Computer Science — Chapter 3

Memory & Storage — RAM, ROM, Data Units and Storage Devices

Why this chapter matters: Chapter 2 taught you the devices that get data in and out; this chapter teaches you where the computer keeps data while it works and where it stores it afterwards. The memory questions form their own tight MCQ family — RAM vs ROM, volatile vs non-volatile, the 1024 ladder, and the capacity of a floppy, a CD and a DVD. Our verified banks hold 33 questions matching this chapter directly, led by storage devices (14), then RAM/ROM and cache/register memory (10) and data units (9). Learn the three tables in Sections 1, 4 and 6 and most of these marks are already yours.

How this chapter relates to the rest of Computer Science: the network devices, network types and internet material taught in the same lecture (Lecture 3, Sections 4, 7–8) belong to Chapter 6 (Internet & Networking), and the software material (Lecture 3, Section 9) belongs to Chapter 4 (Software & Operating Systems) — they are cross-linked from here, not re-taught. The CPU architecture from Chapter 2 (CU + ALU) is assumed: cache and register memory only make sense once you know where the CPU sits.

1. Primary memory — RAM and ROM

Primary memory is also called main memory. Its job is to store data and instructions while the computer works. It has two types: RAM and ROM.

• Firmware resides in ROM — “firm” means strong, and ROM is the firm, permanent memory. The papers ask this directly: which memory is non-volatile and often used for firmware? ROM.
• RAM sits on the motherboard, plugging into its expansion slots (Chapter 2). The tower/desktop box is not the CPU — the processor chip inside is.

Types of RAM

• DRAM = Dynamic RAM — it keeps changing (dynamic).
• SRAM = Static RAM — it stays as it is until you change it yourself.
• DDR is a type of RAM (asked: DDR — RAM or ROM? → RAM).
• The papers ask which volatile RAM is faster: SRAM is faster than DRAM (it needs no constant refreshing) — that is the standard answer behind the “which type of volatile RAM is faster” question.

Types of ROM — the evolution

1. PROM = Programmable Read Only Memory — once programmed, it cannot be changed.
2. EPROM = Erasable Programmable Read Only Memory — the E stands for Erasable.
3. EEPROM = Electrically Erasable Programmable Read Only Memory — the improved form, erasable electrically, without removing the chip.

• Virtual memory — when RAM fills up, the operating system borrows space on the hard disk and uses it as if it were RAM. It is not a separate chip and not ROM: it is hard disk used as RAM, slower than real RAM but what keeps the machine running when many programs are open at once. Trap: “virtual memory is” with Physical RAM in the options → the answer is still the hard-disk answer, not RAM itself.

2. Cache memory and register memory — the fastest, smallest memories

• Fastest memory INSIDE the CPU → Register.
• Outside the CPU (sitting with RAM, “RAM-like”) → Cache (pronounced “cash”).
• Smallest memory → Cache (2–4 MB). When it fills up, the system offers to “clear cache memory”.
• Cache is temporary memory: it holds what you use again and again so it opens fast — typing “pp” in Google instantly suggests PPSC because that search is sitting in cache.
• The journey of data: a movie on the hard disk (secondary memory) is played in primary memory, and cache is what carries it from secondary to primary.

The memory hierarchy (fastest → slowest)

The papers ask “in the memory hierarchy the fastest memory is?” — register. And “cache memory is…?” — the small, fast, temporary memory outside the CPU.

3. Secondary memory — storage devices

Secondary memory (secondary storage) keeps data after the power goes off. It is non-volatile, slower than primary memory, and far larger.

HDD vs SSD

• The common setup: a 128/256 GB SSD for Windows and software, and an HDD for the rest of the data (movies, pictures) — that keeps the machine fast.
• NVMe is the modern fast interface for SSDs — the papers now ask what an NVMe card is for: it is an SSD connection standard, faster than the older SATA route.
• The first hard disk was made by IBM (Chapter 1).
• A hard disk’s surface is logically organised into tracks (concentric circles) divided into sectors — that is how the papers phrase “the surface of a hard disk is logically divided into” and “the capacity of a hard disk depends upon” (tracks, sectors and the recording surface).

