Lis chaque section en anglais. Le resume en francais ("En bref") est juste un filet de securite : essaie de comprendre en anglais d'abord.
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Software needs memory to run. The main type of memory on a motherboard is RAM (random-access memory), and specifically DRAM (dynamic RAM). There is also a second, faster type called SRAM (static RAM), also known as cache memory.
DRAM is cheaper but slower than SRAM. The reason is refreshing: the electrical charge inside a DRAM chip (which stores the 1s and 0s) leaks away over time, so the chip must constantly rewrite its own data to keep it. This refresh process takes time, which is why DRAM is slower.
SRAM does not need refreshing, so it is faster - but it is more expensive, so only a small amount is used, usually inside or right next to the processor, as cache memory. Cache memory holds the data the CPU uses most often, so the CPU does not have to wait for the slower DRAM. For example, on one system, reaching data in DRAM could take about 90 nanoseconds, while reaching the same data in cache could take only about 23 nanoseconds (a nanosecond is one-billionth of a second).
Memory today comes packaged as a DIMM (dual in-line memory module), with 168, 184, 240, or 288 pins depending on the type. A memory module is also informally called a memory stick, or just 'a stick of RAM'. Laptops and printers use a smaller version called a SODIMM (small outline DIMM).
DRAM must be constantly refreshed (its charge leaks away over time); SRAM does not need refreshing.
Inside or right next to the processor.
A smaller version of a DIMM, used in laptops and printers.
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Planning a memory installation/upgrade means: check the documentation to see what memory type is supported, determine what features are supported, determine how much memory is needed, determine which module(s) and how many, then research prices and buy.
Memory technology keeps getting faster through DDR (double data rate) generations. CompTIA calls these 'DDR iterations'. Each DDR generation uses a different number of pins and is NOT compatible with the others - a DDR5 module simply does not fit into a DDR3 or DDR4 slot, even though all DDR modules are the same physical size.
DDR4 and DDR5 modules can also come in a lower-voltage version, marked with an 'L' at the end (for example DDR4L). The motherboard must support this feature. Lower-voltage modules are commonly used in laptops and mobile devices.
| Technology | Key facts |
|---|---|
| SDRAM (Synchronous DRAM) | Synchronizes with the CPU clock signal for faster access; used with DIMMs. |
| DDR | Sends data on both the rising and falling edge of the clock signal (twice the data of plain SDRAM). |
| DDR2 | Uses 240-pin DIMMs; not compatible with DDR. |
| DDR3 | Uses 240-pin DIMMs; not compatible with DDR2 or DDR; supports multicore CPUs; ~1.5V. |
| DDR4 | Uses 288-pin DIMMs (unique slot); lower voltage (~1.2V), faster than DDR3. |
| DDR5 | Uses 288-pin DIMMs (unique slot, not DDR4-compatible); lower voltage (~1.1V); more bank groups/bandwidth than DDR4. |
| DDR6 | Expected: lower voltage, faster speeds, more channels than DDR5 (upcoming technology). |
No - each DDR generation is physically incompatible with the others, even though they are the same physical size.
It means a lower-voltage version; commonly used in laptops and mobile devices.
288 pins.
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Beyond the DDR generation, memory modules can have different features. ECC RAM (error correction code) uses a mathematical check to detect up to 4-bit errors and correct 1-bit errors; it is used in high-end computers and servers. Non-ECC RAM does no error checking at all - if an ad doesn't mention ECC, assume the module is non-ECC.
Unbuffered memory sends data immediately and is faster. Buffered memory (registered memory) has extra chips that delay data transfers slightly to improve accuracy; it is used in servers and high-end computers, and is sometimes called fully buffered (FBDIMM). If a system allows both, and you install one registered module, ALL installed modules must be registered.
Single rank means the module has one set of DRAM chips; dual rank means it has two sets (the CPU normally accesses one rank at a time) - ads showing '2R' mean dual rank; if rank is not mentioned, assume single rank.
SPD (serial presence detect) is an extra chip on the module that stores its specifications (capacity, voltage, refresh rate...); the BIOS/UEFI reads this automatically for best performance. XMP (extreme memory profile) lets the BIOS/UEFI apply special voltage/timing settings to overclock the memory. A module can combine several of these features at once (for example: DDR4, registered, and ECC together).
