary
Storage
(Tape)
10,000,000,000s
(10s sec)
Ts

11.  Static Random Access Memory (SRAM)
- use for cache.
 Dynamic Random Access Memory (DRAM)
- use for main memory

12.  ‘S’ stands for static.
 No need to be refresh , so access time is close to
cycle time.
 Uses 6 transistor per bit.
 Bits stored as on/off switches
 No charges to leak
 More complex construction
 Larger per bit
 More expensive
 Faster

14.  Transistor arrangement gives stable logic state
 State 1
 C1 high, C2 low
 T1 T4 off, T2 T3 on
 State 0
 C2 high, C1 low
 T2 T3 off, T1 T4 on
 Address line transistors T5 T6 is switch
 Write – apply value to B & compliment to B
 Read – value is on line B

15.  Bits stored as charge in capacitors
 Charges leak
 Need refreshing even when powered
 Simpler construction
 Smaller per bit
 Less expensive
 Need refresh circuits
 Slower
 Cycle time is longer than the access time

17.  Address line active when bit read or written
 Transistor switch closed (current flows)
 Write
 Voltage to bit line
 High for 1 low for 0
 Then signal address line
 Transfers charge to capacitor
 Read
 Address line selected
 transistor turns on
 Charge from capacitor fed via bit line to sense amplifier
 Compares with reference value to determine 0 or 1
 Capacitor charge must be restored

18.  Addresses divided into 2 halves (Memory as a 2D matrix):
 RAS or Row Access Strobe
 CAS or Column Access Strobe
Fig : Internal Organization of a DRAM

19.  DRAM had an asynchronous interface to the
memory controller so every transfer involved
overhead to synchronize with controller.
 Adding clock signal to the DRAM interface
reduces the overhead, such optimization
called
“Synchronous DRAM “ i.e SDRAMs

20.  Double Data Rate i.e DDR was a later development
of SDRAM, used in PC memory beginning in 2000.
 DDR SDRAM internally performs double-width
accesses at the clock rate, and uses a double data
rate interface to transfer one half on each clock
edge.
 Further version of DDR(2 data transfer/cycle)
-DDR2(4 data transfer/cycle)
-DDR3 (8 data transfer/cycle)

21.  Review: 6 Basic Cache Optimizations
 • Reducing hit time
 1.Address Translation during Cache Indexing
 • Reducing Miss Penalty
 2. Multilevel Caches
 3. Giving priority to read misses over write
misses
 • Reducing Miss Rate
 4. Larger Block size (Compulsory misses)
 5. Larger Cache size (Capacity misses)
 6. Higher Associativity (Conflict misses)

22.  Reducing hit time
 1. Small and simple caches
 2. Way prediction
 3. Trace caches
 • Increasing cache bandwidth
 4. Pipelined caches
 5. Multibanked caches
 6. Nonblocking caches
 • Reducing Miss Penalty
 7. Critical word first
 8. Merging write buffers
 • Reducing Miss Rate
 9.Compiler optimizations
 • Reducing miss penalty or miss rate via parallelism
 10.Hardware prefetching
 11.Compiler prefetching

23.  Advanced cache optimizations into the following categories:
 Reducing the hit time: small and simple caches, way prediction, and trace
 caches
 Increasing cache bandwidth: pipelined caches, multibanked caches, and
nonblocking
 caches
 Reducing the miss penalty: critical word first and merging write buffers
 Reducing the miss rate: compiler optimizations
 Reducing the miss penalty or miss rate via parallelism: hardware prefetching
 and compiler prefetching
 We will conclude with a summary of the implementation complexity and the
performance

24.  First Optimization: Small and Simple Caches to Reduce
Hit Time
 A time-consuming portion of a cache hit is using the
index portion of the address
 to read the tag memory and then compare it to the
address. Smaller hardware can
 be faster, so a small cache can help the hit time. It is
also critical to keep an L2
 cache small enough to fit on the same chip as the
processor to avoid the time penalty
 of going off chip.
 The second suggestion is to keep the cache simple, such
as using direct mapping.
 One benefit of direct-mapped caches is that the designer
can overlap the tag
 check with the transmission of the data. This effectively
reduces hit time

26.  Compiler techniques to reduce cache misses+ 0 Software is a
challenge; some computers have compiler option
 Hardware prefetching of instructions and data + + 2 instr.,3
data
 Many prefetch instructions;Opteron and Pentium 4 prefetch
data
 Compiler-controlled prefetching + + 3 Needs nonblocking
cache;
 possible instruction overhead; in many CPUs

27.  Figure 5.11 Summary of 11 advanced cache
optimizations showing impact on cache performance
and complexity.
 Although generally a technique helps only one factor,
prefetching can reduce misses if done sufficiently early; if
not,
 it can reduce miss penalty. + means that the technique
improves the factor, – means it hurts that factor, and blank
 means it has no impact. The complexity measure is
subjective, with 0 being the easiest and 3 being a challenge

28.  The techniques to improve hit time, bandwidth, miss
penalty
 Miss rate generally affect the other components of the
average memory access equation as well as the complexity
of the memory hierarchy.
 Estimates the impact on complexity, with + meaning that
the technique
 Improves the factor, – meaning it hurts that factor, and
blank meaning it has no impact.
 Generally, no technique helps more than one category.

29.  Computer Architecture - A Quantitative
Approach