SS 2 First Term

Learning Objectives

• Distinguish between RAM and ROM.
• Describe cache memory and the memory hierarchy.
• Explain how the processor uses main memory.
• Explain why a computer uses virtual memory.

RAM and ROM

Main memory is the primary working memory inside a computer that the CPU accesses directly. It stores the instructions and data that the CPU is currently working with. Without main memory, the CPU would have no instructions to execute and no data to process. The two main categories of main memory are RAM and ROM, and each serves a different purpose.

RAM (Random Access Memory) is volatile memory, meaning it loses all its contents when the computer is switched off. It temporarily holds the programmes and data that are currently in use. When you open a web browser, a Word document or a game, its contents are loaded from secondary storage into RAM so that the CPU can access them quickly. There are two main types of RAM. DRAM (Dynamic RAM) is the most common type used in personal computers. It stores each bit of data in a tiny capacitor that leaks charge over time, so it must be refreshed thousands of times per second to maintain its data. DRAM is relatively cheap and dense, which is why it is used for the main system memory in laptops and desktops. A typical school computer in Abuja may have 4 GB or 8 GB of DRAM. SRAM (Static RAM) uses flip-flop circuits to store each bit and does not need refreshing. It is much faster than DRAM but also more expensive and takes up more space on the chip. Because of its speed, SRAM is used for cache memory rather than as the main system RAM.

ROM (Read-Only Memory) is non-volatile memory, meaning it retains its contents even when the power is switched off. It stores the permanent instructions the computer needs to start up, such as the BIOS (Basic Input/Output System) or UEFI firmware. ROM is called read-only because under normal operation the CPU can read from it but cannot easily write new data to it. When you press the power button on a school computer, the ROM provides the first set of instructions that tell the CPU how to initialise the hardware, check memory and load the operating system from the hard drive. Some modern forms of ROM, such as EEPROM and flash memory, allow limited writing, which is how firmware updates work.

Virtual memory means space on storage used as extra memory when RAM is full.

Cache Memory and the Memory Hierarchy

Cache memory is very fast, small memory located on or very close to the CPU chip. It stores copies of frequently accessed data and instructions from RAM so that the CPU can retrieve them much faster than going to main memory every time. There are typically three levels of cache. L1 cache is inside each CPU core, very small (32 KB to 256 KB) and the fastest. L2 cache is inside or near each core, larger (256 KB to 1 MB) and very fast. L3 cache is shared among all cores, the largest (2 MB to 64 MB), fast but slower than L1 and L2.

When the CPU needs data, it checks L1 first. If the data is there, this is called a cache hit and the data is retrieved instantly. If it is not there, this is called a cache miss, and the CPU then checks L2, then L3, and finally goes to RAM. The closer the data is to the CPU, the faster it can be used. Memory in a computer is arranged in a hierarchy from the fastest, smallest and most expensive at the top to the slowest, largest and cheapest at the bottom. CPU registers are at the top, followed by L1, L2 and L3 cache, then main memory (RAM) with 4 GB to 32 GB, and secondary storage (hard drives and SSDs) at the bottom with 256 GB to several terabytes. The hierarchy exists because fast memory is expensive. By keeping the most used data close to the CPU in cache and the rest in RAM or on disk, the computer achieves a good balance of speed and capacity.

Addressable memory refers to the total amount of memory that the CPU can uniquely address using memory addresses. The number of bits in the address bus determines how many unique addresses the CPU can create. A 32-bit address bus can address up to 2 to the power of 32 bytes, which is about 4 GB of RAM. A 64-bit address bus can address far more, theoretically up to 16 exabytes. If a school computer has 8 GB of RAM installed but the operating system only supports 32-bit addressing, it may only be able to use 4 GB of that RAM.

Memory hierarchy means the order of memory from the fastest and smallest to the slowest and largest.

How the CPU Uses Main Memory

During the fetch-execute cycle, the CPU uses main memory at several points. During the fetch step, the Program Counter sends the address of the next instruction to the MAR, which places it on the address bus. The instruction is then read from RAM into the MDR and placed in the Instruction Register. During the decode and execute steps, if the instruction requires data, the data address is sent to the MAR and the data is fetched from RAM into the MDR. During the store step, results are written back to RAM by placing the address in the MAR and the data in the MDR, with a write signal sent on the control bus. Cache speeds up this entire cycle by keeping frequently used instructions and data close to the CPU, reducing the need to wait for slower RAM accesses.

Practical Work

1. Draw a table comparing RAM and ROM.
2. Arrange cache, RAM, and storage in order of speed.
3. Explain why cache is placed close to the processor.
4. Describe what happens when RAM is full.

Scenario-Based Questions

1. A student opens a web browser, a Word document and a game at the same time on a computer with 4 GB of RAM. Explain why these programs must be loaded from secondary storage into RAM before the CPU can use them, and what happens to their contents when the computer is switched off.
2. A technician is choosing between DRAM and SRAM for different roles inside a computer. Compare DRAM and SRAM in terms of speed, cost and typical location inside the computer.
3. A school purchases new desktops with 8 GB of RAM. The technician says a 32-bit operating system would not use all of it. Explain why a 32-bit OS can address at most 4 GB and how a 64-bit OS solves this.
4. When a computer starts up, it first loads instructions from ROM. Explain what BIOS or UEFI does during boot and why ROM is used instead of RAM for this purpose.
5. A developer is designing a system and must choose between L1, L2 and L3 cache. Explain the memory hierarchy from L1 to main memory and why faster cache is always smaller.
6. During the fetch-execute cycle, the CPU copies an instruction address from the PC into the MAR. Explain why the MAR and MDR are needed rather than sending data directly between the PC and the CPU.
7. A student says RAM is faster than ROM so ROM should be replaced with RAM. Explain why this would prevent the computer from starting up and why both types of memory are necessary.