Cache memory exists to make computers faster, but simply knowing what cache is is not enough for IB Computer Science. Students are expected to understand how cache is used, and especially the ideas of cache hits and cache misses.
These concepts explain why cache improves performance and are often tested in system performance and CPU efficiency questions. Clear explanations using correct terminology are essential for full marks.
Why Cache Is Needed in the First Place
Modern CPUs operate far faster than RAM. Without cache memory:
- The CPU would frequently wait for data
- Processing speed would be limited by memory access times
Cache acts as a high-speed buffer between the CPU and RAM, storing data and instructions that are likely to be needed next.
What Is a Cache Hit?
A cache hit occurs when the CPU looks for data and finds it in the cache.
This means:
- The data is immediately available
- No access to RAM is required
- The CPU can continue processing without delay
Cache hits are desirable because cache memory is extremely fast compared to main memory.
In IB answers, students should explain that a cache hit:
- Reduces latency
- Improves overall system performance
- Allows the CPU to operate efficiently
What Is a Cache Miss?
A cache miss occurs when the CPU looks for data and does not find it in the cache.
When this happens:
- The CPU must request the data from RAM
- The data is transferred from RAM to the cache
- The CPU then accesses the data from cache
This process takes significantly longer than a cache hit.
IB examiners expect students to recognise that:
- Cache misses slow down execution
- Cache misses are unavoidable but should be minimised
Types of Cache Misses (HL Awareness)
At a higher level, students may be aware that cache misses occur for different reasons, such as:
- Data not yet loaded into cache
- Cache being too small
- Previously cached data being replaced
However, IB students are not expected to memorise advanced classifications — understanding the general concept is sufficient.
How Cache Improves Performance Overall
Even though cache misses occur, cache is effective because:
- Programs often reuse the same data
- Instructions are frequently accessed repeatedly
- Loops benefit heavily from caching
This behaviour means that most data requests result in cache hits, not misses.
Cache Hits, Cache Misses, and the Fetch–Decode–Execute Cycle
During the fetch stage:
- Instructions may be fetched from cache instead of RAM
- A cache hit speeds up instruction retrieval
- A cache miss introduces delay
This directly links cache behaviour to CPU performance.
Common Student Mistakes
Students often:
- Confuse cache with RAM
- Say cache permanently stores data
- Ignore the performance impact of misses
- Fail to explain what happens after a miss
Precise cause-and-effect explanations are required.
Final Thoughts
Cache hits and cache misses explain why cache memory is so important in modern computers. Cache hits allow the CPU to run at high speed, while cache misses introduce delays by forcing access to RAM.
Understanding this balance helps IB Computer Science students explain system performance clearly and accurately — exactly what examiners are looking for.
