Memory¶

Exercise cpu-motherboard-memory-annex.pdf · PDF

Look at the box of any phone: "8 GB + 256 GB". Two numbers, both in gigabytes, both called "memory" by the salesperson. But they are completely different things — and if you open 40 browser tabs on a laptop with too little of the first one, you will feel the difference in seconds.

A computer does not have one memory. It has a whole family of them, from tiny and lightning-fast to huge and slow. The chef from the architecture page uses all of them, every second. This page puts them in order.

Volatile or non-volatile?¶

The first question to ask about any memory: what happens when the power goes off?

Volatile memory loses its contents without power. Non-volatile memory keeps them.

That is the answer to the phone box: 8 GB is the RAM (volatile workspace for running apps) and 256 GB is the storage (non-volatile flash for your photos and apps). Both are "memory", with opposite jobs.

The memory hierarchy¶

If fast memory is so good, why not build a computer with only fast memory? Because in memory you can never have all three at once: fast, big and cheap. Faster memory is more expensive per byte, so you can only afford a little of it.

The solution is a hierarchy: a small amount of very fast memory next to the CPU, and bigger, slower, cheaper layers further away.

The memory hierarchy orders a computer's memories by speed, size and cost: the closer to the CPU, the faster, smaller and more expensive per byte.

flowchart TB
    R["<b>Registers</b><br>in the chef's hands<br>bytes · inside each core"]
    C["<b>Cache</b> (L1, L2, L3)<br>trays at the chef's elbow<br>KB to tens of MB · inside the CPU"]
    M["<b>RAM</b><br>the countertop<br>GB · on the motherboard"]
    S["<b>Storage</b> (SSD, HDD)<br>the pantry<br>hundreds of GB to TB"]
    R -->|"slower, bigger,<br>cheaper per byte"| C --> M --> S

The kitchen stays the same as always: what the chef holds in their hands (registers), the trays at their elbow (cache), the countertop (RAM) and the pantry (storage). The chef works fast because the things used most are always the closest. Storage has its own page: Storage.

RAM: the countertop¶

RAM (Random Access Memory) is the computer's main working memory: it holds the programs that are running and the data they use. "Random access" means any position can be read directly, in the same time, without going through the others.

When you open an app, the operating system copies it from storage into RAM, and the CPU runs it from there. Close it, and its space is freed. Switch the computer off, and everything in RAM disappears — that is why unsaved work is lost in a power cut.

DRAM and SRAM: two ways to store a bit¶

There are two main ways to build a bit of volatile memory:

A capacitor is like a tiny glass that holds an electric charge: full = 1, empty = 0. The problem is that the glass leaks. After a few milliseconds, a 1 would turn into a 0. So the memory refreshes itself: it reads every bit and writes it back, about every 64 milliseconds, all the time. That is the "dynamic" in DRAM.

DDR: two transfers per tick¶

The RAM in today's computers is DDR SDRAM. You only need to remember what DDR means:

DDR (Double Data Rate) memory transfers data twice per clock tick — once when the signal goes up, once when it goes down.

That is why RAM speed is given in MT/s (megatransfers per second), not MHz. DDR4-3200 means 3,200 million transfers per second, while its clock actually ticks at 1,600 MHz. Many shops write "3200 MHz" anyway; now you know it is a small lie.

Each generation is faster and uses less power:

Modules: DIMM and SO-DIMM¶

RAM chips come soldered onto small boards called modules:

When you buy a thin laptop, check whether its RAM is soldered: if it is, the amount you buy is the amount you keep forever.

Channels: more corridors to the countertop¶

A memory channel is an independent path between the CPU's memory controller and the RAM. Most desktop CPUs have two (dual channel).

With one module, the CPU uses one channel. With two modules in the right slots, it uses both at the same time — twice the corridors, twice the traffic. The maths for DDR4-3200:

One channel:  3,200 million transfers/s × 8 bytes = 25.6 GB/s
Two channels: 25.6 GB/s × 2                      = 51.2 GB/s

So 2 × 8 GB is usually faster than 1 × 16 GB, even though the total is the same. And the slots matter: on a board with four slots, the manual tells you which two to use first (often labelled A2 and B2).

ROM, flash and firmware¶

When you press the power button, RAM is empty. So where does the very first instruction come from? From a memory that never forgets.

ROM (Read-Only Memory) is non-volatile memory whose contents are written once and then only read.

The first ROMs were written at the factory and could never change. Over time, engineers wanted to update them, so ROM evolved:

Flash memory was invented at Toshiba by Fujio Masuoka, who presented it in 1984. A colleague suggested the name because erasing a whole block at once reminded him of a camera's flash. Today flash is everywhere: SSDs, USB sticks, memory cards, your phone's storage.

It is also where the firmware lives.

Firmware is software stored in non-volatile memory inside a device, which controls the hardware directly. It is the first thing that runs when the device is switched on.

Your motherboard's UEFI (the modern version of the old BIOS) is firmware, stored in a flash chip on the board. It is not "read-only" any more — you can update it — but the name ROM stuck. Your router, printer, keyboard and even your SSD have their own firmware too. We will see what the UEFI does when a PC boots later in this unit.

See it in your own computer¶

On Linux, how much RAM there is and how much is free:

free -h

total is the size of the countertop, available is what apps can still use, and the Swap line is the overflow space on the disk.

What modules are installed, their type (DDR4/DDR5), speed, and which slots are empty:

sudo dmidecode -t memory | grep -E 'Size|Type:|Speed|Locator' | grep -v Unknown

On Windows: Task Manager (Ctrl+Shift+Esc) → Performance → Memory shows the speed, the slots used and the form factor (DIMM or SO-DIMM).

Key takeaways¶

• Volatile memory (registers, cache, RAM) forgets without power; non-volatile memory (ROM, flash, disks) keeps its data.
• The memory hierarchy mixes a little fast, expensive memory near the CPU with lots of slow, cheap memory further away: registers → cache → RAM → storage.
• Cache is SRAM (6 transistors per bit, fast, expensive); RAM is DRAM (1 transistor + 1 capacitor, cheap, must be refreshed).
• DDR transfers twice per tick; DDR4-3200 means 3,200 MT/s. DDR4 and DDR5 are not interchangeable.
• DIMM for desktops, SO-DIMM for laptops; two modules in dual channel double the bandwidth.
• Firmware such as the UEFI lives in flash memory, the modern descendant of ROM.

Quick check¶

RAM forgets everything when the power goes off. So where do your files really live? Next: the pantry — storage.