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Difficulty: Very hardRAM Types and Characteristics

A network administrator is upgrading a financial data server that currently contains two 16 GB16\text{ GB} DDR4 unbuffered ECC (ECC UDIMM) modules installed in memory slots A1 and B1. To expand system capacity to 64 GB64\text{ GB}, two additional 16 GB16\text{ GB} DDR4 registered ECC (RDIMM) modules with matching clock speed (3200 MT/s3200\text{ MT/s}) and operating voltage (1.2 V1.2\text{ V}) are inserted into slots A2 and B2. Upon powering on, the system fails to Power-On Self-Test (POST) and emits a memory initialization error beep code. Which of the following is the primary cause of this system failure?

  1. Unbuffered memory (UDIMM) and registered memory (RDIMM) technologies are fundamentally incompatible and cannot be mixed within the same system.Answer
  2. B
    Populating slots A2 and B2 creates an asymmetrical channel population that disables dual-channel mode and halts POST execution.
  3. C
    Expanding system memory to four populated slots requires small outline DIMM (SO-DIMM) modules to comply with motherboard power rail limits.
  4. D
    The operating system corrupted its bootloader configuration because the memory beep code indicates a missing storage driver.

Answer

Unbuffered (UDIMM) and registered (RDIMM) memory modules cannot be mixed within the same system.
Unbuffered RAM (UDIMM) and Registered RAM (RDIMM) have different electrical signaling characteristics. RDIMMs incorporate an onboard register chip to buffer control and address signals between the memory modules and the memory controller, reducing electrical load on the host bus. UDIMMs lack this register and communicate directly with the memory controller. Because the memory controller cannot simultaneously adjust its signaling timing and electrical parameters for both buffered and unbuffered modules, mixing UDIMM and RDIMM results in a failed POST routine.

Step-by-Step Solution

1
Analyze the existing memory specifications and the newly installed modules.
Identified existing modules as 16 GB16\text{ GB} DDR4 ECC UDIMM and new modules as 16 GB16\text{ GB} DDR4 ECC RDIMM.
Although both sets share identical capacities (16 GB16\text{ GB}), speeds (3200 MT/s3200\text{ MT/s}), and operating voltages (1.2 V1.2\text{ V}), UDIMMs and RDIMMs differ in physical buffer architecture.
2
Evaluate memory controller buffering rules.
Determined that RDIMMs contain a hardware register chip that buffers control/address lines, while UDIMMs connect directly to the memory controller.
Motherboard memory controllers cannot operate in a hybrid mode handling both buffered and unbuffered signaling simultaneously.
3
Identify the cause of the POST failure.
Mixing UDIMM and RDIMM modules causes the memory controller initialization check to fail during POST.
All installed RAM modules must match in buffering type (all UDIMM or all RDIMM) for the motherboard memory bus to function.

Key Concept

RAM Types and Compatibility (UDIMM vs. RDIMM Mixing Restrictions)
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Topics

RAM Types and Characteristics

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