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Advanced Computer Architecture-CS501
Advanced Computer Architecture
Lecture No. 39
Reading Material
Vincent P. Heuring & Harry F. Jordan
Chapter 7
Computer Systems Design and Architecture
7.4, 7.5
 Cache Organization and Functions
 Cache Controller Logic
 Cache Strategies
Cache Organization and Functions:
The working of the cache is based on the principle of locality which has two aspects.
Spatial Locality: refers to the fact when a given address has been referenced, the next
address is highly probable to be accessed within a short period of time.
Temporal Locality refers to the fact that once a particular data item is accessed, it is
likely that it will be referenced again within a short period of time.
To exploit these two concepts, the data is transferred in blocks between cache and the
main memory. For a request for data, if the data is available in the cache it results in a
cache hit. And if the requested data is not present in the cache, it is called a cache miss. In
the given example program segment, spatial locality is shown by the array ALPHA, in
which next variable to be accessed is adjacent to the one accessed previously. Temporal
locality is shown by the reuse of the loop variable 100 times in For loop instruction.
Int ALPHA [100], SUM;
For (i=0; i<100; i++)
Cache Management
To manage the working of the cache, cache control unit is implemented in hardware,
which performs all the logic operations on the cache. As data is exchanged in blocks
between main memory and cache, four important cache functions need to be defined.
 Block Placement Strategy
 Block Identification
 Block Replacement
 Write Strategy
Block Diagram of a Cache System
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In the figure, the block diagram of a system using cache is shown. It consists of two
 Fast Memory
 Control Logic Unit
Control logic is further divided into two parts.
Determine and Comparison Unit: For determining and comparisons of the different
parts of the address and to evaluate hit or miss.
Tag RAM: Second part consists of tag memory which stores the part of the memory
address (called tag) of the information (block) placed in the data cache. It also contains
additional bits used by the cache management logic.
Data Cache: is a block of fast memory which stores the copies of data and instructions
frequently accessed by the CPU.
Cache Strategies
In the next section we will discuss various cache functions, and strategies used to
implement these functions.
Block Placement
Block placement strategy needs to be defined to specify where blocks from main memory
will be placed in the cache and how to place the blocks. Now various methods can be
used to map main memory blocks onto the cache .One of these methods is the associative
mapping explained below.
Associative Mapping:
In this technique, block of data from main memory can be placed at any location in the
cache memory. A given block in cache is identified uniquely by its main memory block
number, referred to as a tag, which is stored inside a separate tag memory in the cache.
To check the validity of the cache blocks, a valid bit is stored for each cache entry, to
verify whether the information in the corresponding block is valid or not.
Main memory address references have two fields.
 The word field becomes a "cache address" which specifies where to find the word
in the cache.
 The tag field which must be compared against every tag in the tag memory.
Associative Mapping Example
Refer to Book Ch.7 Section (7.5) Figure 7.31(page 350-351) for detailed explanation.
Mechanism of the Associative Cache Operation
For details refer to book Ch.7, Section 7.5, Figure 7.32 (Page 351-352).
Direct Mapping
In this technique, a particular block of data from main memory can be placed in only one
location into the cache memory. It relies on principle of locality.
Cache address is composed of two fields:
 Group field
 Word field
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Valid bit specifies that the information in the selected block is valid.
For a direct mapping example, refer to the book Ch.7, Section 7.5, Figure 7.33 (page 352
­ 353).
Logic Implementation of the Controller for Direct Mapping
Logic design for the direct mapping is simpler as compared to the associative mapping.
Only one tag entry needs to be compared with the part of the address called group field.
Tasks Required For Direct Mapping Cache:
For details refer to the book Ch. 7, Section 7.5, Figure 7.34 (Page 353-354).
Cache Design: Direct Mapped Cache
To understand the principles of cache design, we will discuss an example of a direct
mapped cache.
The size of the main memory is 1 MB. Therefore 20 address bits needs to be specified.
Assume that the block size is 8 bytes. Cache memory is assumed to be 8 KB organized as
1 K lines of cache memory. Cache memory addresses will range from 0 up to 1023. Now
we have to specify the number of bits required for the tag memory. The least significant
three bits will define the block. The next 10 bits will define the number of bits required
for the cache. The remaining 7 bits will be the width of the tag memory.
Main memory is organized in rectangular form in rows and columns. Number of rows
would be from 0 up to 1023 defined by 10 bits. Number of rows in the main memory will
be the same as number of lines in the cache. Number of columns will correspond to 7 bits
address of the tag memory. Total number of columns will be 128 starting from 0 up to
127. With direct mapping, out of any particular row only one block could be mapped into
the cache. Total number of cache entries will be 1024 each of 8 bytes.
Only a single block from a given group is present in cache at any time. Direct
map Cache imposes a considerable amount of rigidity on cache organization.
Set Associative Mapping
In this mapping scheme, a set consisting of more than one block can be placed in the
cache memory.
The main memory address is divided into two fields. The Set field is decoded to select
the correct group. After that the tags in the selected groups are searched. Two possible
places in which a block can reside must be searched associatively. Cache group address is
the same as that of the direct-mapped cache.
For details of the Set associative mapping example, refer to the book Ch.7, Section 7.5,
Figure 7.35 (Page 354-355).
Replacement Strategy
For a cache miss, we have to replace a cache block with the data coming from main
memory. Different methods can be used to select a cache block for replacement.
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Always Replacement: For Direct Mapping on a miss, there is only one block which
needs replacement called always replacement.
For associative mapping, there are no unique blocks which need replacement .In this case
there are two options to decide which block is to be replaced.
 Random Replacement: To randomly select the block to be replaced
 LFU: Based on the statistical results, the block which has been least used in the
recent past, is replaced with a new block.
Write Strategy
When a CPU command to write to a memory data will come into cache, the writing into
the cache requires writing into the main memory also.
Write Through: As the data is written into the cache, it is also written into the main
memory called Write Through. The advantages are:
 Read misses never result in writes to the lower level.
 Easy to implement than write back
Write Back: Date resides in the cache, till we need to replace a particular block then the
data of that particular block will be written into the memory if that needs a write, called
write back. The advantages are:
 Write occurs at the speed of the cache
 Multiple writes with in the same block requires only one write to the lower
 This strategy uses less memory bandwidth, since some writes do not go to the
lower level; useful when using multi processors.
Cache Coherence
Multiple copies of the same data can exist in memory hierarchy simultaneously. The
Cache needs updating mechanism to prevent old data values from being used. This is the
problem of cache coherence. Write policy is the method used by the cache to deal with
and keep the main memory updated.
Dirty bit is a status bit which indicates whether the block in cache is dirty (it has been
modified) or clean (not modified). If a block is clean, it is not written on a miss, since
lower level contains the same information as the cache. This reduces the frequency of
writing back the blocks on replacement.
Writing the cache is not as easy as reading from it e.g., modifying a block can not begin
until the tag has been checked, to see if the address is a hit. Since tag checking can not
occur in parallel with the write as is the case in read, therefore write takes longer time.
Write Stalls: For write to complete in Write through, the CPU has to wait. This wait state
is called write stall.
Write Buffer: reduces the write stall by permitting the processor to continue as soon as
the data has been written into the buffer, thus allowing overlapping of the instruction
execution with the memory update.
Write Strategy on a Cache Miss
On a cache miss, there are two options for writing.
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Write Allocate: The block is loaded followed by the write. This action is similar to the
read miss. It is used in write back caches, since subsequent writes to that particular block
will be captured by the cache.
No Write Allocate: The block is modified in the lower level and not loaded into the
cache. This method is generally used in write through caches, because subsequent writes
to that block still have to go to the lower level.
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Table of Contents:
  1. Computer Architecture, Organization and Design
  2. Foundations of Computer Architecture, RISC and CISC
  3. Measures of Performance SRC Features and Instruction Formats
  4. ISA, Instruction Formats, Coding and Hand Assembly
  5. Reverse Assembly, SRC in the form of RTL
  6. RTL to Describe the SRC, Register Transfer using Digital Logic Circuits
  7. Thinking Process for ISA Design
  8. Introduction to the ISA of the FALCON-A and Examples
  9. Behavioral Register Transfer Language for FALCON-A, The EAGLE
  10. The FALCON-E, Instruction Set Architecture Comparison
  11. CISC microprocessor:The Motorola MC68000, RISC Architecture:The SPARC
  12. Design Process, Uni-Bus implementation for the SRC, Structural RTL for the SRC instructions
  13. Structural RTL Description of the SRC and FALCON-A
  14. External FALCON-A CPU Interface
  15. Logic Design for the Uni-bus SRC, Control Signals Generation in SRC
  16. Control Unit, 2-Bus Implementation of the SRC Data Path
  17. 3-bus implementation for the SRC, Machine Exceptions, Reset
  18. SRC Exception Processing Mechanism, Pipelining, Pipeline Design
  19. Adapting SRC instructions for Pipelined, Control Signals
  20. SRC, RTL, Data Dependence Distance, Forwarding, Compiler Solution to Hazards
  21. Data Forwarding Hardware, Superscalar, VLIW Architecture
  22. Microprogramming, General Microcoded Controller, Horizontal and Vertical Schemes
  23. I/O Subsystems, Components, Memory Mapped vs Isolated, Serial and Parallel Transfers
  24. Designing Parallel Input Output Ports, SAD, NUXI, Address Decoder , Delay Interval
  25. Designing a Parallel Input Port, Memory Mapped Input Output Ports, wrap around, Data Bus Multiplexing
  26. Programmed Input Output for FALCON-A and SRC
  27. Programmed Input Output Driver for SRC, Input Output
  28. Comparison of Interrupt driven Input Output and Polling
  29. Preparing source files for FALSIM, FALCON-A assembly language techniques
  30. Nested Interrupts, Interrupt Mask, DMA
  31. Direct Memory Access - DMA
  32. Semiconductor Memory vs Hard Disk, Mechanical Delays and Flash Memory
  33. Hard Drive Technologies
  34. Arithmetic Logic Shift Unit - ALSU, Radix Conversion, Fixed Point Numbers
  35. Overflow, Implementations of the adder, Unsigned and Signed Multiplication
  36. NxN Crossbar Design for Barrel Rotator, IEEE Floating-Point, Addition, Subtraction, Multiplication, Division
  37. CPU to Memory Interface, Static RAM, One two Dimensional Memory Cells, Matrix and Tree Decoders
  38. Memory Modules, Read Only Memory, ROM, Cache
  39. Cache Organization and Functions, Cache Controller Logic, Cache Strategies
  40. Virtual Memory Organization
  41. DRAM, Pipelining, Pre-charging and Parallelism, Hit Rate and Miss Rate, Access Time, Cache
  42. Performance of I/O Subsystems, Server Utilization, Asynchronous I/O and operating system
  43. Difference between distributed computing and computer networks
  44. Physical Media, Shared Medium, Switched Medium, Network Topologies, Seven-layer OSI Model