What is the difference between RAM and ROM?

RAM is volatile working memory that you can read and write; it loses data when power goes off. ROM is non-volatile, mostly read-only, and holds start-up programs such as the BIOS.

What is volatile memory?

Volatile memory needs power to keep its data, so everything is lost when the power is turned off. RAM and cache are volatile.

How many bytes are there in 1 KB?

1 KB = 1024 bytes. Likewise 1 MB = 1024 KB, 1 GB = 1024 MB and 1 TB = 1024 GB. One byte is 8 bits.

What is the difference between primary and secondary memory?

Primary memory (RAM, ROM, cache) is accessed directly by the CPU; it is fast, costly and limited. Secondary memory (hard disk, SSD, pen drive, CD) is slower and cheaper, and it keeps files permanently.

Where is the BIOS stored?

In ROM on the motherboard (in modern computers, a flash chip). It is non-volatile, so the BIOS is ready the moment the computer is switched on.

What is seek time in a hard disk?

Seek time is the time the read-write head takes to move to the required track. Disk access time is seek time plus rotational latency plus transfer time.

What are tracks and sectors?

Tracks are the circles on a disk platter, one inside another. Sectors are the pie-shaped segments that divide each track.

Is a pen drive primary or secondary memory?

Secondary memory. A pen drive (USB flash drive) uses non-volatile flash memory and keeps files without power.

Is a DVD magnetic or optical storage?

Optical. CDs, DVDs and Blu-ray discs are read by a laser. Hard disks, floppy disks and magnetic tape are magnetic.

Punjab Police ASI Storage — Computer Memory: RAM, ROM, Cache and Storage Devices Notes

Q: What is cache memory?

Cache is a small, very fast memory between the CPU and RAM. It keeps copies of the data and instructions the CPU uses most, in levels L1, L2 and often L3.

A long green RAM stick with black memory chips in a row and gold contacts along the bottom edge — A **DDR4 RAM module**. RAM is the computer's working memory. It is fast, but it forgets everything when the power goes off. Image: ElooKoN, Wikimedia Commons · CC BY-SA 4.0

Memory is the part of a computer that holds data and programs. Some memory is fast but forgets when power goes off, like RAM. Some is slower but keeps data for years, like a hard disk or a pen drive. Punjab Police papers ask about this topic often: memory units, primary vs secondary memory, RAM vs ROM, cache, and the parts of hard disks and magnetic tape.

8

Bits in one byte

4 bits = 1 nibble

1024

KB in one MB

Each unit is 2¹⁰ times the last

3

Parts of disk access time

Seek + latency + transfer

1.44 MB

A 3.5-inch floppy disk

The common high-density size

Memory units and the two families of memory

From one bit to a terabyte, and primary vs secondary memory.

KEY TERM

Bit and byte

A bit (binary digit) is the smallest unit of data. It is a single 0 or 1. 4 bits make a nibble and 8 bits make a byte. One byte can hold one character, such as the letter A.

Bit→Nibble (4 bits)→Byte (8 bits)

Unit	Equals
1 Kilobyte (KB)	1024 bytes
1 Megabyte (MB)	1024 KB
1 Gigabyte (GB)	1024 MB
1 Terabyte (TB)	1024 GB
1 Petabyte (PB)	1024 TB
Then Exabyte (EB), Zettabyte (ZB), Yottabyte (YB)	Each is 1024 of the one before

In exams, always take 1 KB = 1024 bytes (2¹⁰). Drive makers often count 1 GB as 1000 MB, so a new drive shows a little less space on the computer.

How many bytes? Each unit as a power of 2

KB

2¹⁰ bytes

MB

2²⁰ bytes

GB

2³⁰ bytes

TB

2⁴⁰ bytes

PB

2⁵⁰ bytes

Every step up adds 10 to the power, which means ×1024.

REMEMBER

"Kind Men Give To Poor" = KB → MB → GB → TB → PB (small to big).

