Example 4: Real World Processor Overview
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Direct Mapped Cache Overview
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Today, we will explore the concept of direct mapped caches. Can anyone tell me what a cache is and its purpose?
A cache is a small-sized type of volatile computer memory that provides high-speed data access to a processor.
Correct! Caches help speed up data retrieval. Now, in a direct mapped cache, how is the memory address structured?
It consists of 's' bits for the address, 'w' bits for the word offset, and 'r' bits for the cache index.
Exactly! The tag, which is 's - r' bits long, is essential for checking if a line in the cache contains the required data. Let's remember this by using the acronym TWI: Tag, Word offset, and Index.
That helps, TWI for Tag, Word offset, and Index!
Great! Remembering this will aid in better understanding cache behavior.
Understanding Cache Hits and Misses
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Now, let's talk about cache hits and misses. Can anyone explain what happens during a cache hit?
When the data requested is found in the cache, we call it a hit, right?
Correct! During a hit, we access the cached data immediately. What about a cache miss?
A cache miss occurs when the data is not in the cache, and we have to fetch it from the main memory!
Exactly! And after fetching the data, it is stored in the cache for future access. Let’s do a quick check: if we access an address and it’s a miss, where do we get the data from?
From the main memory!
Awesome! You’ve got it. Remember, hits improve performance, while misses can slow things down.
Cache Examples with Real-World Processor
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Now, let’s look at an example of a real-world processor. Who can tell me about the Intrinsity FastMATH processor?
It has separate instruction and data caches, right? And each cache is 16 KB.
Exactly! How are these caches organized?
Each cache has 4 words per block, right? So the line size becomes 64 bytes.
Right again! Note that a cache with 16 words has a significant organization which improves efficiency. Can anyone explain how data retrieval works?
We send the address from either the PC or ALU to the cache, and if it matches the tag, we retrieve the data, right?
Exactly! And this process showcases how direct mapped caches help improve access time using locality of reference.
Introduction & Overview
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Quick Overview
Standard
The section breaks down the concepts behind direct mapped caches, highlighting how cache indexes and tags are used to identify memory addresses. It includes examples of memory accesses, cache hits and misses, and explanations of a specific real-world processor's cache organization.
Detailed
In this section, we delve into the organization and functionality of direct mapped caches, illustrating the interplay of memory addressing and cache organization. A direct mapped cache features a specific number of cache lines, each identified by an index derived from a portion of the memory address. Key components discussed include the tag field, line index, and word offset, which work together to determine whether a requested memory address results in a cache hit or miss. Several practical examples follow, detailing specific memory accesses and their results, elucidating how data is retrieved from either the cache or main memory. Finally, the section highlights a real-world example featuring the Intrinsity FastMATH processor, discussing its architecture and cache organization, particularly emphasizing how read requests are handled, including the retrieval of data and efficiency considerations related to cache size and access times.
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Direct Mapped Cache Overview
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Chapter Content
As a fourth and last example we take the example of a real word processor which uses direct mapped cache. So, we take the example of in Intrinsity FastMATH processor which is a fast embedded processor based on MIPS architecture. The direct mapped cache organization of this processor is shown in the figure here. So, the cache uses separate 16 KB instruction and data caches there is the mem the organization has 16 KB instruction and data caches separate.
Detailed Explanation
The Intrinsity FastMATH processor is an embedded processor that utilizes a direct mapped cache system to manage data efficiently. This particular organization uses dedicated cache for both instructions and data, each being 16 kilobytes (KB) in size. Direct mapped cache is a straightforward caching method where each block of main memory maps to exactly one cache line. This structure is beneficial for quick data retrieval as it minimizes the time taken to access frequently used instructions and data.
Examples & Analogies
Imagine a library where each book must only be placed on a specific shelf (representing the cache lines), determined by the book's category (the program's data addresses). If another book of the same category is added, it must replace the existing book in that shelf (the original book gets pushed out), which illustrates how direct mapped cache replaces existing entries based on memory requests.
Cache Specifications
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We have 32 bits per word. So, therefore, we have 4 byte words. We have 4K words in the cache ok and we have 16 word lines. So, each line contains 16 words. So, line size is 64 bytes. So, 16 words, each containing 4 bytes is 64 bytes and 64 bytes or 512 bits. We have a 8 bit wide line index. So, therefore, we have 256 lines in the cache. We have a 18 bit wide tag field. So, 218 possible blocks can map to each cache line ok.
