Cache介紹

1. Introduction

The purpo of this paper is two fold. The first part gives an overview of cache, while the

cond part explains how the Pentium Processor implements cache.

A simplified model of a cache system will be examined first. The simplified model is expanded

to explain: how a cache works, what kind of cycles a cache us, common architectures, major

components, and common organization schemes.

The implementation may differ in actual designs, however the concepts remain the same. This

eliminates unnecessary detail and background in hardware design or system dependency. The

cond part of this paper gives more detail and specifics of how the internal caches work on the

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2.1 Basic Model

CPU

Cache Memory

Main

DRAM

Memory

System Interface

Figure 2-1 Basic Cache Model

Figure 2-1 shows a simplified diagram of a system with cache. In this system, every time the

CPU performs a read or write, the cache may intercept the bus transaction, allowing the cache to

decrea the respon time of the system. Before discussing this cache model, lets define

some of the common terms ud when talking about cache.

2.1.1 Cache Hits

When the cache contains the information requested, the transaction is said to be

a cache hit.

2.1.2 Cache Miss

When the cache does not contain the information requested, the transaction is

said to be a cache miss.

2.1.3 Cache Consistency

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Now that we have some names for cache functions lets e how caches are designed and how

this effects their function.

2.2 Cache Architecture

Caches have two characteristics , a read architecture and a write policy. The read architecture

may be either “Look Aside” or “Look Through.” The write policy may be either “Write-Back” or

“Write-Through.” Both types of read architectures may have either type of write policy,

depending on the design. Write policies will be described in more detail in the next ction. Lets

examine the read architecture now.

2.2.1 Look Aside

CPU

SRAM

Cache Controller

Tag RAM

System Interface

Figure 2-2 Look Aside Cache

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2.2.2 Read Architecture: Look Through

CPU

SRAMCache ControllerTag RAM

System Interface

Figure 2-3 Look Through Cache

Figure 2-3 shows a simple diagram of cache architecture. Again, main memory is located

opposite the system interface. The discerning feature of this cache unit is that it sits between

the processor and main memory. It is important to notice that cache es the processors bus

cycle before allowing it to pass on to the system bus.

2.2.2.1 Look Through Read Cycle Example

When the processor starts a memory access, the cache checks to e if that address is a

cache hit.

HIT:

The cache responds to the processor’s request without starting an access to

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therefore less expensive to implement. The performance with a Write-Through policy is lower

since the processor must wait for main memory to accept the data.

2.3 Cache Components

The cache sub-system can be divided into three functional blocks: SRAM, Tag RAM, and the

Cache Controller. In actual designs, the blocks may be implemented by multiple chips or all

may be combined into a single chip.

2.3.1 SRAM

Static Random Access Memory (SRAM) is the memory block which holds the data. The size of

the SRAM determines the size of the cache.

2.3.2 Tag RAM

Tag RAM (TRAM) is a small piece of SRAM that stores the address of the data that is stored

in the SRAM.

2.3.3 Cache Controller

The cache controller is the brains behind the cache. Its responsibilities include: performing the

snoops and snarfs, updating the SRAM and TRAM and implementing the write policy. The

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cache controller is also responsible for determining if memory request is cacheable

and if a

request is a cache hit or miss.

2.4 Cache Organization

Main Memory

Cache

Page

Cache Line

Cache Line

Cache Line

Cache Line

:

:

Cache Line

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called a

cache line

. The size of a cache line is determined by both the processor and the cache

design. Figure 2-4 shows how main memory can be broken into cache pages and how each

cache page is divided into cache lines. We will discuss cache organizations and how to

determine the size of a cache page in the following ctions.

2.4.1 Fully-Associative

Main Memory

Line m

:

:

Line 2

Line 1

Line 0

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2.4.2 Direct Map

Main Memory Pages

Line n

.

Page m

:

Line n

Line 0

.

Line n

Page 1

.

Page 0

:

:

Line 0

Line 0

Cache Memory

Line n

.

.

.