PRIMARY VS SECONDARY MEMORY

Primary (main / internal)

Secondary (auxiliary)

Examples

RAM, ROM, cache

Hard disk, SSD, pen drive, CD

CPU access

Direct

Data first goes to RAM

Speed

Fast

Slower

Cost per GB

High

Low

Size

Limited (GBs)

Large (often TBs)

Power off

RAM loses data

Data stays (files)

All magnetic (hard disk, floppy, tape) and optical (CD, DVD) media are secondary memory. So are flash drives and memory cards; papers call them memory storage devices.

QUICK CHECK

________ can be referred to as primary storage and ________ can be referred to as secondary storage in a computer system.

Tap an option to answer

QUICK CHECK

In which type of computer storage are data and programs stored permanently in the form of files?

Tap an option to answer

QUICK CHECK

State whether the following statements are true or false for RAM. (i) RAM is a volatile memory as the data is lost when the power is turned off. (ii) RAM is also called the secondary memory.

Tap an option to answer

RAM and ROM: the two kinds of primary memory

Volatile vs non-volatile, SRAM vs DRAM, and the ROM family.

KEY TERM

Volatile and non-volatile

Volatile memory needs power to keep its data. When power goes, the data is lost. RAM and cache are volatile. Non-volatile memory keeps data without power. ROM, flash memory, SSD and hard disk are non-volatile.

Power off→RAM: data lost→ROM: data stays

RAM VS ROM

RAM

ROM

Full form

Random Access Memory

Read Only Memory

Without power

Volatile: data lost

Non-volatile: data stays

Read / write

Read and write

Mostly read only

Holds

Programs running now

BIOS / start-up program

BIOS must run the moment you switch on, so it sits in ROM (today, a flash chip). RAM is empty at start-up.

TWO KINDS OF RAM

SRAM (Static)

DRAM (Dynamic)

Built with

Flip-flops (transistors)

Tiny capacitors

Refresh needed?

No

Yes, again and again

Speed / cost

Faster, costlier

Slower, cheaper

Used as

Cache memory

Main memory (RAM sticks)

DDR = Double Data Rate. DDR3, DDR4 and DDR5 are types of DRAM used as main memory. They are primary, not secondary.

ROM type	Full form	How it is written or erased
ROM (mask ROM)	Read Only Memory	Written once at the factory. Cannot be changed.
PROM	Programmable ROM	The user can write it only once.
EPROM	Erasable Programmable ROM	Erased with ultraviolet (UV) light, then written again.
EEPROM	Electrically Erasable Programmable ROM	Erased electrically, without taking the chip out.
Flash memory	A kind of EEPROM	Erased in large blocks. Used in pen drives, SSDs, memory cards and BIOS chips.

Close-up of a grid of tiny dark rings threaded on fine crossing wires: magnetic-core memory — **Magnetic-core memory**, the main memory of many computers from the mid-1950s into the 1970s. Each tiny ring stored **one bit**. It kept its data even without power. Image: Bubba73 (Jud McCranie), Wikimedia Commons · CC BY-SA 4.0

QUICK CHECK

What distinguishes ROM from RAM in terms of functionality?

Tap an option to answer

QUICK CHECK

In the context of computer architecture, what is the primary distinction between volatile and non-volatile memory?

Tap an option to answer

QUICK CHECK

Which of the following options representing non-volatile memory stores the Basic Input Output System (BIOS) program of the computer?

Tap an option to answer

The memory hierarchy and cache

Fast and small at the top, slow and big at the bottom.

MEMORY HIERARCHY: FROM FASTEST TO SLOWEST

fastest
Registers
Inside the CPU. Fastest and smallest.
Cache (L1 → L2 → L3)
Very fast SRAM between the CPU and RAM.
Main memory (RAM)
Holds the programs that are open now.
SSD / hard disk
Keeps all your files, even when switched off.
slowest
CD/DVD, magnetic tape
Backup and archives. Slowest, cheapest per GB.

Going down: speed falls, cost per GB falls, capacity rises.

