ECC Memory: Registered vs. Unregistered DIMMs

Gatsby

Modern memory modules differ not only in size and speed, but also in architecture. Two key distinctions are registered (RDIMM) vs unregistered (UDIMM) and whether they support ECC (Error-Correcting Code).

Why No Registered, Non-ECC Modules?

All registered DIMMs include ECC because:
– ECC requires 1 extra bit for every 8 bits of data
– ECC DIMMs typically have an extra RAM chip to store this parity information
– CPUs that support RDIMMs always support ECC, so there’s no market for RDIMMs without ECC

ECC in a Nutshell

ECC adds redundancy to detect and correct memory errors:

8 bits data + 1 ECC bit => error detection/correction

CPUs with ECC support have the extra data lanes and logic to handle this.

Unregistered (Unbuffered) DIMMs

In unregistered DIMMs, the CPU directly communicates with each RAM chip on the module.

Diagram:

CPU
 |------------------------|
 |      Memory Controller |
 |------------------------|
     |   |   |   |
     v   v   v   v
   [RAM][RAM][RAM][RAM]   <- Unbuffered (direct)

Pros:

  • Lower latency
  • Ideal for desktops and small workstations

Cons:

  • Limited scalability (fewer modules per channel)

Registered DIMMs (RDIMMs)

RDIMMs add a register chip between the CPU and the RAM chips for address and command signals (not data).

Diagram:

CPU
 |------------------------|
 |      Memory Controller |
 |------------------------|
           |
           v
       [Register]
         | | |
         v v v
      [RAM][RAM][RAM]     <- Registered

Pros:

  • Higher capacity per module (up to 36 chips)
  • More DIMMs per channel (up to 3)
  • Improved signal integrity

Cons:

  • Slightly more latency
  • More power consumption

Fully Buffered / Load-Reduced DIMMs

In this design, the register chip also handles data in addition to addresses and commands.

Diagram:

CPU
 |------------------------|
 |      Memory Controller |
 |------------------------|
           |
           v
      [Buffer Chip]
         | | |
         v v v
      [RAM][RAM][RAM]     <- Fully Buffered / LRDIMM

Pros:

  • Very high memory density
  • Ideal for enterprise systems

Cons:

  • Higher latency
  • More power usage

Summary Table

Feature UDIMM RDIMM LRDIMM / FB-DIMM
ECC Support Optional Always ECC Always ECC
Register/Buffer Chip None Address/command only Address, command & data
Latency Low Medium High
Capacity Limited High Very high
Power Consumption Low Medium High
Use Case Desktops, homelab Servers Enterprise-grade servers

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