Computer ScienceCBSE Class 11 · Computer System

CPU, Memory Hierarchy and Units of Memory Class 11: Concepts and Conversions

Updated 17 October 2026 · Fact-checked

The CPU has an ALU that performs arithmetic and logic, a control unit that directs operations, and registers that hold data being processed. Memory forms a hierarchy: registers, cache, RAM, then secondary storage; speed and cost per byte fall and capacity rises down the hierarchy. Units: 1 byte = 8 bits; 1 KB = 1024 bytes; 1 MB = 1024 KB; 1 GB = 1024 MB.

Going down the pyramid, memory gets slower and cheaper but bigger; converting a unit down (GB to MB to KB to Byte) multiplies by 1024 each step.

Illustration drawn for QuestionBuddy.

Understand CPU, Memory Hierarchy and Units of Memory with Conversions

CPU. (1) *Arithmetic Logic Unit (ALU)*: arithmetic (add, subtract) and logic (compare, AND, OR). (2) *Control Unit (CU)*: fetches instructions, decodes them, and directs the other parts. (3) *Registers*: very small, very fast storage inside the CPU (for example holding the current instruction or an operand).

Memory types.

*Cache*: small, very fast memory between the CPU and RAM that stores frequently used data and instructions.
*RAM (primary memory)*: volatile; holds programs and data in use; contents lost when power is off.
*ROM*: non-volatile; holds the start-up (boot) instructions.
*Secondary memory*: hard disk, SSD, pen drive, CD/DVD; non-volatile, large, slower.

Hierarchy. From top: registers, cache, RAM, secondary storage. Moving down the hierarchy, access gets slower and cost per byte falls, while capacity increases.

Units. 1 nibble = 4 bits; 1 byte = 8 bits; 1 KB = 1024 B; 1 MB = 1024 KB; 1 GB = 1024 MB; 1 TB = 1024 GB; 1 PB = 1024 TB.

Conversions. Larger to smaller: multiply by 1024 for each step. Smaller to larger: divide. 3 GB in KB = 3 x 1024 x 1024 = 31,45,728 KB (3,145,728).

Image size. Number of pixels x bits per pixel, divided by 8 for bytes. A 1024 x 768 image at 24 bits per pixel uses 1024 x 768 x 3 = 23,59,296 bytes (2,359,296 bytes), which is 2.25 MB.

Fetch-decode-execute. The CU fetches an instruction from memory, decodes it and the ALU executes it; the result is stored back.

CPU, Memory Hierarchy and Units of Memory with Conversions: key facts

CPU parts: ALU arithmetic and logic
CU controls and decodes
Registers fastest storage inside the CPU; Three parts.
Memory order: Registers
Cache
RAM
Secondary storage
Speed falls and capacity rises down the list; Hierarchy.
Units: 1 byte = 8 bits
1 KB = 1024 bytes
1 MB = 1024 KB
1 GB = 1024 MB
1 TB = 1024 GB; Powers of 1024.
Image size: Width x height x bits per pixel = bits | Divide by 8 for bytes; Then convert to KB or MB.

ALU	arithmetic and logic
CU	controls and decodes
Registers	fastest storage inside the CPU

How to answer CPU, Memory Hierarchy and Units of Memory with Conversions questions

Conversion questions are multiplication or division by 1024; identify the direction first.

1Write the unit you have and the unit you want.
2Count the steps between them on the ladder byte, KB, MB, GB, TB.
3Multiply by 1024 per step going down; divide going up.
4For images, find total bits first, then divide by 8.
5State the unit in the answer.

CPU, Memory Hierarchy and Units of Memory with Conversions in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Photos on a phone

Suppose a phone has 2 GB of free space and each photo takes 4 MB.

How many photos fit?

Try it first, then show the solutionSolution

2 GB = 2 x 1024 = 2048 MB.
2048 ÷ 4 = 512.

Answer: 512 photos.

Memory hook: GB to MB multiply by 1024, then divide by the size.

Tricks to remember CPU, Memory Hierarchy and Units of Memory with Conversions

Short memory aids. Say each one aloud once, then use it on a question the same day.

Fast is small, big is slow
Registers fastest and smallest; secondary storage largest and slowest.
Use it when: Hierarchy.
1024 steps
Each step in the unit ladder is 1024.
Use it when: Conversions.

CPU, Memory Hierarchy and Units of Memory with Conversions: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Volatile memory · 1 mark · MCQBoard level

Which of the following loses its contents when the power is switched off?
(a) ROM
(b) RAM
(c) hard disk
(d) SSD

Try it first, then show the solutionSolution

RAM is volatile.

Answer: (b) RAM

Example 2: Role of the parts · 3 marks · Short answerBoard level

State one function each of the ALU, the control unit and the registers. (3 marks)

Try it first, then show the solutionSolution

ALU: performs arithmetic and logical operations.
Control unit: fetches and decodes instructions and directs the other parts.
Registers: hold the data and instructions currently being processed inside the CPU.

Answer: ALU computes; CU controls; registers hold current data.

