📑 Topics on this page (Jump to Topic):
📌 1. Memory Units
| Unit | Value |
|---|---|
| Bit | 0 or 1 (binary digit) — smallest unit |
| Nibble | 4 bits |
| Byte | 8 bits |
| KB | 1024 Bytes |
| MB | 1024 KB |
| GB | 1024 MB |
| TB | 1024 GB |
| PB | 1024 TB |
| EB | 1024 PB |
| ZB | 1024 EB |
| YB | 1024 ZB |
💡 Memory Tip: Order: K-M-G-T-P-E-Z-Y (Kilo, Mega, Giga, Tera, Peta, Exa, Zetta, Yotta).
⚠️ Exam Traps:
- 1 byte = 8 bits, 1 nibble = 4 bits. 1 KB = 1024 bytes (not 1000, in binary).
📌 2. Primary Memory: RAM & ROM
| RAM | ROM | |
|---|---|---|
| Full form | Random Access Memory | Read Only Memory |
| Nature | Volatile — data lost when power goes | Non-volatile — data stays |
| Use | Running programs and data | Boot instructions, BIOS |
| Types | SRAM (fast, used in cache), DRAM (needs refreshing, main memory) | PROM (write once), EPROM (erase with UV), EEPROM (erase electrically) |
- Flash memory is a form of EEPROM — used in pen drives, SSDs, memory cards.
- Cache memory is very fast memory (SRAM) between the CPU and RAM; it keeps frequently used data. Levels: L1, L2, L3.
- Virtual memory = using part of the hard disk as if it were RAM.
Memory hierarchy (fast → slow)
Register → Cache → RAM → SSD/Hard disk → Optical/Tape. Going up, speed and cost per bit rise and capacity falls.
⚠️ Exam Traps:
- DRAM needs refreshing; SRAM does not — SRAM is faster and costlier.
- The fastest memory = register (not cache).
- Virtual memory is not physical RAM; it is part of the hard disk.
📌 3. Secondary Memory (Storage Devices)
| Device | Fact |
|---|---|
| Hard Disk Drive (HDD) | Magnetic; platters, tracks, sectors; first HDD — IBM 350 (1956) |
| SSD (Solid State Drive) | Flash memory; no moving parts; faster than HDD |
| Magnetic Tape | Sequential access; cheap for backups |
| Floppy Disk | Old magnetic; 3.5 inch = 1.44 MB |
| CD | Optical; about 700 MB |
| DVD | Optical; 4.7 GB (single layer) |
| Blu-ray | Blue-violet laser; 25 GB (single layer) |
| Pen drive / Memory card | Flash memory, USB |
| Cloud storage | On internet servers — Google Drive, OneDrive, iCloud |
- Access time = seek time + rotational latency (+ transfer time).
- Formatting prepares a disk into tracks and sectors; defragmentation brings scattered file pieces together.
⚠️ Exam Traps:
- Magnetic tape = sequential access; hard disk = direct/random access.
- A CD-ROM is optical, not magnetic.
📌 4. Practice MCQs
📝 Practice MCQs (prepared by SF Prep — not PYQs)
Q1. How many bits are there in 1 byte? Practice
Correct answer: (B)
1 byte = 8 bits; 4 bits = 1 nibble.
1 byte = 8 bits; 4 bits = 1 nibble.
Q2. 1 nibble is equal to: Practice
Correct answer: (C)
Nibble = half a byte = 4 bits.
Nibble = half a byte = 4 bits.
Q3. 1 TB is equal to: Practice
Correct answer: (D)
1 TB = 1024 GB.
1 TB = 1024 GB.
Q4. Which of these is the largest unit? Practice
Correct answer: (A)
Order: TB < PB < EB < ZB < YB.
Order: TB < PB < EB < ZB < YB.
Q5. Which memory loses its data when power is switched off? Practice
Correct answer: (B)
RAM is volatile; ROM, hard disk and flash are non-volatile.
RAM is volatile; ROM, hard disk and flash are non-volatile.
Q6. Which is the fastest memory in a computer? Practice
Correct answer: (C)
Registers are inside the CPU, so they are fastest. Cache comes next.
Registers are inside the CPU, so they are fastest. Cache comes next.
Q7. Cache memory works between: Practice
Correct answer: (D)
Cache keeps frequently used data close to the CPU to increase speed.
Cache keeps frequently used data close to the CPU to increase speed.
Q8. Which RAM must be refreshed repeatedly? Practice
Correct answer: (A)
DRAM stores data in capacitors that leak, so it needs refreshing.
DRAM stores data in capacitors that leak, so it needs refreshing.
Q9. ROM that can be erased with ultraviolet (UV) light is: Practice
Correct answer: (B)
EPROM = Erasable PROM, erased with UV. EEPROM is erased electrically.
EPROM = Erasable PROM, erased with UV. EEPROM is erased electrically.
Q10. What kind of memory is used in a pen drive? Practice
Correct answer: (C)
Pen drives, SSDs and memory cards use flash memory (a form of EEPROM).
Pen drives, SSDs and memory cards use flash memory (a form of EEPROM).
Q11. Using part of the hard disk as if it were RAM is called: Practice
Correct answer: (D)
When RAM is full the OS uses a page file/swap on disk as virtual memory.
When RAM is full the OS uses a page file/swap on disk as virtual memory.
Q12. Which storage accesses data sequentially? Practice
Correct answer: (A)
On tape you must pass over earlier data to reach the required data.
On tape you must pass over earlier data to reach the required data.
Q13. What is the usual capacity of a single-layer DVD? Practice
Correct answer: (B)
CD ≈ 700 MB, DVD = 4.7 GB, Blu-ray = 25 GB (single layer).
CD ≈ 700 MB, DVD = 4.7 GB, Blu-ray = 25 GB (single layer).
Q14. The usual capacity of a 3.5-inch floppy disk was: Practice
Correct answer: (C)
3.5-inch HD floppy = 1.44 MB.
3.5-inch HD floppy = 1.44 MB.
Q15. Why is an SSD faster than an HDD? Practice
Correct answer: (D)
An SSD uses flash chips; an HDD has spinning platters and a moving head.
An SSD uses flash chips; an HDD has spinning platters and a moving head.
Q16. What colour of laser is used in a Blu-ray disc? Practice
Correct answer: (A)
A shorter-wavelength blue-violet laser allows more data to be stored.
A shorter-wavelength blue-violet laser allows more data to be stored.
Q17. BIOS is usually stored in: Practice
Correct answer: (B)
BIOS/UEFI lives in a ROM (nowadays flash ROM) chip on the motherboard.
BIOS/UEFI lives in a ROM (nowadays flash ROM) chip on the motherboard.
Q18. The process of bringing scattered file pieces together on a disk is: Practice
Correct answer: (C)
Defragmentation improves the reading speed of an HDD.
Defragmentation improves the reading speed of an HDD.
Q19. Which of these is a cloud storage service? Practice
Correct answer: (D)
Google Drive, OneDrive, iCloud and Dropbox store data on the internet.
Google Drive, OneDrive, iCloud and Dropbox store data on the internet.
Q20. Moving up the memory hierarchy (towards registers): Practice
Correct answer: (A)
Higher levels are faster and costlier but smaller.
Higher levels are faster and costlier but smaller.