Line 0

Figure 2-6 Direct Mapped

Direct Mapped cache is also referred to as 1-Way t associative cache. Figure 2-6 shows a

diagram of a direct map scheme. In this scheme, main memory is divided into cache pages.

The size of each page is equal to the size of the cache. Unlike the fully associative cache, the

direct map cache may only store a specific line of memory within the same line of cache. For

example, Line 0 of any page in memory must be stored in Line 0 of cache memory. Therefore if

Line 0 of Page 0 is stored within the cache and Line 0 of page 1 is requested, then Line 0 of

Page 0 will be replaced with Line 0 of Page 1. This scheme directly maps a memory line into an

equivalent cache line, hence the name Direct Mapped cache.

A Direct Mapped cache scheme is the least complex of all three caching schemes. Direct

Mapped cache only requires that the current requested address be compared with only one

cache address. Since this implementation is less complex, it is far less expensive than the other

caching schemes. The disadvantage is that Direct Mapped cache is far less flexible making the

performance much lower, especially when jumping between cache pages.

2.4.3 Set Associative

Main Memory Pages

Line n

.

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Associate cache scheme. In this scheme, two lines of memory may be stored at any time. This

helps to reduce the number of times the cache line data is written-over?

This scheme is less complex than a Fully-Associative cache becau the number of comparitors

is equal to the number of cache ways. A 2-Way Set-Associate cache only requires two

comparitors making this scheme less expensive than a fully-associative scheme.

3. The Pentium(R) Processors Cache

This ction examines internal cache on the Pentium(R) processor. The purpo of this ction

is to describe the cache scheme that the Pentium(R) processor us and to provide an overview

of how the Pentium(R) processor maintains cache consistency within a system.

The above ction broke cache into neat little categories. However, in actual implementations,

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CPU

L1 Cahce

Memory

L2 Cache

Memory

Main

DRAM

Memory

System Interface

Figure 3-1 Pentium Processor with L2 cache

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When developing a system with a Pentium(R) processor, it is common to add an external

cache. External cache is the cond cache in a Pentium(R) processor system, therefore it is

called a Level 2 (or L2) cache. The internal processor cache is referred to as a Level 1 (or L1)

cache. The names L1 and L2 do not depend on where the cache is physically located,( i.e.,

internal or external). Rather, it depends on what is first accesd by the L1 cache

is accesd before L2 whenever a memory request is generated). Figure 3-1 shows how L1 and

L2 caches relate to each other in a Pentium(R) processor system.

3.1 Cache Organization

Main Memory Pages

Line 127

.

Page m

:

Line 127

Line 0

.

Line 127

.

Page 1

:

Page 0

Line 0

:

Line 0

Cache Memory

Way 0Way 1

Line 127Line 127

..

..

..

Line 0Line 0

Figure 3-2 Internal Pentium Processor Cache Scheme

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Both caches are 2-way t-associative in structure. The cache line size is 32 bytes, or 256 bits.

A cache line is filled by a burst of four reads on the processor’s 64-bit data bus. Each cache way

contains 128 cache lines. The cache page size is 4K, or 128 lines. Figure 3-2 shows a diagram

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suggests, the CD bit allows the ur to disable the Pentium(R) processors internal cache. When

CD = 1, the cache is disabled, CD = 0 cache is enabled. The NW bit allows the cache to be

either write-through (NW = 0) or write-back (NW = 1).

The Pentium(R) processor maintains cache consistency with the MESI protocol. MESI is ud

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to allow the cache to decide if a memory entry should be updated or invalidated. With the

Pentium(R) processor, two functions are performed to allow its internal cache to stay consistent,

Snoop Cycles and Cache Flushing.

The Pentium(R) processor snoops during memory transactions on the system bus

. That is,

when another bus master performs a write, the Pentium(R) processor snoops the address. If the

Pentium(R) processor contains the data, the processor will schedule a write-back.

Cache flushing is the mechanism by which the Pentium(R) processor clears its cache. A cache

flush may result from actions in either hardware or software. During a cache flush, the

Pentium(R) processor writes back all modified (or dirty) data. It then invalidates its cache,(i.e.,