1. TK 2123 COMPUTER ORGANISATION & ARCHITECTURE
Lecture 7: CPU and Memory (3)

2. Contents
This lecture will discuss:
Cache.
Error Correcting Codes.

3. The Memory Hierarchy Trade-off: cost, capacity and access time.
Faster access time, greater cost per bit.
Greater capacity, smaller cost per bit.
Greater capacity, slower access time.
Access time - the time it takes to perform a read or write operation.
Memory Cycle time -Time may be required for the memory to “recover” before next access, i.e. access + recovery.
Transfer Rate - rate at which data can be moved.

4. A five-level memory hierarchy.
Memory Hierarchies
A five-level memory hierarchy.

5. Hierarchy List Registers L1 Cache L2 Cache Main memory Disk cache Disk
Optical
Tape
decreasing cost/bit, increasing capacity, and slower access time
Internal memory
external memory

6. Hierarchy List
It would be nice to use only the fastest memory, but because that is the most expensive memory,
we trade off access time for cost by using more of the slower memory.
The design challenge is to organise the data and programs in memory so that the accessed memory words are usually in the faster memory.
In general, it is likely that most future accesses to main memory by the processor will be to locations recently accessed.
So the cache automatically retains a copy of some of the recently used words from the DRAM.
If the cache is designed properly, then most of the time the processor will request memory words that are already in the cache.

7. Hierarchy List
No one technology is optimal in satisfying the memory requirements for a computer system.
As a consequence, the typical computer system is equipped with a hierarchy of memory subsystems;
some internal to the system (directly accessible by the processor) and
some external (accessible by the processor via an I/O module).

8. Cache Small amount of fast memory
Sits between normal main memory and CPU
May be located on CPU chip or module
or cache line.

9. Cache The cache contains a copy of portions of main memory.
When the processor attempts to read a word of memory, a check is made to determine if the word is in the cache.
If so (hit), the word is delivered to the processor.
If not (miss), a block of main memory, consisting of some fixed number of words, is read into the cache and then the word is delivered to the processor.
Because of the phenomenon of locality of reference, when a block of data is fetched into the cache to satisfy a single memory reference, it is likely that there will be future references to that same memory location or to other words in the block.
The ratio of hits to the total number of requests is known as the hit ratio.

10. Cache/Main Memory Structure

11. 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
Cache includes tags to identify which block of main memory is in each cache slot

12. Cache Operation

13. Cache Design Size Mapping Function Replacement Algorithm Write Policy
Block Size
Number of Caches – L1, L2, L3 etc.

14. Size does matter Cost More cache is expensive Speed
More cache is faster (up to a point)
Checking cache for data takes time
We would like the size of the cache to be small enough so that the overall average cost per bit is close to that of main memory alone and large enough so that the overall average access time is close to that of the cache alone.
The larger the cache, the larger the number of gates involved in addressing the cache. The result is that large caches tend to be slightly slower than small ones.

15. Comparison of Cache Sizes
Comparison of Cache Sizes
Processor
Type
Year of Introduction
L1 cachea
L2 cache
L3 cache
IBM 360/85
Mainframe
1968
16 to 32 KB —
PDP-11/70
Minicomputer
1975
1 KB
VAX 11/780
1978
16 KB
IBM 3033
64 KB
IBM 3090
1985
128 to 256 KB
Intel 80486 PC
1989
8 KB
Pentium
1993
8 KB/8 KB
256 to 512 KB
PowerPC 601
32 KB
PowerPC 620
1996
32 KB/32 KB
PowerPC G4
PC/server
1999
256 KB to 1 MB
2 MB
IBM S/390 G4
1997
256 KB
IBM S/390 G6
8 MB
Pentium 4
2000
IBM SP
High-end server/ supercomputer
64 KB/32 KB
CRAY MTAb
Supercomputer
Itanium
2001
16 KB/16 KB
96 KB
4 MB
SGI Origin 2001
High-end server
Itanium 2
2002
6 MB
IBM POWER5
2003
1.9 MB
36 MB
CRAY XD-1
2004
64 KB/64 KB
1MB
a Two values seperated by a slash refer to instruction and data caches
b Both caches are instruction only; no data caches

16. Cache: Mapping Function
Cache lines < main memory blocks:
An algorithm is needed for mapping main memory blocks into cache lines.
Three techniques:
Direct
Associative
set associative

17. Direct Mapping Each block of main memory maps to only one cache line
i.e. if a block is in cache, it must be in one specific place.
pros & cons
Simple
Inexpensive
Fixed location for given block
If a program accesses 2 blocks that map to the same line repeatedly, cache misses are very high

18. Associative Mapping
A main memory block can load into any line of cache
Memory address is interpreted as tag and word
Tag uniquely identifies block of memory
Every line’s tag is examined for a match
Disadvantage:
Cache searching gets expensive
The complex circuitry is required to examine the tags of all cache lines in parallel.

19. Set Associative Mapping
A compromise that exhibits the strengths of both the direct and associative approaches while reducing their disadvantages.
Cache is divided into a number of sets.
Each set contains a number of lines.
A given block maps to any line in a given set
e.g. Block B can be in any line of set i.
With fully associative mapping, the tag in a memory address is quite large and must be compared to the tag of every line in the cache.
With k-way set associative mapping, the tag in a memory address is much smaller and is only compared to the k tags within a single set.

