Unit 3 Session 4: Types of Memory | IT 231 BBA Slides

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Unit 3 · Session 4

Types of Memory

IT 231: IT and Applications (BBA)

🎯 Learning Objectives

By the end of this session, you will be able to:

  • ✅ Distinguish volatile from non-volatile memory with examples.
  • ✅ Compare SRAM and DRAM in terms of speed, cost, and typical use.
  • ✅ Identify the main types of ROM and where each is used.
  • ✅ Contrast RAM and ROM by their key attributes.
  • ✅ Describe the role each type of memory plays in the boot process.

🔌 Two Families of Memory

Volatile memory

Needs power to keep its data. Switch off, and the data is gone.

Holds what you're working on right now: open browser tabs, the game you're playing, a document you haven't saved.

Non-volatile memory

Keeps its data with no power at all.

Holds what must survive: saved files, photos, installed apps, and the instructions a computer needs to start up.

Power cut test: The power goes off while you're typing an assignment. What's lost, and what's safe?

Everything since your last save is lost, because it was only in volatile memory. The saved file is safe, because it's on non-volatile storage.

Volatile vs Non-Volatile: Sort It!

Click each memory type to assign it to the correct category.

VOLATILE
NON-VOLATILE

SRAM vs DRAM: Which is Which?

Click a property card. Select SRAM or DRAM, then reveal the answer.

Score: 0 / 0

🎬 How SRAM and DRAM Cells Are Built

⚖️ SRAM vs DRAM at a Glance

SRAM (Static RAM)DRAM (Dynamic RAM)
One bit is stored inA flip-flop made of about 6 transistors1 transistor and 1 tiny capacitor
Refresh needed?No. It holds data while poweredYes. The capacitor leaks, so every cell is recharged many times a second
SpeedFastestSlower
Cost and size per bitHigh, and it takes lots of chip spaceLow, so many bits fit in a small space
Where you find itCPU cache (L1, L2, L3)Main memory: the DDR4 or DDR5 sticks

"Static" means it stays put. "Dynamic" means it must be constantly refreshed.

🧮 Worked Example: Why Is Cache So Small?

A high-end desktop CPU has about 36 MB of L3 cache, but the PC has 32 GB of RAM, about 900 times more. Why not build all 32 GB from fast SRAM?

  1. 32 GB is about 32 × 8 = 256 billion bits (rounded)
  2. In SRAM, each bit needs about 6 transistors: 256 billion × 6 ≈ 1.5 trillion transistors
  3. Apple's M3 Max, one of the largest laptop chips, has about 92 billion transistors in total. That's about 16 M3 Max chips, just for memory
  4. In DRAM, each bit needs only 1 transistor and 1 capacitor, so it's cheap enough to fit on a stick that costs a few thousand rupees

Computers use a little expensive SRAM for speed, and a lot of cheap DRAM for space.

ROM Types: Name That Memory!

Read the description, then click the matching ROM type to reveal if you are correct.

Loading...

EPROM: The Window Is for UV Light

A 42-pin ST M27C160 UV-erasable EPROM chip with a clear quartz window over the silicon die.

🎬 The First EPROM

A UV EPROM Eraser

A blue UV EPROM eraser box with a timer dial and its drawer open, holding five EPROM chips.

🌳 The ROM Family at a Glance

TypeHow it's writtenCan it be erased?
Mask ROMBuilt into the chip at the factoryNever
PROMWritten once by the user with a programmer deviceNever
EPROMWritten electricallyYes, under UV light through the window, out of the circuit
EEPROMWritten electricallyYes, electrically, byte by byte, in the circuit
FlashWritten electricallyYes, electrically, in large blocks, which makes it fast and cheap

Each step made ROM easier to change. Today "read-only" memory is updated whenever you install a firmware update.

💾 Flash Memory Is Everywhere

Flash is the youngest member of the ROM family, and the one you use every day:

📱 Phones and tablets

All your apps, photos and messages

💻 SSDs, USB drives, SD cards

Fast storage with no moving parts

🧩 Firmware chips

The BIOS/UEFI in a PC, and the software inside routers, TVs and cars

  • Strength: non-volatile, small, shock-proof and silent
  • Limit: each cell wears out after a limited number of writes, typically thousands, so SSDs spread writes evenly across all cells

RAM vs ROM: Attribute Challenge

Each row shows an attribute. Click the correct winner — RAM or ROM.

