Computer Memory Notes for SSC Exams: RAM, ROM and Cache

Computer memory is the most scoring part of SSC Computer Knowledge, because the questions are direct: volatile or non-volatile, which memory is fastest, and the units of memory. These computer memory notes for SSC cover RAM, ROM, cache, registers, units and the memory hierarchy with tables and practice questions. Read it with the Operating System Notes and Input and Output Devices Notes notes.

What Is Computer Memory?

Computer memory is the place where data, instructions and results are stored. Memory is divided into two main groups.

• Primary memory (main memory) is directly accessed by the CPU. It is fast but limited in size. Examples: RAM, ROM, cache.
• Secondary memory (auxiliary memory) is used for permanent storage. It is slower but large. Examples: hard disk, SSD, pen drive, CD, DVD.
• Volatile memory loses its data when the power is off (RAM). Non-volatile memory keeps its data without power (ROM, hard disk).

RAM and ROM Compared

Point	RAM	ROM
Full form	Random Access Memory	Read Only Memory
Volatile	Yes	No
Read and write	Both	Mostly read only
Use	Stores running programs and data temporarily	Stores the startup program (BIOS)
Speed	Fast	Slower than RAM
Also called	Main memory, read-write memory	Firmware memory

Types of RAM and ROM

Type	Meaning and fact
SRAM	Static RAM; faster and costlier; does not need refreshing; used in cache memory
DRAM	Dynamic RAM; slower and cheaper; needs constant refreshing; used as main memory
SDRAM / DDR	Synchronous and double data rate RAM used in modern computers
PROM	Programmable ROM; can be programmed only once by the user
EPROM	Erasable PROM; erased by ultraviolet light
EEPROM	Electrically erasable PROM; erased by electric signals
Flash memory	A type of EEPROM used in pen drives, SSDs and memory cards

Cache Memory and Registers

• Cache memory is a very fast memory placed between the CPU and the RAM. It stores the data that is used again and again, so the CPU works faster.
• Cache is made with SRAM and is divided into levels L1, L2 and L3. L1 is the fastest and the smallest.
• Registers are the fastest memory inside the CPU. They hold the data being processed at that moment.
• Important registers: program counter (address of the next instruction), instruction register, accumulator, MAR (memory address register) and MDR (memory data register).

Memory Units

Unit	Value
1 bit	Smallest unit (0 or 1)
1 nibble	4 bits
1 byte	8 bits
1 KB (kilobyte)	1024 bytes
1 MB (megabyte)	1024 KB
1 GB (gigabyte)	1024 MB
1 TB (terabyte)	1024 GB
1 PB (petabyte)	1024 TB
1 EB (exabyte)	1024 PB

Memory Hierarchy and Virtual Memory

The memory hierarchy from the fastest to the slowest is: registers, cache, RAM, secondary storage (SSD or hard disk). As we go down, memory becomes cheaper, bigger and slower. Virtual memory is a technique in which a part of the hard disk is used as an extension of RAM when the RAM is full. The computer swaps data between the RAM and the disk, and this area is called the page file or swap space.

Primary Memory in More Detail

Primary memory is the memory that the CPU can read directly. It is made with semiconductors, so it is fast, but it is costly and limited in size. It has three main parts: RAM, ROM and cache. The CPU always works with the data kept in RAM, which is why more RAM makes a computer smoother when many programs are running.

Feature	RAM	ROM	Cache
Volatile	Yes	No	Yes
Speed	Fast	Slower than RAM	Fastest after registers
Size	GB	MB	KB to a few MB
Made of	DRAM (SDRAM, DDR)	Fixed circuits (PROM, EPROM, EEPROM)	SRAM
Purpose	Holds the running programs and data	Holds the start-up program	Holds the most used data for the CPU

Types of ROM and Their Erasing Methods

Type	Can the user write it?	How is it erased?
Mask ROM	No, written at the factory	Cannot be erased
PROM	Yes, once	Cannot be erased
EPROM	Yes	Ultraviolet (UV) light
EEPROM	Yes	Electric signal; can be erased byte by byte
Flash memory	Yes	Electric signal in blocks; used in pen drives, SSDs and memory cards

Flash memory is non-volatile, so a pen drive keeps the data without power. It is a type of EEPROM, but it is faster because it erases data in blocks.

