Computer Memory, Storage Devices and Units: RAM, ROM, Cache, Hard Disk, SSD and Byte Conversions for SSC
Rahul Kumar
SSC Exam Expert & Content Editor
In this note
- 1.Units of Memory
- 2.Primary Memory (Main Memory)
- 3.Types of RAM
- 4.Types of ROM
- 5.Memory Hierarchy (Fastest to Slowest, Smallest to Largest)
- 6.Secondary Storage Devices
- 7.Virtual Memory
- 8.Volatile and Non-Volatile
- 9.Solved Examples
- 10.Quick Tips
- 11.Quick Revision Points
- 12.Practice Questions With Answers
- 13.How to Practise This Chapter
- 14.Frequently Asked Questions
PDF Notes: Computer Memory, Storage Devices and Units: RAM, ROM, Cache, Hard Disk, SSD and Byte Conversions for SSC
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Memory and storage are among the most frequently asked topics in computer awareness. This guide on computer memory, storage devices and units for SSC explains the units of memory with conversions, the types of RAM and ROM, cache memory, the secondary storage devices and their capacities, with solved examples, practice questions and FAQs.
Units of Memory
| Unit | Value |
|---|---|
| Bit (binary digit) | The smallest unit: 0 or 1 |
| Nibble | 4 bits |
| Byte | 8 bits (one character) |
| Kilobyte (KB) | 1,024 bytes |
| Megabyte (MB) | 1,024 KB |
| Gigabyte (GB) | 1,024 MB |
| Terabyte (TB) | 1,024 GB |
| Petabyte (PB) | 1,024 TB |
| Exabyte (EB) | 1,024 PB |
| Zettabyte (ZB) | 1,024 EB |
| Yottabyte (YB) | 1,024 ZB |
In computing, each step is 210 = 1,024. Storage makers often use 1,000 for the sake of simplicity.
Primary Memory (Main Memory)
| Memory | Features |
|---|---|
| RAM (Random Access Memory) | Volatile (the data is lost when the power is off); read and write; holds the data and programs currently in use; the more the RAM, the faster the computer |
| ROM (Read Only Memory) | Non-volatile; the data is stored permanently; holds the start-up instructions (BIOS) |
| Cache memory | Very fast, small memory between the CPU and RAM; stores the frequently used data and instructions |
| Registers | Fastest and smallest memory inside the CPU |
Types of RAM
| Type | Features |
|---|---|
| SRAM (Static RAM) | Faster and costlier; no refreshing needed; used for cache memory |
| DRAM (Dynamic RAM) | Slower and cheaper; needs refreshing; used as the main memory |
| SDRAM, DDR (DDR2, DDR3, DDR4, DDR5) | Synchronous DRAM; newer DDR versions are faster |
Types of ROM
| Type | Features |
|---|---|
| MROM (Masked ROM) | Programmed by the manufacturer; cannot be changed |
| PROM (Programmable ROM) | Can be programmed once by the user |
| EPROM (Erasable PROM) | Can be erased by ultraviolet light and reprogrammed |
| EEPROM (Electrically Erasable PROM) | Can be erased and rewritten electrically |
| Flash memory | A type of EEPROM; used in pen drives, memory cards and SSDs |
Memory Hierarchy (Fastest to Slowest, Smallest to Largest)
Registers → Cache → RAM → Secondary storage (SSD, hard disk) → Tertiary storage (tape, optical). Faster memory is more expensive per byte.
