Primary Storage

This lesson covers primary storage — the memory that the CPU can access directly, including RAM, ROM, cache and virtual memory. For the OCR H446 exam, you must understand how each type works, the differences between SRAM and DRAM, the cache hierarchy, and the role of virtual memory.

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What Is Primary Storage?

Primary storage (also called main memory or internal memory) is memory that the CPU can access directly via the system bus. It includes RAM, ROM and cache. Primary storage is fast but generally volatile (except ROM) and limited in capacity compared to secondary storage.

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RAM — Random Access Memory

RAM is volatile — it loses all its contents when the power is switched off. It stores the currently running programs and data that the CPU needs to access quickly.

DRAM (Dynamic RAM)

Feature	Detail
How it works	Each bit is stored as a charge in a tiny capacitor. The charge leaks over time, so the memory must be refreshed thousands of times per second
Density	High — each cell requires only one transistor and one capacitor, so many bits can be packed into a small area
Speed	Slower than SRAM due to refresh cycles
Cost	Cheaper per bit than SRAM
Power	Moderate — needs continuous refreshing
Use	Main memory (RAM) in PCs, laptops, servers

SRAM (Static RAM)

Feature	Detail
How it works	Each bit is stored using a flip-flop circuit made of 4-6 transistors. The data is held as long as power is supplied — no refreshing needed
Density	Low — each cell requires multiple transistors, so fewer bits per unit area
Speed	Very fast — no refresh delay
Cost	More expensive per bit than DRAM
Power	Lower idle power (no refresh), but more transistors per bit
Use	CPU cache (L1, L2, L3)

DRAM vs SRAM Comparison

Feature	DRAM	SRAM
Storage mechanism	Capacitor (charge)	Flip-flop (transistors)
Needs refreshing?	Yes	No
Speed	Slower	Faster
Density	Higher	Lower
Cost per bit	Cheaper	More expensive
Power consumption	More (refresh cycles)	Less (no refresh)
Used for	Main memory (RAM)	CPU cache

Exam Tip: A common question asks why cache uses SRAM rather than DRAM. Answer: SRAM is faster (no refresh delay), which is essential for cache because the CPU needs data from cache in as few clock cycles as possible. The higher cost and lower density are acceptable because cache is small.

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ROM — Read-Only Memory

ROM is non-volatile — it retains its contents when the power is switched off. It stores permanent, essential software that rarely (or never) changes.

Variant	Description
ROM	Contents set during manufacture; cannot be changed
PROM	Programmable ROM — can be written to once after manufacture using a special programmer
EPROM	Erasable PROM — can be erased by exposure to UV light and then reprogrammed
EEPROM	Electrically Erasable PROM — can be erased and reprogrammed electrically, byte by byte
Flash ROM	A form of EEPROM that can be erased and rewritten in blocks. Used in firmware storage

Common Uses of ROM

• BIOS / UEFI — the Basic Input/Output System or Unified Extensible Firmware Interface. The first code that runs when a computer powers on. It initialises hardware and loads the operating system from secondary storage.
• Embedded system firmware — the permanent program in devices like washing machines, microwaves and routers.

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Cache Memory

Cache is a small amount of very fast SRAM located inside or very close to the CPU. Its purpose is to store copies of frequently accessed data and instructions so the CPU does not have to wait for slower main memory (DRAM).

Cache Hierarchy

Level	Location	Size	Speed	Purpose
L1	Inside each CPU core	Very small (32-128 KB per core)	Fastest	Stores the most frequently accessed data/instructions. Often split into L1-I (instructions) and L1-D (data)
L2	Inside or close to each core	Small-medium (256 KB - 1 MB per core)	Fast	Stores data that does not fit in L1
L3	Shared between all cores	Medium-large (4-64 MB)	Moderate (slower than L1/L2)	Shared cache; reduces the need to access main memory

How Cache Works

When the CPU needs data: