When you see memory speed reported as "30 ns" in dmidecode output, this actually represents the access time in nanoseconds, not the clock speed. To convert this to MHz:

Speed (MHz) = 1000 / (ns value × 2)
For 30 ns: 1000 / (30 × 2) ≈ 16.67 MHz

This shows why you're seeing a discrepancy - the ns value refers to the underlying DRAM cell access time, not the bus speed (400/533/667 MHz). Modern DDR memory uses techniques like prefetching to achieve higher effective speeds.

Here are several reliable methods to check RAM speed without opening your case:

# Method 1: Using dmidecode with proper parsing
sudo dmidecode --type 17 | grep -i speed

# Method 2: Using lshw (may require admin privileges)
sudo lshw -class memory | grep clock

# Method 3: Decoding SPD data via i2c-tools
sudo modprobe eeprom
sudo decode-dimms

# Method 4: Checking kernel messages
dmesg | grep -i memory

The most accurate method is reading the Serial Presence Detect (SPD) data from your modules:

# Install necessary tools
sudo apt-get install i2c-tools

# Detect I2C buses (may vary by system)
sudo i2cdetect -l

# Read SPD data (replace X with your bus number)
sudo i2cdump -y X 0x50

Let's examine what proper DDR3 speed detection looks like:

# Sample output from decode-dimms
Memory Serial Presence Detect Decoder
=====================================

Decoding EEPROM: /sys/bus/i2c/drivers/eeprom/0-0050
Guessing DIMM is in                             bank 1

---=== SPD EEPROM Information ===---
EEPROM CRC of bytes 0-116                       OK (0x7C7D)
# of bytes written to SDRAM EEPROM              176
Total width (per module)                        64 bits
Data width (per module)                         64 bits
SDRAM type                                      DDR3 SDRAM
Module speed                                    1066 MHz (DDR3-1066)

If all tools report "Unknown" speed (as in your dmidecode output), this typically indicates either:

• SPD data isn't being read correctly (try i2c-tools)
• Motherboard limitations in reporting
• Very old or non-standard memory modules

In such cases, checking the manufacturer's specifications (as you ultimately did) becomes necessary. The model number you found (RM12864AA667) confirms DDR2-667 operation.

For developers wanting direct hardware access, here's a Python snippet using ctypes to read memory information:

import ctypes

class MemoryInfo(ctypes.Structure):
    _fields_ = [
        ("length", ctypes.c_ulong),
        ("memory_load", ctypes.c_ulong),
        ("total_phys", ctypes.c_ulonglong),
        ("avail_phys", ctypes.c_ulonglong),
        ("total_page", ctypes.c_ulonglong),
        ("avail_page", ctypes.c_ulonglong),
        ("total_virtual", ctypes.c_ulonglong),
        ("avail_virtual", ctypes.c_ulonglong)
    ]

def get_memory_info():
    mem_info = MemoryInfo()
    ctypes.windll.kernel32.GlobalMemoryStatusEx(ctypes.byref(mem_info))
    return mem_info

# Note: This provides capacity info but not speed - 
# demonstrates the challenge of low-level memory detection

When working with Linux systems, identifying actual RAM speed can be surprisingly tricky. While BIOS and hardware specifications provide theoretical support for certain speeds (like 400/533/667 MHz in your case), the operational speed might differ due to various factors.

Here are the most effective command-line methods to check RAM speed on Linux systems:

# Method 1: dmidecode (Detailed Memory Information)
sudo dmidecode --type memory

# Method 2: Decoding specific DMI types
sudo dmidecode -t 5,6,16,17

# Method 3: Alternative tool - hardinfo (GUI available)
sudo hardinfo -r > memory_report.txt

The confusion about "30 ns" versus MHz ratings stems from how DMI reports memory timings. Here's how to interpret the values:

# Converting nanosecond ratings to MHz:
# 30 ns = (1 / (30 * 10^-9)) ≈ 33.33 MHz
# This represents the access time, not the clock speed

# Actual DDR speed calculation:
# DDR2-667 operates at 333 MHz clock (667 MT/s data rate)
# The ns rating refers to latency characteristics

For more precise detection when dmidecode shows "Unknown":

# Check kernel ring buffer for memory initialization
dmesg | grep -i memory

# Parse SPD data (requires root)
sudo decode-dimms

# Check memory controller info
lspci -v | grep -i memory -A 10

# CPU-specific memory info (Intel)
sudo cpuid | grep -i "memory profile"

As you discovered, sometimes physical inspection is necessary. However, we can combine multiple techniques:

# Create comprehensive memory report
{
  echo "===== DMI Information ====="
  sudo dmidecode --type memory
  echo "\n===== Kernel Memory Info ====="
  dmesg | grep -i -A 5 -B 5 "memory"
  echo "\n===== Current Memory Usage ====="
  free -h
  echo "\n===== Processor Cache Info ====="
  lscpu | grep -i cache
} > system_memory_report.txt

The motherboard's supported speeds (400/533/667 MHz) represent theoretical maximums. Actual operational speed depends on:

• Lowest common denominator between installed DIMMs
• CPU memory controller capabilities
• BIOS configuration settings
• Potential downclocking due to stability issues