Detailed Explanation
In this cache organization, each word is 32 bits, which translates to 4 bytes per word. With a total of 4,096 words in the cache, the structure consists of lines, where each line can hold 16 words, resulting in a total line size of 64 bytes (or 512 bits). The line index is 8 bits wide, enabling the existence of 256 distinct lines in the cache. Additionally, an 18-bit tag field allows for a maximum of 262,144 (or 2^18) possible memory blocks to be stored in the cache.
Examples & Analogies
Think of a large filing cabinet (the cache) divided into multiple drawers (the cache lines). Each drawer can only hold a limited number of files (the words). If you have a filing system that allows for multiple categories (the tags), you can quickly store and retrieve documents. The number of drawers and their size determines how quickly you can find what you're looking for, just like how the cache size and line size increase the speed of data access.
Reading from Cache
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What are the steps for a read request on this? We send the address to cache, either the instruction cache or the data cache. Addresses are sent from the PC for the instruction cache and from the ALU for the data cache. On a hit, that means, the tag bits and valid bits match the tag bits and the valid bits match. On a hit when we have the tag bits and the valid bits matching, the data is made available on the data lines.
Detailed Explanation
When the processor needs data, it sends an address to either the instruction cache or the data cache. This address comes from the program counter (PC) for instructions or the Arithmetic Logic Unit (ALU) for data. If the data is found (referred to as a 'hit'), the tag bits and valid bits that help verify the location match perfectly, allowing the desired data to be quickly fetched from the cache and delivered via the data lines.
Examples & Analogies
Picture a waiter at a restaurant retrieving a food order from a specific table for a customer. The order (the address) originates from the kitchen (PC or ALU) who requests a dish. If the dish is currently on the table (the data cache hit), the waiter delivers it directly. However, if the dish isn't there, a different retrieval process involving the kitchen (main memory) would begin, illustrating cache hits and misses.
Word Selection in Cache
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We have 16 words per line; that means, a line offset 16 words per line. So, we need to identify which word in the line is required. So, what we have? We have a line offset which is used to select which word in the line is desired by the memory. So, this line offset is used as a selector in a 16×1 mux and we have a 4 bit line access because we have 16 words in the line.
Detailed Explanation
With each cache line containing 16 words, when the processor determines a hit has occurred, the system must then identify precisely which word among those in the line is being requested. To achieve this, a line offset was utilized. This offset acts as an index to select the appropriate word using a multiplexer (mux), which allows for efficient and immediate access to the specific word required for processing.
Examples & Analogies
Consider a pizza with 16 slices (the words), where each slice can have a different topping. When a customer orders a topping (the specific word), the waiter checks which slice to serve (the line offset) using a distinct method (the mux). The faster this identification occurs, the quicker the customer receives their desired slice, similar to how line offsets streamline word retrieval in cache.
Key Concepts
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Direct Mapped Cache: A cache structure where each memory block maps to a unique cache line.
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Cache Hits and Misses: Understanding whether data is successfully retrieved from the cache or must be fetched from memory.
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Memory Address Structure: Comprises tag, cache index, and word offset to identify data locations.
Examples & Applications
To retrieve a word from memory address 22, when checked against an initially empty direct mapped cache, it results in a miss, and the data is loaded into the cache.
In a practical case with the Intrinsity FastMATH processor, a different cache line holding the tag from a previous access leads to a cache miss and replacement of data.
Memory Aids
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Rhymes
Cache fast and hit right, data found brings delight!
Stories
Imagine a librarian (the cache) who quickly fetches books (data) stored nearby wherever they are needed in a library (main memory), never getting lost, making life easier for readers (the processor).
Memory Tools
HIT for 'Hit' - H for 'House of Data', I for 'Instant retrieval', T for 'Tag matches'.
Acronyms
TWI
Tag
Word offset
Index - key components to remember in cache addresses.
Flash Cards
Glossary
- Direct Mapped Cache
A type of cache where each memory block maps to exactly one cache line.
- Cache Hit
Occurs when the requested data is found in the cache.
- Cache Miss
Occurs when the requested data is not found in the cache and must be fetched from main memory.
- Tag
Part of the cache that is used to identify which specific memory block is stored in a cache line.
- Cache Index
The portion of the memory address used to determine where in the cache a block resides.
- Word Offset
The portion of the address that determines which word within a cache block is being accessed.
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