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A short class with Sarah and her students. The other ways are under More.
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Welcome class! Today, we will learn about direct mapped cache systems. Can anyone tell me what a cache is?
Isn't it a type of memory that stores frequently accessed data?
Exactly! A cache stores data to speed up access times. Now, in a direct mapped cache, each block has a unique line in the cache. Who can remember what 's', 'r', and 'w' stand for in memory addresses?
's' is the total bits in a memory address, 'r' is the bits for the cache index, and 'w' is the number of bits per word!
Excellent! We will use these terms often today. Now, let's explore how these components function together.
Let's talk about what happens when we access data in the cache. Can anyone explain the difference between a cache hit and a cache miss?
A cache hit occurs when the data is already in the cache, while a cache miss happens when it needs to be fetched from the main memory.
Correct! In the case of a hit, we can read from the cache. If there's a miss, we must retrieve the data from main memory. Can anyone give an example of when we have a hit?
When we access the same address twice, the second access would be a hit if it was brought into the cache the first time!
Well done! Now, let's analyze a specific example of memory accesses and see how hits and misses occur.
For our example, let's look at the access sequence: 22, 26, 16, 3, 16, and 18. What happens when we access the address 22 first?
The binary for 22 is 10110. Since the cache is initially empty, it will be a cache miss, and we will load it from the main memory.
Precisely! So we will store it in line 110 with the tag 10. Now, what happens when we access 26 next?
We again have a cache miss since 26 hasn't been loaded yet, and we load it into line 010 with the tag 11.
Great job! Let's continue with the rest of the accesses and analyze how the cache updates.
Now, let's explore a 16 KB cache with 4-word blocks. Can anyone tell me how we calculate the total bits in this cache?
We need to consider the block size, the number of lines, and the tag bits!
Exactly! If the block size is 4 words each, and we find the total number of blocks in main memory, how do we calculate the number of tag bits?
We subtract the bits used for the cache index from the total bits representing the main memory blocks!
Exactly right! This understanding is crucial in analyzing cache efficiency. Now, let's summarize our discussion.
Each memory block maps to one cache line.
When the required data is found in the cache.
When the required data must be fetched from main memory.
Used to verify data correspondence in the cache.
A row in the cache that holds a block of memory data.
Cache line's where blocks align, Shapes a structure, oh so fine!
Once in a town of Memory, a cache stood proud with blocks. Whenever a cache hit occurred, the townsfolk cheered, ‘Data is here!’ But when a cache miss rang, they rushed to fetch the data from the Memory Mountain.
H.M. - Hit Memory, Miss Main memory.
TAM - Tag, Address, Memory - Remember your cache structure!
Term
What happens during a cache hit?
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Term
Define cache line.
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A cache structure where each block of main memory maps to exactly one cache line.
An occurrence when data requested by the CPU is found in the cache.
An occurrence when data requested by the CPU is not found in the cache and must be retrieved from main memory.
Bits in the memory address used to identify if a block in the cache corresponds to a block in main memory.
A single row in the cache memory used to store data from main memory.
This section explains the organization of a direct mapped cache and illustrates its functioning through examples and calculations.
The section delves into the structure of a direct mapped cache, including memory address breakdowns and examples of cache hits and misses. It also provides practical cases and structured calculations to demonstrate how data is managed within the cache.
In this section, we explore the organization of a direct mapped cache system. The memory address for cache systems comprises 's' plus 'w' bits, where the tag contains 's - r' bits, the cache index, and each word within a cache block is identified by a word offset. The mechanism for checking if a line is present in the cache involves matching the tag field with the corresponding bits from the main memory address.
The section presents a simple example with an initial state of an 8-block cache where all data is initially empty. The example proceeds through a sequence of memory accesses (22, 26, 16, 3, 16, 18), demonstrating hits and misses. For instance, accessing the first address (22) results in a cache miss and prompts loading data from the main memory. The example continues to showcase how the subsequent accesses are managed within the cache, including situations where the data is already present (hits) and the need to replace existing data (cache misses).
Additionally, the section covers a more complex example involving a 16 KB direct mapped cache and calculations associated with cache lines, block sizes, and total bits required, as well as how specific byte addresses map to cache lines. The principles of locality of reference, cache hits, and misses are emphasized to underline the efficiency gains from using cache memory.