KEY TERM

Cache memory

Cache is a small, very fast memory between the CPU and RAM. It keeps copies of the data and instructions the CPU uses most. L1 is the smallest and fastest, inside each core. L2 is bigger and a little slower. Many CPUs also have a shared L3. If the CPU does not find an instruction in L1 but finds it in L2, the instruction is first copied from L2 into L1, and the CPU then uses it.

L1 miss→Found in L2→Copied to L1→Executed

Primary memory trait

Volatile

RAM forgets everything in a power cut.

Primary memory trait

High cost

Costs more per GB than a disk.

Primary memory trait

Limited size

A few GB, not TB.

Primary memory trait

Fast access

Nanoseconds. That is why the CPU works from it.

"Slow access" is not a trait of internal memory. It is fast.

QUICK CHECK

A program is run and the CPU needs to fetch the next instruction. If the instruction is not found in the L1 cache but is present in the L2 cache, what is the next step before the instruction is finally executed?

Tap an option to answer

QUICK CHECK

Which of the following is NOT a main characteristic of the internal memory of a computer?

Tap an option to answer

Secondary storage: magnetic, optical and flash

Three ways to keep data for years.

Three floppy disk drives side by side, from a large 8-inch drive to a small 3.5-inch drive

Shiny CDs and DVDs reflecting rainbow colours

Family	How it stores	Examples
Magnetic	Tiny magnetised spots on a coated surface	Hard disk, floppy disk, magnetic tape
Optical	Pits on a shiny layer, read by a laser	CD, DVD, Blu-ray
Flash (solid state)	Electric charge in chips; no moving parts	Pen drive, memory card, SSD

Hard disk = magnetic disk. DVD = optical disk. Both pairs are correct.

Usual capacity of one disc or disk (single layer)

3.5-inch floppy

1.44 MB

CD

about 700 MB

DVD

4.7 GB

Blu-ray

25 GB

Blu-ray holds more because its blue-violet laser reads smaller pits. A DVD with two layers holds more than 4.7 GB.

KEY TERM

Pen drive

A flash drive is also called a pen drive, USB drive or thumb drive. It uses flash memory, so it keeps data without power. It is secondary memory. Flash drives and memory cards are called memory storage devices.

Flash drive→= pen drive→= USB / thumb drive

COMMON TRAPS

SSD, not "SDD". SSD = Solid State Drive. It replaces a hard disk inside the computer. It is not another name for a pen drive.
DDR, SRAM are RAM (primary). Flash ROM holds firmware. Of these four, only the pen drive is secondary storage.
"Light drive" and "pencil drive" are not real terms.

QUICK CHECK

Which of the following pair of storage device type - device is correct? I. Magnetic disk - Hard disk II. Optical disk - DVD

Tap an option to answer

QUICK CHECK

A computer's secondary memory permanently stores data and programs even when the computer is turned off. Identify which of the following is a type of secondary memory.

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QUICK CHECK

Which of the following storage devices contains plates made of aluminium or glass coated with a layer of magnetic material?

Tap an option to answer

Inside a hard disk: tracks, sectors and access time

Platters, heads, and the three waits.

An opened laptop hard drive showing a shiny round platter and the arm with the read-write head over it — An opened 2.5-inch **hard disk drive**. The shiny disc is the **platter**. The arm carries the **read-write head**, which moves across the spinning platter. Image: Evan-Amos, Wikimedia Commons · CC BY-SA 3.0

Ring

Track

A circle on the platter. Tracks sit one inside another.

Pie slice

Sector

A pie-shaped segment of a track. Classically 512 bytes.

Stack

Cylinder

The same track number on every platter, one above another.

"Segment" is not a disk term. A disk pack is a stack of several platters on one spindle.

DISK ACCESS TIME = THREE WAITS ADDED TOGETHER

1
Seek time
The head moves to the right track.
2
Latency
The disk spins the right sector under the head. Also called rotational delay.
3
Transfer time
The data is read or written. It depends on the transfer rate.