Rule of thumb: always check the motherboard documentation before buying memory - don't guess the specifications.
| Feature | What it means |
|---|---|
| ECC RAM | Detects up to 4-bit errors, corrects 1-bit errors; used in servers/high-end PCs |
| Non-ECC RAM | No error checking (assume this if ECC isn't mentioned) |
| Unbuffered memory | Sends data immediately; faster than buffered/registered |
| Buffered / registered memory | Delays data by one clock tick for accuracy; used in servers; sometimes called FBDIMM |
| Single rank | One set of DRAM chips on the module (the default if not stated) |
| Dual rank (2R) | Two single-rank sets on one module |
| SPD (serial presence detect) | A chip that tells the BIOS/UEFI the module's specifications |
| Dual-voltage memory | Can run at a lower voltage if the motherboard supports it (may be marked L) |
| XMP (extreme memory profile) | Lets the BIOS/UEFI overclock the memory with special voltage/timing |
ECC can detect and correct small errors (used in servers/high-end PCs); non-ECC does no error checking.
All installed modules must also be registered.
It lets the module tell the BIOS/UEFI its specifications (capacity, voltage, etc.) automatically.
Letting the BIOS/UEFI apply special voltage/timing settings to overclock the memory.
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The amount of memory needed depends on the operating system, the applications used, how many are open at once, and the maximum the motherboard allows. Memory module capacities double in a series (for example 512 MB, 1 GB, 2 GB, 4 GB...). If a motherboard supports a maximum of 32 GB across four slots, using two 16 GB sticks is usually better than one 32 GB stick, because it allows dual-channel operation (covered next) and leaves room to upgrade later. Using matching stick sizes also tends to give better stability.
Before upgrading, you need to know: how much memory you already have, how many RAM slots are used/free, and the motherboard's maximum supported memory. There is no way around a motherboard's maximum - if you need more, you must upgrade the motherboard itself.
Windows editions also have their own memory limits, separate from the motherboard's limit - even if the motherboard allows more, the OS might not use it. Important warning: never install more than 4 GB of RAM on a 32-bit operating system (like 32-bit Windows) - it simply cannot recognize memory above 4 GB.
To see how much memory is installed, search for and run msinfo32 (System Information) or check Task Manager's Performance tab.
| Operating system | Minimum RAM to start calculations |
|---|---|
| Windows 10 | 2 GB |
| Windows 11 | 4 GB |
| macOS Sierra/Catalina/Big Sur | 4 GB |
| macOS Ventura/Sonoma/Sequoia | 8 GB |
| Linux | From 64 MB, depending on the version |
| Operating system edition | Memory limit |
|---|---|
| Windows 10/11 Home | 128 GB |
| Windows 10 Pro/Enterprise | 2 TB |
| Windows 11 Pro/Education | 2 TB |
| Windows 11 Pro for Workstations/Enterprise | 6 TB |
4 GB (it cannot recognize anything above that).
Upgrade to a different motherboard - there is no workaround.
Run msinfo32 (System Information) or check Task Manager's Performance tab.
It allows dual-channel operation and leaves slots free for future upgrades.
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Most motherboards today support dual-channel memory: the memory controller uses two memory paths at once instead of one, which improves performance. For example, a motherboard with four memory slots divided into two channels has two slots per channel.
Dual-channeling works best when the modules match exactly: same type, features, speed, and capacity. Some motherboards require an exact match to enable dual-channeling at all; others allow mismatched modules but then run everything at the lower/slower specification. To guarantee a match, buy a memory 'kit' - modules pre-tested and sold together as a matching pair.
Some newer and server motherboards support triple-channel (three modules working together) or quad-channel (four modules at once) memory, mostly found on server motherboards.
Documentation sometimes uses codes like '1DPC' or '2DPC', meaning 1 or 2 DIMMs Per Channel; '1R' or '2R' means single-rank or dual-rank modules.
Motherboard manufacturers label their slots differently, so always check the documentation to know which slots form which channel - never assume from the physical layout alone.