Secondary Storage Devices
| Device | Features and Typical Capacity |
|---|---|
| Hard disk drive (HDD) | Magnetic; spinning platters; capacity from hundreds of GB to many TB |
| Solid-state drive (SSD) | Flash-based; no moving parts; faster, quieter and more durable than HDD |
| CD-ROM | Optical; about 700 MB |
| DVD | Optical; about 4.7 GB (single layer) and 8.5 GB (dual layer) |
| Blu-ray disc | Optical; 25 GB (single layer) and 50 GB (dual layer); a blue-violet laser is used |
| Pen drive (USB flash drive) | Flash; from a few GB to hundreds of GB |
| Memory card (SD card) | Flash; used in cameras and phones |
| Magnetic tape | Sequential access; used for backups |
| Floppy disk | Old magnetic disk; 1.44 MB (3.5 inch) |
| Cloud storage | Online storage on remote servers, e.g. Google Drive, OneDrive, Dropbox |
Virtual Memory
When the RAM is full, the operating system uses a part of the hard disk as an extension of the RAM; this is called virtual memory (paging file or swap space). It is slower than the real RAM.
Volatile and Non-Volatile
| Type | Examples |
|---|---|
| Volatile (lost without power) | RAM, cache, registers |
| Non-volatile (kept without power) | ROM, hard disk, SSD, CD, DVD, pen drive |
Solved Examples
Example 1. How many MB are there in 2 GB?
2 × 1,024 = 2,048 MB.
Example 2. How many bytes are there in 16 bits?
16/8 = 2 bytes.
Example 3. How many KB are there in 4 MB?
4 × 1,024 = 4,096 KB.
Example 4. Which memory is lost when the computer is switched off?
RAM (volatile).
Example 5. Which memory is used to store the BIOS?
ROM.
Quick Tips
- Remember that each unit is 1,024 times the previous one.
- Remember that RAM is volatile and ROM is non-volatile.
- Remember that SSD is faster than HDD.
- Learn the capacities of CD, DVD and Blu-ray.
Quick Revision Points
- 1 byte = 8 bits; 1 KB = 1,024 bytes.
- RAM is volatile; ROM is non-volatile.
- Cache is faster than RAM.
- CD about 700 MB; DVD 4.7 GB; Blu-ray 25 GB.
- Flash memory is a type of EEPROM.
Practice Questions With Answers
- How many bits are there in one byte?
Answer: 8 - How many KB are there in 1 MB?
Answer: 1,024 - Which memory is volatile?
Answer: RAM - Which memory is faster: cache or RAM?
Answer: Cache - What is the capacity of a CD?
Answer: About 700 MB - What is the capacity of a single-layer DVD?
Answer: 4.7 GB - Which memory can be erased by ultraviolet light?
Answer: EPROM - What is 1 TB in GB?
Answer: 1,024 GB - Which storage has no moving parts?
Answer: SSD - What is the temporary extension of RAM on the hard disk called?
Answer: Virtual memory
How to Practise This Chapter
Practise 10 questions every day and write the steps on paper. Then attempt timed sets from our practice papers and the previous year papers page. See all topics on the Computer Awareness notes page, read the previous topic Types and Generations of Computers and the next topic MS Excel and PowerPoint Basics.
Frequently Asked Questions
What is a bit?
The smallest unit of data, a 0 or a 1.
What is a byte?
A group of 8 bits.
What is RAM?
The temporary working memory of a computer, which is volatile.
What is ROM?
A permanent read-only memory that holds the start-up instructions.
What is cache memory?
A small fast memory between the CPU and RAM.
What is the difference between SRAM and DRAM?
SRAM is faster and costlier and needs no refresh; DRAM is cheaper and needs refreshing.
What is the difference between HDD and SSD?
HDD has spinning magnetic disks; SSD uses flash memory and is faster.
What is virtual memory?
A part of the hard disk used as RAM when the RAM is full.
What is flash memory?
A non-volatile memory that can be erased and rewritten electrically.
Is this topic asked in SSC?
Yes. Memory and storage are very common in computer awareness.
How many bytes are in a KB?
1,024.
What is the capacity of a Blu-ray disc?
25 GB for a single layer.
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Rahul Kumar
SSC Exam Expert & Content Editor
Rahul is a senior SSC exam strategist with 8+ years of experience helping aspirants crack CGL, CHSL and MTS exams. He writes in-depth notification breakdowns, exam pattern guides and preparation strategies.