← IGCSE Computer Science Paper 1

Cache memory

A small amount of very fast memory in or near the CPU that stores frequently used instructions and data, so the CPU can get them faster than from RAM.

Part of: IGCSE Computer Science 0478 — 3.1 Computer Architecture — Module 27, ACARA Digital Technologies Years 7–8 — Hardware and Performance — Module 12

Understanding Cache memory in IGCSE Computer Science Paper 1

What it does: This shows the order of lookup: cache first because it is faster, and RAM only after a cache miss. This compares the two memories: cache has less capacity than RAM but is much quicker to access. The CPU checks the cache first. If the item is there, it avoids the slower trip to RAM. Cache holds only the instructions and data used most often. Everything else stays in RAM or secondary storage. Level 1 (L1) cache is the smallest and fastest. Levels L2 and L3 get bigger and slightly slower.

Common mistakes: If the instruction is found in the cache, the CPU uses it from the cache, which is faster. Only the ELSE branch fetches from RAM, so line 3 should say 'use it from the cache'. When the data is not in the cache, the CPU goes to RAM (main memory), not ROM. Line 4 should be 'ELSE read data from RAM'.

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This module's key terms are also in a printable crossword (Modules 228–239) and word search (Modules 218–239).

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Learning
Duo
Which statement about cache memory is correct?
Cache is slower than RAMCache is faster than RAM
Matches
Cache is larger than RAM, so it can store all of the programs
↓
Cache is much smaller than RAM, not larger. It stores only frequently used instructions and data, not every program.
Applying
Meanings
Term
cache
Meaning
A small amount of very fast memory in or near the CPU that stores frequently used instructions and data, so the CPU can get them faster than from RAM.
Practice
IF instruction IS IN cache THEN
Testing
Gaps
IF item IS IN THEN
Check Answer
Spots
2IF it is found in cache THEN
3 use it from RAM

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Quick Questions

1. Which statement about cache memory is correct?
A. Cache is faster than RAM ✓
B. Cache is slower than RAM
Cache is very fast memory built in or near the CPU, so the CPU can access it more quickly than RAM.
2. Compared with RAM, how much storage does cache usually have?
A. Much more (it is bigger than RAM)
B. Much less (it is smaller than RAM) ✓
Cache is small because very fast memory is expensive. A computer may have gigabytes of RAM but only a few megabytes of cache.
3. What does the CPU do first when it needs an instruction or data?
A. Goes straight to RAM
B. Checks the cache ✓
The CPU checks the cache first. If the item is there, it avoids the slower trip to RAM.
4. What does cache store?
A. Frequently used instructions and data ✓
B. Every program on the computer
Cache holds only the instructions and data used most often. Everything else stays in RAM or secondary storage.
5. Which description of cache levels is correct?
A. L1 is the smallest and fastest; L2 and L3 are larger but slower ✓
B. L3 is the smallest and fastest; L1 is the largest but slowest
Level 1 (L1) cache is the smallest and fastest. Levels L2 and L3 get bigger and slightly slower.

Match the Pairs

cache → A small amount of very fast memory in or near the CPU that stores frequently used instructions and data
L1 cache → The smallest and fastest level of cache, found closest to the CPU core
The CPU checks cache first, and only goes to RAM if the data is not found there → if data in cache: use it, else: read RAM
Cache is smaller but faster than RAM, so it only holds the most frequently used data → cache size < RAM size, cache speed > RAM speed
Cache is larger than RAM, so it can store all of the programs → Cache is much smaller than RAM, not larger. It stores only frequently used instructions and data, not every program.
The CPU looks in RAM first, then checks cache if RAM is too slow → The CPU checks cache first because it is faster, and only goes to RAM if the data is not in cache.

Key Term

cache
A small amount of very fast memory in or near the CPU that stores frequently used instructions and data, so the CPU can get them faster than from RAM.

Fill in the Blanks

1. IF item IS IN cache THEN
Hint: Type the name of the small, very fast memory that the CPU checks first.
2. cacheSize < ramSize
Hint: Type the comparison operator showing that cache is smaller than RAM.
3. cacheSpeed > ramSpeed
Hint: Type the comparison operator showing that cache is faster than RAM.
4. CHECK L1, then L2, then L3, then RAM
Hint: Type the level of cache that comes between level 1 and level 3.
5. IF cacheHit = FALSE THEN READ RAM
Hint: Type the larger, slower memory the CPU uses when the data is not found in cache.

Spot the Error

Question 1
A CPU needs an instruction
B IF it is found in cache THEN
C use it from RAM
D ELSE
E fetch it from RAM
Line C — If the instruction is found in the cache, the CPU uses it from the cache, which is faster. Only the ELSE branch fetches from RAM, so line 3 should say 'use it from the cache'.
Question 2
A CPU needs data
B CHECK cache first
C IF found THEN use cache data
D ELSE read data from ROM
Line D — When the data is not in the cache, the CPU goes to RAM (main memory), not ROM. Line 4 should be 'ELSE read data from RAM'.
Question 3
A Cache levels:
B L1 - smallest and fastest
C L2 - bigger, slower than L1
D L3 - smaller, faster than L2
Line D — Higher cache levels are bigger but slower. L3 is larger and slower than L2, not smaller and faster.
Question 4
A cacheSize ← 8
B ramSize ← 16000
C IF cacheSize > ramSize THEN
D OUTPUT "Cache is smaller"
Line C — Cache is smaller than RAM, so the condition must test cacheSize < ramSize. With > the message would not be output.
Question 5
A cacheTime ← 1
B ramTime ← 0.5
C IF cacheTime < ramTime THEN
D OUTPUT "Cache is faster"
Line B — Cache is faster than RAM, so RAM must have the longer access time. ramTime should be bigger than cacheTime, such as 100.

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