Serial Presence Detect - Reading and Writing SPD Information

Reading and Writing SPD Information

Memory module manufacturers write the SPD information to the EEPROM on the module. Motherboard BIOSes read the SPD information to configure the memory controller. There exist several programs that are able to read and modify SPD information on most, but not all motherboard chipsets.

• dmidecode program that can decode information about memory (and other things) and runs on Linux, FreeBSD, NetBSD, OpenBSD, BeOS, Cygwin and Solaris. dmidecode does not access SPD information directly; it reports the BIOS data about the memory. This information may be limited or incorrect.

• On Linux systems, the user space program decode-dimms provided with i2c-tools decodes and prints information on any memory with SPD information in the computer. It requires SMBus controller support in the kernel, the EEPROM kernel driver, and also that the SPD EEPROMs are connected to the SMBus. On older Linux distributions, decode-dimms.pl was available as part of lm_sensors.

• OpenBSD has included a driver (spdmem(4)) since version 4.3 to provide information about memory modules. The driver was ported from NetBSD, where it is available since release 5.0.

• Coreboot reads and uses SPD information to initialize all memory controllers in a computer with timing, size and other properties.

• On Windows systems, there are programs like HWiNFO32, CPU-Z and Speccy which are capable of reading and displaying DRAM module information from SPD.

Chipset-independent reading and writing of SPD information is done by accessing the memory's EEPROM directly with eeprom programmer hardware and software.

• A not so common use for old laptops is as generic SMBus readers, as the internal EEPROM on the module can be disabled once the BIOS has read it so the bus is essentially available for use.

The method used is to de-assert the /CE line so the internal memory shuts down, allowing the external device to access the SMBus. Once this is done, a custom Linux build or DOS application can then access the external device; a common use is recovering data from LCD panel memory chips in order to retrofit a generic panel into a proprietary laptop.