4. Direct-mapped cache

Basic Cache

On a request from the processor for the data in memory address $m$, we search the tags in cache:

• $\text{Hit:}$ $m$ is a tag
  1. Return data.
• $\text{Miss:}$ $m$ is not a tag
  1. Read data at address $m$ from memory.
  2. Return data.
  3. Select an address in the cache to hold $m$ and its data (according to replacment policy).
  4. Store $m$ in tag with data.

We use a number of approaches to organise the cache described over the next few topics, including this one.

Direct-mapped cache

Direct-mapped cache

In direct-mapped cache, each block in memory maps to a **single**, **specific** cache block.

For a direct mapped cache consisting of $N$ blocks: ?

• Block $X$ of main memory maps to cache block $Y$.
• $Y=X\mathrm{mod}N$

To find out which block actually resides in a cache block at any given time we use a tag field which distinguishes one memory block from another.

A direct-mapped cache address is composed of the following: ?

• The $\text{block field}$: number of bits required to address all cache blocks.
• The $\text{offset field (word field)}$: the number of bits required to address all words within a block.
• The $\text{tag field}$: the number of bits left over after the block and offset. Formatted as such: $001101110$ ($\text{tag}$, $\text{block}$, $\text{word}$).

title: Remember the order: $\color{blue}\textsf{tag}$, $\color{red}\textsf{block}$, $\color{green}\textsf{word}$

Link to original

title: Example 1
![](https://git.is.horse/insert/university/obsidian-notes/-/raw/9d95afcbbdb7a4c77ca9f62b32e35c6f31e269e7/University/Year%201/Semester%201/4CCS1CS1%20Computer%20Systems%201/Week%209.%20Memory%20hierarchy%20and%20cache%20systems/Diagrams/Pasted%20image%2020211206123621.png)

title: Example 2
![](https://git.is.horse/insert/university/obsidian-notes/-/raw/9d95afcbbdb7a4c77ca9f62b32e35c6f31e269e7/University/Year%201/Semester%201/4CCS1CS1%20Computer%20Systems%201/Week%209.%20Memory%20hierarchy%20and%20cache%20systems/Diagrams/Pasted%20image%2020211206123733.png)