The storage timeline: tape → floppy → CD → DVD → USB

• Magnetic tape (the tape recorder) — reel-based, sequential access: to hear the 7th song you wait through the first six. If the reel snapped mid-way, the cassette was dead.
• Floppy disk — shrunk to 3.5 inches, capacity 1.44 MB. Asked many times — memorise both figures together.
• CD = Compact Disc — capacity 700 MB / 680 MB. The papers ask it both ways (once 700 without 680 as an option, once 680 without 700). Rule: if both appear, tick the one present; tick “none of these” only if neither appears. Sony and Philips developed the compact disc independently (1979) and jointly launched it in 1982 — a bank-verified fact.
• DVD = Digital Versatile Disc — four capacities, all asked:

• USB = Universal Serial Bus (the flash drive / pen drive) — sizes now reach TBs. The papers ask both “USB stands for” and “USB is a type of what storage?” — a secondary storage device (and, in Chapter 2’s frame, a port/interface — the same letters answer both families, so read the question stem).
• Memory card is also a storage device (no MCQ from it yet in the lecture’s count).

4. Data units — the 1024 ladder

• Bit = Binary Digit — the smallest unit of data (0 or 1). Asked directly.
• 4 bits = 1 nibble. Asked MCQ: “32 bits = how many nibbles?” → 32 ÷ 4 = 8. And “1 nibble contains…?” → 4 bits.
• 8 bits = 1 byte. “1 byte consists of…?” → 8 bits.

• The papers stop at TB, and they ask the ladder both ways: “1024 bytes equal to…?” → 1 KB; “one megabyte is equal to how many bytes?” → work it up the ladder: 1 MB = 1024 KB = 1024 × 1024 bytes = 1,048,576 bytes. “A terabyte represents about…?” → 1024 GB (about a trillion bytes).
• Note the trap: computer memory uses 1024, not 1000 — a “1 GB” drive in shop arithmetic (1000-based) and a “1 GB” in paper arithmetic (1024-based) are different numbers, and the papers always use 1024.

5. Master table — the whole chapter in one view

6. Rapid-fire one-liners

• Main/primary memory → stores data and instructions; two types: RAM and ROM.
• RAM → Random Access Memory; volatile; temporary; read + write.
• ROM → Read Only Memory; non-volatile; permanent; read only.
• Firmware resides in → ROM.
• DRAM → Dynamic RAM (keeps changing); SRAM → Static RAM (stays until changed).
• DDR is a type of → RAM.
• RAM is located on the → motherboard (expansion slots).
• PROM → Programmable Read Only Memory; EPROM → Erasable; EEPROM → Electrically Erasable.
• Virtual memory → hard disk used as RAM (not a chip, not ROM).
• Fastest memory inside CPU → Register; outside CPU (RAM-like) → Cache.
• Smallest memory → Cache (2–4 MB).
• HDD → Hard Disk Drive (spinning disk); SSD → Solid State Drive (chips only); SSD is faster.
• Floppy disk → 3.5 inches, 1.44 MB.
• CD → Compact Disc; 700 MB / 680 MB (tick the one present).
• DVD → Digital Versatile Disc: 4.7 / 9.4 / 8.5 / 17.08 GB.
• USB → Universal Serial Bus.
• Bit → Binary Digit; smallest unit of data.
• 4 bits → 1 nibble; 32 bits → 8 nibbles; 8 bits → 1 byte.
• 1024 bytes → 1 KB; 1024 KB → 1 MB; 1024 MB → 1 GB; 1024 GB → 1 TB.
• 1 MB = 1,048,576 bytes (1024 × 1024).
• Hard disk surface → organised into tracks and sectors.
• First hard disk → made by IBM.

7. Most-asked facts (from our verified past-paper banks)

• RAM vs ROM in every form: expansion, volatile vs non-volatile, read+write vs read-only, and firmware in ROM — the single most repeated memory family (10 bank questions, and the same pair recurs inside Chapter 2’s CPU questions).
• The 1024 ladder and the nibble: “1024 bytes equal to” (asked three times in the banks), “1 nibble contains”, “1 byte consists of”, “one megabyte in bytes”.
• Floppy 1.44 MB, CD 700/680 MB, DVD capacities, USB expansion — the storage-capacity family (14 bank questions), including the Sony & Philips CD question and the NVMe-SSD question from recent papers.
• Cache and register: “fastest memory in the hierarchy” and “cache memory is” — small in count, certain in wording.

8. Traps & confusions

• Volatile vs non-volatile: RAM loses everything at power-off; ROM keeps it. If the question says “temporary” or “aarzi”, it is RAM; “permanent” is ROM. Do not swap them under exam pressure.
• Fastest vs smallest: fastest = register (inside the CPU); smallest = cache (2–4 MB, outside the CPU). Papers ask both, and students swap them.
• CD 700 vs 680 MB: both are asked, in different papers. Tick whichever figure the options offer; “none of these” only if neither appears.
• DVD 9.4 vs 8.5 GB: 9.4 = double-sided, single-layer (4.7 + 4.7); 8.5 = single-sided, double-layer. The side/layer order flips the answer — read both words.
• USB in two chapters: as a port/interface it belongs to Chapter 2’s ports family; as a flash drive it belongs here, in storage. The stem decides: “USB stands for” can appear in either family, but “USB is a type of what storage?” is this chapter.
• 1024, not 1000: every ladder step in the papers is 1024. A shop’s “1 GB = 1000 MB” will cost you the mark.
• System unit vs CPU: the box is the system unit; the processor chip inside is the CPU (Chapter 2’s trap, repeated here because RAM-on-the-motherboard questions sit next to it).
• Odd bank key (log only in spirit — read carefully): one bank paper keys “the top and bottom track of the hard disk does not contain data”. The standard teaching kept in this chapter is that a hard disk’s surface is divided into tracks and sectors, and tracks hold data — do not let that single odd key overwrite the standard picture.

9. Memory tricks (from the lectures)

• Flip the A: flip the letter A upside down → it becomes a V shape → V = Volatile = RAM. If RAM is volatile, ROM must be non-volatile. One letter, and the whole RAM/ROM table locks.
• “Firm”ware in the “firm” memory: firmware lives in ROM because ROM is the firm, permanent one — the word itself tells you the answer.
• Cache = “cash”: pronounce it “cash” and remember that cash is what you keep close at hand for what you buy again and again — exactly what cache memory does for your repeated files and searches.
• The 1024 ladder chant: bytes → KB → MB → GB → TB, each step ×1024. Say it as “one-oh-two-four” and the megabyte-in-bytes question (1024 × 1024) works itself out.
• Floppy’s pair: 3.5 and 1.44 — the disk that shrank (3.5 inches) and the number that never changes (1.44 MB). Learn them as one pair, never separately.
• DVD doubling: single-sided single-layer 4.7 is the base; double the side → 9.4; double the layer → 8.5; double both → 17.08. One base number, three doublings.

10. Visuals (site build specs)

• DIAGRAM — The memory hierarchy pyramid: four stacked layers, fastest and smallest at the top (register → cache → RAM → secondary storage), each labelled with location (inside CPU / outside CPU / motherboard / external) and the volatile/non-volatile split — placed in Section 2.
• TABLE-INFOGRAPHIC — RAM vs ROM: the Section 1 comparison table rendered as a two-column visual (volatile/temporary/red vs non-volatile/permanent/green) with the firmware and motherboard anchors — placed in Section 1.
• INFOGRAPHIC — The storage timeline: tape → floppy (3.5″, 1.44 MB) → CD (700/680 MB) → DVD (4.7/9.4/8.5/17.08 GB) → USB/SSD, capacity growing left to right — placed in Section 3.
• DIAGRAM — The 1024 ladder: vertical ladder bit → nibble (4) → byte (8) → KB → MB → GB → TB, each rung ×1024 (×4 and ×8 for the first two) — placed in Section 4.

11. Practice this topic

• Practice set: 33 verified past-paper questions in the PPSC + FPSC banks are tagged to this topic’s sub-topics (RAM/ROM & cache/register 10, data units 9, storage devices 14) — the site serves them as this chapter’s practice set, with the IBES Computer capsule questions behind them. Bank-only extras inside the set (NVMe SSD, Sony & Philips CD history, hard-disk tracks/sectors) are taught in Sections 3 above and flagged here so the set holds no surprises.
• Book mode: ← Previous: Hardware — Input Devices, Output Devices, the CPU and Ports · Next: Software & Operating Systems →

Goldmine Notes · Computer Science · Chapter 3 · Sources: TBT Computer Lecture 3 (memory, storage & data-units sections; network/software sections cross-linked to Chapters 6/4), IBES Capsule Computer, verified PPSC/FPSC past-paper banks. Virtual-memory line added 3 November 2026 from the PMS Screening 2026 Computer & IT mock (Q19, standard definition: hard disk used as RAM). Standard answers only.