Access time = seek time + latency + transfer time. All three count.

REMEMBER

"Seek the street, wait for the house, then walk in." Seek = go to the track. Latency = wait for the sector. Transfer = read the data.

A hard disk drive is also called a hard drive, fixed disk or simply a disk drive. It is secondary storage, so "primary drive" is wrong.
A disk is a direct (random) access device: the head can jump straight to any track. A tape is sequential: you must wind past everything before it.

QUICK CHECK

In magnetic disks, the time taken by the read-write head to move to the specified track is known as ______.

Tap an option to answer

QUICK CHECK

Latency time is also known as ______.

Tap an option to answer

QUICK CHECK

The disk access time depends on which of the following parameters. I. Seek time II. Latency III. Transfer rate

Tap an option to answer

Magnetic tape: channels, frames and blocking

Old, slow, cheap, and still asked.

ROWS AND COLUMNS ON A MAGNETIC TAPE

Channel (track)

Frame

Direction

Horizontal row, along the tape

Vertical column, across the tape

Holds

One bit position

One character

On 9-track tape

9 channels

8 data bits + 1 parity bit

Horizontal = channel. Vertical = frame. "Sectors" belong to disks, not tapes.

KEY TERM

Blocking

A tape drive needs blank tape each time it starts and stops. So records are grouped: two or more records joined into one block is called blocking. The number of records in a block is the blocking factor. The blank space between two blocks is the inter-block gap (IBG). Blocking saves tape and time.

Record→Record→= 1 block→gap (IBG)

QUICK CHECK

A magnetic tape contains perpendicular (vertical) columns which are called ______.

Tap an option to answer

QUICK CHECK

The process of grouping two or more records together in a magnetic tape is known as ______.

Tap an option to answer

Quick revision

Full forms, traps and one-line facts.

FLASHCARDS

Full forms: tap to check

RAM: Random Access Memory
ROM: Read Only Memory
PROM: Programmable Read Only Memory
EPROM: Erasable Programmable Read Only Memory
EEPROM: Electrically Erasable Programmable Read Only Memory
SRAM / DRAM: Static RAM / Dynamic RAM
DDR: Double Data Rate
SSD: Solid State Drive
HDD: Hard Disk Drive
CD / DVD: Compact Disc / Digital Versatile Disc
USB: Universal Serial Bus
IBG: Inter-Block Gap

COMMON TRAPS

RAM is primary, never secondary. Flash memory (pen drive) is secondary.
Volatile = needs power. Don't flip it: RAM forgets, ROM remembers.
Seek = to the track. Latency = to the sector.
Disk: tracks and sectors. Tape: channels and frames.
Internal memory is fast, not slow.

Also asked onceTap to open

A flash drive is also called a pen drive.
Another name for a hard disk drive is disk drive.
Flash drives and memory cards are examples of memory storage devices.
Magnetic and optical memory are examples of secondary memory.
The pie-shaped segments on a magnetic disk are sectors.
The horizontal rows on a magnetic tape are channels.

CHECK YOURSELF

Arrange from fastest to slowest: hard disk, cache, register, RAM. Then say which of them are volatile.

Show answer

Register → cache → RAM → hard disk. Registers, cache and RAM are volatile. The hard disk is non-volatile.

Computer Memory: Units, RAM, ROM, Cache, Hard Disk and Storage Devices

Memory units and the two families of memory

RAM and ROM: the two kinds of primary memory

The memory hierarchy and cache

Secondary storage: magnetic, optical and flash

Inside a hard disk: tracks, sectors and access time

Magnetic tape: channels, frames and blocking

Quick revision

Frequently Asked Questions

What is the difference between RAM and ROM?

What is volatile memory?

How many bytes are there in 1 KB?

What is the difference between primary and secondary memory?

What is cache memory?

Where is the BIOS stored?

What is seek time in a hard disk?

What are tracks and sectors?

Is a pen drive primary or secondary memory?

Is a DVD magnetic or optical storage?