20. Replacement Algorithms
When cache memory is full, some block in cache memory must be selected for replacement.
Direct mapping :
No choice
Each block only maps to one line
Replace that line

21. Replacement Algorithms (2) Associative & Set Associative
Hardware implemented algorithm (speed)
Least Recently used (LRU)
An LRU algorithm, keeps track of the usage of each block and replaces the block that was last used the longest time ago.
First in first out (FIFO)
replace block that has been in cache longest
Least frequently used (LFU)
replace block which has had fewest hits
Random

22. Write Policy
Issues:
Must not overwrite a cache block unless main memory is up to date
Multiple CPUs may have individual caches
I/O may address main memory directly

23. Write through All writes go to main memory as well as cache
Multiple CPUs can monitor main memory traffic to keep local (to CPU) cache up to date
Disadvantage:
Lots of traffic
Slows down writes
Create a bottleneck.

24. Cache: Line Size
As the block size increases from very small to larger sizes, the hit ratio will at first increase because of the principle of locality.
Two issues:
Larger blocks reduce the number of blocks that fit into a cache. Because each block fetch overwrites older cache contents, a small number of blocks results in data being overwritten shortly after they are fetched.
As a block becomes larger, each additional word is farther from the requested word, therefore less likely to be needed in the near future.

25. Number of Caches external cache: Is it still desirable?
Multilevel Caches:
On-chip cache:
A cache on the same chip as the processor.
Reduces the processor’s external bus activity and therefore speeds up execution times and increases overall system performance.
external cache: Is it still desirable?
Yes - most contemporary designs include both on-chip and external caches.
E.g. two-level cache, with the internal cache (L1) and the external cache (L2). Why?
If there is no L2 cache and the processor makes an access request for a memory location not in the L1 cache, then the processor must access DRAM or ROM memory across the bus – poor performance.

26. Number of Caches
More recently, it has become common to split the cache into two:
one dedicated to instructions and one dedicated to data.
There are two potential advantages of a unified cache:
For a given cache size, a unified cache has a higher hit rate than split caches because it balances the load between instruction and data fetches automatically.
Only one cache needs to be designed and implemented.
The trend is toward split caches, such as the Pentium and PowerPC, which emphasize parallel instruction execution and the prefetching of predicted future instructions. Advantage:
It eliminates contention for the cache between the instruction fetch/decode unit and the execution unit.

27. Processor on which feature first appears
Intel Cache Evolution
Problem
Solution
Processor on which feature first appears
External memory slower than the system bus.
Add external cache using faster memory technology.
386
Increased processor speed results in external bus becoming a bottleneck for cache access.
Move external cache on-chip, operating at the same speed as the processor.
486
Internal cache is rather small, due to limited space on chip
Add external L2 cache using faster technology than main memory
Contention occurs when both the Instruction Prefetcher and the Execution Unit simultaneously require access to the cache. In that case, the Prefetcher is stalled while the Execution Unit’s data access takes place.
Create separate data and instruction caches.
Pentium
Increased processor speed results in external bus becoming a bottleneck for L2 cache access.
Create separate back-side bus that runs at higher speed than the main (front-side) external bus. The BSB is dedicated to the L2 cache.
Pentium Pro
Move L2 cache on to the processor chip.
Pentium II
Some applications deal with massive databases and must have rapid access to large amounts of data. The on-chip caches are too small.
Add external L3 cache.
Pentium III
Move L3 cache on-chip.
Pentium 4

28. Locality
Page 129: Stalling.

29. Internal Memory (revision)

30. Memory Packaging and Types
A group of chips, typically 8 or 16, is mounted on a tiny PCB and sold as a unit.
SIMM - single inline memory module, has a row of connectors on one side.
DIMM – Dual inline memory module, has a row of connectors on both side.
A SIMM holding 256 MB. Two of the chips control the SIMM.

31. Error Correction Hard Failure Soft Error
Permanent defect
Caused by harsh environmental abuse, manufacturing defects, and wear.
Soft Error
Random, non-destructive
No permanent damage to memory
Caused by power supply problems.
Detected using Hamming error correcting code.

32. Error Correction
When reading out the stored word, a new set of K code bits is generated from M data bits and compared with fetch code bits. Results:
No errors – the fetch data bits are sent out.
An error is detected, and it is possible to correct the error.
Data bits + error correction bits corrector sent out the corrected set of M bits.
An error is detected, but it is not possible to correct the error. This condition is reported.

33. Error Correcting Code Function
A function to produce code
Stored codeword: M+K bits

34. Error Correction
Page 73, 74, 75 Tanenbaum.

35. Error Correcting Codes (1)
Number of check bits for a code that can correct a single error

36. Error Correcting Codes (2)
(a) Encoding of 1100
(b) Even parity added
(c) Error in AC

37. Error Correcting Codes (3)
Construction of the Hamming code for the memory word by adding 5 check bits to the 16 data bits.

38. Thank you Q & A