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This lesson covers the two types of primary storage — RAM (Random Access Memory) and ROM (Read-Only Memory) — as specified in OCR J277 Section 1.2.1. You need to understand their characteristics, differences, and roles within a computer system.
Primary storage (also called main memory or internal memory) is memory that the CPU can access directly via the system bus. It is used to store data and instructions that are currently being used or are about to be used.
There are two types of primary storage:
RAM is the main working memory of a computer. When you open a program, the operating system loads it from secondary storage (e.g. hard drive or SSD) into RAM so that the CPU can access it quickly.
| Feature | Detail |
|---|---|
| Volatile | Data is lost when the power is turned off |
| Read and write | Data can be both read from and written to RAM |
| Speed | Much faster than secondary storage, but slower than cache |
| Capacity | Typically 4 GB to 64 GB in modern computers |
| Purpose | Stores currently running programs, the OS, and data being processed |
Secondary storage (HDDs, SSDs) is too slow for the CPU to access directly during program execution. RAM provides a fast, temporary workspace where the CPU can quickly read and write data.
When you open multiple programs, they all need space in RAM. If RAM becomes full, the computer may slow down significantly (see the section on virtual memory).
| Type | Full Name | Key Feature |
|---|---|---|
| DRAM | Dynamic RAM | Needs to be constantly refreshed (re-energised); cheaper, used for main memory |
| SRAM | Static RAM | Does not need refreshing; faster but more expensive; used for cache |
OCR Exam Tip: If asked about the difference between SRAM and DRAM, focus on: SRAM is faster and does not need refreshing, but is more expensive. DRAM is slower, needs refreshing, but is cheaper and used for main memory.
ROM is a type of primary memory that stores data permanently. The data in ROM is typically written during manufacturing and is not easily changed by the user.
| Feature | Detail |
|---|---|
| Non-volatile | Data is retained even when the power is turned off |
| Read-only | Data can be read but generally cannot be written to by the user |
| Speed | Fast, but typically smaller capacity than RAM |
| Purpose | Stores the BIOS/UEFI (boot-up instructions) |
The most important content stored in ROM is the BIOS (Basic Input/Output System) or its modern replacement, UEFI (Unified Extensible Firmware Interface). This firmware:
| Feature | RAM | ROM |
|---|---|---|
| Volatile? | Yes — loses data when power is off | No — retains data when power is off |
| Read/Write? | Read and write | Read only (generally) |
| Speed | Fast | Fast (but typically smaller) |
| Capacity | Large (4-64 GB typically) | Small (a few MB) |
| Contents | Running programs, OS, current data | BIOS/UEFI boot instructions |
| Can be changed? | Yes, constantly | Not by the user (written at manufacture) |
Understanding how RAM and ROM work together during startup is essential:
OCR Exam Tip: A common question asks: "Why does the CPU access ROM first when the computer starts up?" Answer: Because RAM is volatile and empty when the computer is turned on, so the CPU needs the non-volatile BIOS in ROM to start the boot process.
Key Vocabulary: RAM, ROM, volatile, non-volatile, BIOS, UEFI, POST, DRAM, SRAM, primary storage.