NBC 101 • Unit 215 min read★ Very High Exam Frequency
Primary Memory Deep-Dive: RAM (SRAM vs DRAM) & ROM (PROM, EPROM, EEPROM, Flash)
Unit 2: Computer Memory Hierarchy, Storage & I/O Systems • Fundamentals of Computer
👨🏫 Professor's Mental Model: Whiteboard vs Engraved Stone Tablet
RAM ek classroom ke whiteboard jaisa hai jisme teacher active lecture likhta hai aur class khatam hote hi mita deta hai (Volatile, fast read/write). ROM ek patthar par khude hue shila-lekh jaisa hai jisme permanent rules likhe hote hain (Non-volatile). Chahe electricity chali jaye ya laptop switch off ho jaye, ROM me likha BIOS software kabhi nahi mit ta!
1. RAM (Random Access Memory) In-Depth: SRAM vs DRAM
RAM allows any memory cell to be accessed directly in constant time O(1) regardless of its physical position on the chip.
• Static RAM (SRAM):
• Construction: Each bit is stored in a bistable flip-flop latch made of 4 to 6 CMOS transistors.
• Characteristics: Extremely fast (1 - 5 ns), does NOT require periodic electrical refreshing, consumes less dynamic power, but has low density and high cost per bit.
• Application: Used exclusively in CPU Cache (L1, L2, L3).
• Dynamic RAM (DRAM):
• Construction: Each bit is stored as an electrical charge inside a microscopic capacitor paired with a single transistor (1T-1C cell).
• The Refresh Mechanism: Capacitors naturally leak electrical charge over milliseconds. Therefore, DRAM requires periodic dynamic refreshing (reading and rewriting charge every 64 ms) via dedicated refresh circuitry.
• Characteristics: Slower than SRAM (40 - 70 ns), but offers massive memory density and low cost per gigabyte.
• Application: Used as Main System RAM (DDR3, DDR4, DDR5).
2. ROM (Read Only Memory) Classification & Erasure Mechanisms
ROM is non-volatile primary memory that permanently stores bootstrap initialization firmware (BIOS / UEFI POST diagnostic tests):
1. Mask ROM (MROM): Programmed during factory manufacturing by hardwiring diode matrices. Permanent, cannot be rewritten.
2. PROM (Programmable ROM): Shipped blank with tiny fusible microscopic links. Programmed once by burning links using high voltage pulses via a PROM Burner/Programmer. One-Time Programmable (OTP).
3. EPROM (Erasable Programmable ROM): Can be erased and rewritten multiple times. Erased by exposing the chip through its transparent quartz crystal window to high-intensity Ultraviolet (UV) light for 20-30 minutes. All bytes erased simultaneously.
4. EEPROM (Electrically Erasable Programmable ROM): Can be erased and rewritten electrically byte-by-byte without removing from circuit board. Erase time: Milliseconds.
5. Flash Memory: An advanced form of EEPROM that erases and writes data in whole memory blocks (pages) rather than single bytes. Powers modern motherboard BIOS chips, USB drives, and SSDs.
Master SRAM vs DRAM Comparative Matrix (University Exam Table)
| Parameter | Static RAM (SRAM) | Dynamic RAM (DRAM) |
|---|---|---|
| Storage Element | Bistable Flip-Flop (4 - 6 Transistors) | 1 Transistor + 1 Capacitor (1T-1C) |
| Need for Refreshing? | NO (Retains data continuously) | YES (Periodic electrical refresh every 64ms) |
| Access Speed | Ultra Fast (1 – 10 ns) | Moderate (50 – 80 ns) |
| Density & Capacity | Low density (Large cell size) | High density (Tiny capacitor cell) |
| Cost per Bit | Expensive | Inexpensive / Economical |
| Primary Use Case | CPU Internal Cache (L1/L2/L3) | Main Primary RAM Modules (DDR4/DDR5) |
🎯 University Exam Scoring Blueprint
- Draw a 6-parameter comparative table for SRAM vs DRAM.
- Explain the difference between EPROM (UV light erasure) and EEPROM (electrical erasure).
- Define BIOS and explain why it is stored in ROM/Flash memory instead of RAM.
Top Viva Questions on Primary Memory Deep-Dive: RAM (SRAM vs DRAM) & ROM (PROM, EPROM, EEPROM, Flash)
1