Types of RAM

• SRAM (Static RAM) uses flip-flops, does not need to be refreshed, is faster and costlier, and is used for cache.
• DRAM (Dynamic RAM) stores each bit in a capacitor that leaks charge, so it must be refreshed again and again. It is cheaper and used as main memory.
• SDRAM (Synchronous DRAM) works in step with the system clock. DDR (Double Data Rate) SDRAM transfers data twice in each clock cycle; DDR3, DDR4 and DDR5 are its versions.
• VRAM (Video RAM) is used in the graphics card.
• NVRAM (Non-Volatile RAM) keeps data even without power.

Cache Memory Levels

Level	Position	Features
L1 cache	Inside the CPU core	Smallest and fastest
L2 cache	Inside or near the CPU	Larger and a little slower
L3 cache	Shared by all the cores of the CPU	Largest cache, slower than L1 and L2

When the CPU finds the data in the cache, it is called a cache hit. When the data is not found and has to be taken from the RAM, it is called a cache miss.

Registers Inside the CPU

Register	Function
Program Counter (PC)	Holds the address of the next instruction
Instruction Register (IR)	Holds the instruction that is being executed
Memory Address Register (MAR)	Holds the address of the memory location to be read or written
Memory Data Register (MDR)	Holds the data that is being read from or written to the memory
Accumulator (ACC)	Holds the result of the ALU operations
Stack pointer	Points to the top of the stack

Memory Units and Conversions

Unit	Symbol	Size
Bit	b	0 or 1
Nibble		4 bits
Byte	B	8 bits
Kilobyte	KB	1024 bytes
Megabyte	MB	1024 KB
Gigabyte	GB	1024 MB
Terabyte	TB	1024 GB
Petabyte	PB	1024 TB
Exabyte	EB	1024 PB
Zettabyte	ZB	1024 EB
Yottabyte	YB	1024 ZB

• Speed of data transfer is measured in bits per second (bps, Kbps, Mbps, Gbps).
• Memory (storage) is measured in bytes, and the speed of the processor in hertz.
• A word is the number of bits that the CPU processes at one time, such as 32 bits or 64 bits.

Other Memory Terms

• Buffer is a temporary storage area used while data moves between two devices of different speeds.
• Swapping or paging moves data between the RAM and the hard disk in virtual memory.
• Memory leak happens when a program does not release the memory that it no longer needs.
• Flash drive, SSD and memory card are non-volatile secondary memories.
• Memory hierarchy: registers, cache, RAM, SSD or hard disk, optical disks and magnetic tape; speed and cost per byte fall as we go down, while the size rises.

Important Questions Explained

Question 1: Which memory is volatile?
Answer: RAM. It loses its data when the power is switched off.

Question 2: Which memory stores the BIOS (startup program)?
Answer: ROM.

Question 3: Which is the fastest memory in a computer?
Answer: Registers, then cache memory, then RAM.

Question 4: Which RAM is used in cache memory?
Answer: SRAM (static RAM).

Question 5: How many bytes are there in 1 kilobyte?
Answer: 1024 bytes.

Question 6: Which ROM can be erased by ultraviolet light?
Answer: EPROM.

Quick Tips to Remember

• Remember "RAM is volatile and ROM is non-volatile".
• Learn the memory order from fastest to slowest: registers, cache, RAM, hard disk.
• Use the byte ladder: bit, nibble, byte, KB, MB, GB, TB, PB, EB.
• Remember that 1 KB is 1024 bytes and not 1000 bytes in the exam.
• Link EPROM with ultraviolet light and EEPROM with electric signals.

Common Mistakes to Avoid

• Writing ROM as a volatile memory.
• Mixing SRAM (cache) and DRAM (main memory).
• Writing 1 KB as 1000 bytes.
• Confusing the cache memory with the virtual memory.
• Mixing EPROM and EEPROM.

Quick Revision Points

• RAM: volatile, read and write, temporary. ROM: non-volatile, permanent startup instructions.
• SRAM is used in cache; DRAM is used in main memory.
• Fastest to slowest: registers, cache, RAM, secondary storage.
• 1 byte = 8 bits; 1 KB = 1024 bytes.
• Virtual memory uses a part of the hard disk as RAM.