Example 3: Conversions · 3 marks · Short answerHarder board level

Convert: (i) 3 GB to KB; (ii) 1.5 MB to KB; (iii) 5 KB to bits. (3 marks)

Try it first, then show the solutionSolution

(i) 3 x 1024 x 1024 = 3,145,728 KB.
(ii) 1.5 x 1024 = 1,536 KB.
(iii) 5 x 1024 = 5,120 bytes; 5,120 x 8 = 40,960 bits.

Answer: (i) 3,145,728 KB (ii) 1,536 KB (iii) 40,960 bits

Example 4: Image size · 5 marks · Short answerHarder board level

A 1024 x 768 pixel image uses 24 bits per pixel. Find its size in bytes and in MB. How many such images fit in a 9 MB space? (5 marks)

Try it first, then show the solutionSolution

Total bits per pixel is 24, which is 3 bytes.
Size = 1024 x 768 x 3 = 2,359,296 bytes.
In MB: 2,359,296 ÷ 1,048,576 = 2.25 MB.
Images in 9 MB: 9 ÷ 2.25 = 4.

Answer: 2,359,296 bytes = 2.25 MB; 4 images fit.

Example 5: Why a cache? · 4 marks · Short answerHOTS / competency

RAM is faster than a hard disk, yet computers also use cache memory. Explain why cache is faster and smaller than RAM and why the computer still needs RAM. Why can secondary storage not replace RAM? (4 marks)

Try it first, then show the solutionSolution

Cache stores frequently used data close to the CPU, so the CPU waits less.
It is small because very fast memory costs much more per byte.
RAM is larger and holds all running programs and data.
Secondary storage is too slow for the CPU to work from directly, though it keeps data permanently; RAM holds data in use.

Answer: Cache: fast and small; RAM: large working memory; secondary: permanent but slow.

Where students lose marks in CPU, Memory Hierarchy and Units of Memory with Conversions

The slips examiners see most often, and what to write instead.

Mistake: Using 1000 instead of 1024
Decimal habit.
Wrong working
1 KB = 1000 bytes.
Right working
1 KB = 1024 bytes.
Do this: Use powers of 2 in computer science.
Usually costs: 1 mark
Mistake: Forgetting to divide by 8
Leaving the answer in bits.
Wrong working
Image size = 18,874,368 bytes.
Right working
Image size = 18,874,368 bits ÷ 8 = 2,359,296 bytes.
Do this: Bits divided by 8 for bytes.
Usually costs: 1 mark
Mistake: Placing ROM in the volatile group
Confusing RAM and ROM.
Wrong working
ROM loses its contents.
Right working
ROM is non-volatile.
Do this: RAM is volatile; ROM is not.
Usually costs: 1 mark

Exam tips

Write the unit ladder and count steps before multiplying.
Give units in every conversion answer.
In hierarchy answers mention speed, size and cost together.
For image size, state the number of bytes per pixel.

Practice questions on CPU, Memory Hierarchy and Units of Memory with Conversions

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
1 GB equals:
(a) 1000 MB
(b) 1024 MB
(c) 1024 KB
(d) 1024 bytes
Hint
Powers of 1024.
Answer
(b) 1024 MB
Q2 · 3 marks · Short answerHarder board level
How many 4 MB songs can be stored on a 2 GB pen drive? How many bits does a 2 KB message use? (3 marks)
Hint
Convert GB to MB first.
Answer
Songs = 2 x 1024 ÷ 4 = 512. A 2 KB message = 2 x 1024 x 8 = 16,384 bits.
Q3 · 3 marks · Short answerHarder board level
An image is 800 pixels wide and 600 pixels high with 16 bits per pixel. Find its size in KB. (3 marks)
Hint
Pixels x bytes per pixel.
Answer
Pixels = 800 x 600 = 480,000. Bytes = 480,000 x 2 = 960,000. KB = 960,000 ÷ 1024 = 937.5 KB.
Q4 · 1 mark · Assertion-reasonHarder board level
Assertion (A): The CPU uses registers rather than RAM for the operands of the current instruction.
Reason (R): Registers are faster than RAM.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Hint
Top of the hierarchy.
Answer
(a) Both A and R are true, and R is the correct explanation of A.
Q5 · 5 marks · Short answerHOTS / competency
A video of 60 seconds is stored at 1 MB per second. Find the size in GB. If a drive has 8 GB free, how many such videos fit and what space is left? (5 marks)
Hint
60 MB = 60 / 1024 GB.
Answer
Size = 60 MB = 60 ÷ 1024 GB ≈ 0.0586 GB. In MB, 8 GB = 8,192 MB; 8,192 ÷ 60 = 136.5, so 136 videos fit using 8,160 MB; the free space left is 32 MB.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Which part of the CPU decodes instructions?Show answer
Control unit.
2.Fastest memory?Show answer
Registers.
3.Is RAM volatile?Show answer
Yes.
4.1 MB equals how many KB?Show answer
1024.
5.Image size formula?Show answer
Pixels x bits per pixel ÷ 8 for bytes.

CPU, Memory Hierarchy and Units of Memory with Conversions: frequently asked questions

What is cache memory?

Small, very fast memory that stores frequently used data and instructions.

Why is ROM used?

To keep the start-up instructions permanently.

What does the ALU do?

It performs arithmetic and logical operations.