Attribute RAM ROM
Score: 0 / 0

📱 "8 GB RAM, 128 GB ROM"? Reading a Phone Spec

Phone shops and websites often list a phone as "8/128" or "8 GB RAM, 128 GB ROM".

  • 8 GB RAM: volatile DRAM, the phone's working memory. More RAM lets more apps stay open without reloading.
  • 128 GB "ROM": really flash storage, where apps, photos and videos are kept. You write to it every time you take a photo, so it isn't read-only at all.
  • The phone does have true firmware memory, but it's a tiny part that you never see in the spec.

Quick check: A friend's phone says "Storage full" but runs smoothly. Which number is the problem?

The 128 GB, the flash storage. Deleting old videos fixes it. Buying more RAM wouldn't help.

The BIOS Lives in a Flash Chip

A socketed Winbond flash memory chip on a motherboard, labelled "686 AMIBIOS ©1999".

Boot Process: Order the Memory Steps!

Click the steps in the correct order from power-on to OS ready.

Your sequence (click to add):

🚀 The Boot Sequence: Which Memory Does What?

1. Power onRAM is empty, because it's volatile. The CPU needs instructions from somewhere that survived the night.
2. Firmware runsThe CPU reads the BIOS/UEFI from the flash chip (non-volatile).
3. POSTThe Power-On Self-Test checks the CPU, RAM and drives. A beep or error here means faulty hardware.
4. BootloaderThe firmware finds the bootloader on the SSD and copies it into RAM.
5. Operating systemThe bootloader copies the OS kernel from the SSD into RAM, which starts drivers and services.
6. ReadyYour apps load from storage into RAM and run there.

Non-volatile memory starts the computer, and volatile memory runs it.

🎬 BIOS and POST on a Real PC

🎬 From Bootloader to Operating System

🇳🇵 At the Computer Shop: Which RAM Do You Buy?

Three customers walk into a shop in Putalisadak, Kathmandu's computer market. What should each one buy?

  1. Sita has a 2019 laptop with one free DDR4 slot and wants more memory.
    A DDR4 laptop stick (SO-DIMM). DDR5 has its notch in a different place, so it physically won't fit.
  2. Ramesh is building a new desktop on AMD's AM5 platform.
    DDR5 desktop sticks. AM5 boards support DDR5 only.
  3. Anjali has a thin laptop with 8 GB of soldered LPDDR5 memory.
    Nothing. Soldered RAM can't be upgraded, so next time she should buy enough at the start.

The motherboard decides which RAM fits. Check its manual or spec sheet before you buy.

Key Takeaways 🎯

  • Volatile memory (RAM) loses data without power. Non-volatile memory (ROM, flash) keeps it.
  • SRAM is fast and expensive, so it's used for cache. DRAM is cheaper and denser, so it's used for main memory, and it needs constant refresh.
  • The ROM family grew from Mask ROM to PROM, EPROM, EEPROM and flash, getting easier to rewrite at each step.
  • Booting starts from firmware in flash, then loads the OS into RAM.
  • Spec sheets mix up these terms. A phone's "ROM" is really flash storage.

💬 Discussion Questions

  1. Phone makers advertise RAM very loudly. Is more RAM always worth paying for? When isn't it?
  2. Imagine a memory that is as fast as RAM, as cheap as flash, and non-volatile. What would change about how computers start up and work? (Intel tried this with Optane, and stopped making it in 2022.)
  3. A customer says, "My phone has 256 GB of ROM." How would you explain what that really means, in one sentence?

Image & Video Credits

Images (Wikimedia Commons)

Videos (YouTube)

  • BitLemon, "SRAM vs DRAM: The Speed Difference between Cache and RAM (Animation)" (0:32 to 2:01)
  • lcfgroup, "Vintage First EPROM Erasable Programmable Read Only Memory 1702" (0:13 to 0:57)
  • Dave's Garage, "How Computers BOOT: From Startup to Viruses" (6:21 to 7:05 and 7:35 to 9:00)

Exam Practice 📝

Short answer

  1. Differentiate between RAM and ROM.
  2. Differentiate between SRAM and DRAM.
  3. What is flash memory? Where is it used?

Long answer

  1. Explain the types of RAM and ROM.
  2. Describe the role of RAM and ROM in the boot process of a computer.

Revise the answers in the full note linked on the last slide.

Thank You

Questions before we wrap this hour?

Next: Unit 3 · Session 5 — Storage Devices

Course Home · Next session

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