Practice Questions With Answers

1. What is the full form of RAM?
   Answer: Random Access Memory (Volatile memory).
2. Which memory is non-volatile?
   Answer: ROM (Keeps data without power).
3. What is the smallest unit of memory?
   Answer: Bit (0 or 1).
4. How many bits make one byte?
   Answer: 8 (A nibble is 4 bits).
5. Which memory is faster than RAM and sits between the CPU and RAM?
   Answer: Cache memory (Made with SRAM).
6. Which memory is erased by ultraviolet rays?
   Answer: EPROM (Erasable PROM).
7. 1 GB is equal to how many MB?
   Answer: 1024 MB (Binary system).
8. Where is the BIOS stored?
   Answer: ROM (Startup program).
9. Which is the fastest memory inside the CPU?
   Answer: Registers (Holds data being processed).
10. Which technique uses the hard disk as an extension of RAM?
    Answer: Virtual memory (Page file).

More Practice Questions With Answers

1. Which memory needs to be refreshed again and again?
   Answer: DRAM (Capacitor based).
2. Which RAM is faster and used in cache?
   Answer: SRAM (No refresh).
3. What does the program counter hold?
   Answer: The address of the next instruction (CPU register).
4. Which ROM can be erased electrically?
   Answer: EEPROM (Byte by byte).
5. Which memory is used in a graphics card?
   Answer: VRAM (Video RAM).
6. What do we call data found in the cache?
   Answer: Cache hit (Fast access).
7. What is 1 petabyte equal to?
   Answer: 1024 TB (Binary).
8. Which register holds the result of the ALU?
   Answer: Accumulator (CPU).
9. What is the bps unit used for?
   Answer: Data transfer speed (Bits per second).
10. Which memory is used as main memory in a PC?
    Answer: DRAM (DDR RAM).
11. What does DDR stand for?
    Answer: Double Data Rate (Transfers data twice per cycle).
12. What is a temporary storage area between two devices called?
    Answer: Buffer (Speed matching).

How to Practise This Chapter

1. Read the tables once and learn the key facts by heart.
2. Revise the quick revision points the next day.
3. Attempt the practice questions above without looking at the answers.
4. Solve a full SSC CGL Tier 1 mini mock to test your general awareness.
5. Check the previous year papers to see how SSC asks computer questions.

Related Notes and Practice Papers

• Input and Output Devices Notes
• Operating System Notes
• Generations of Computers Notes
• Computer Knowledge Notes for SSC
• SSC CGL Tier 1 Mini Mock

Explore More SSC Subjects

SSC exams test many subjects, so connect this topic with the related subjects below:

• General Science Notes
• General Awareness Notes
• Quantitative Aptitude Notes
• Reasoning Notes
• Electrical Engineering (SSC JE) Notes
• All SSC subjects and notes

Frequently Asked Questions

What is the difference between RAM and ROM?

RAM is volatile and stores running programs temporarily, while ROM is non-volatile and stores permanent startup instructions.

What is volatile memory?

Volatile memory loses its data when the power is turned off, such as RAM.

What is cache memory?

Cache is a very fast memory between the CPU and RAM that stores frequently used data to speed up processing.

What is the difference between SRAM and DRAM?

SRAM is faster and costlier and is used in cache, while DRAM is slower and cheaper and is used as main memory.

How many bytes are in a kilobyte?

There are 1024 bytes in one kilobyte.

What is a register?

A register is a very small and very fast memory inside the CPU that holds the data being processed.

What is virtual memory?

Virtual memory uses a part of the hard disk as an extension of RAM when the RAM is full.

Which is the largest unit of memory asked in SSC?

Questions usually go up to terabyte, petabyte or exabyte.

How many questions come from computer memory in SSC exams?

One or two questions usually come from memory in the Computer Knowledge part.

What is the difference between SRAM and DRAM?

SRAM is faster, costlier and does not need refreshing, so it is used for cache. DRAM is slower and cheaper and must be refreshed, so it is used as main memory.

What is a cache hit and a cache miss?

A cache hit means the CPU found the data in the cache. A cache miss means the data was not there and had to be taken from the RAM.

What is flash memory?

Flash memory is a non-volatile memory that can be erased and written electrically in blocks, as in pen drives and SSDs.