JEDEC memory standards

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The JEDEC Solid State Technology Association is the semiconductor engineering standardization body of the Electronic Industries Alliance (EIA). Semiconductor memory is a very active area of standardization today. JEDEC Standard 21 specifies semiconductor memories from the 256 bit static RAM (".25Kb") [1] to the latest DDR3 SDRAM modules

This standard, JESD21-C "Configurations for Solid State Memories", is maintained by JEDEC committee JC41. The members are from companies that make microprocessors, memory ICs, memory modules, and other components. The members also come from companies that design these components into systems such as video cards and personal computers. There are additional committees for other aspects of memory standards.

Most JEDEC standards are published as a complete document. When a revision is required the document is republished. Additions to Standard 21 are so frequent that it is published in loose-leaf format and comes in a three-ring binder.

In the 1980s the configuration of a family of memories could be specified on a single page. Modern memory modules require over 100 pages; standards for the memory IC [2] and a reference design of the module.[3] The standards specify the physical dimensions for the module, the electrical characteristics for the module and even the data for doing computer simulations of the memory module operating in a system.

A memory module like the DDR2-SDRAM is available for laptop, desktop, and server computers. There is also a wide selection of memory capacities and speeds. The standards specify memory module label formats for "End User Markets".[4]For example:

1GB 2Rx4 PC2-3200P-333-11-D2 is a 1GB DDR2 Registered DIMM, with address/command parity function, using 2 ranks of x4 SDRAMs operational to PC2-3200 performance with CAS Latency = 3, tRCD = 3, tRP = 3, using JEDEC SPD revision 1.1, raw card reference design file D revision 2 used for the assembly.

JEDEC Standard 100B.01^[1] defines the "prefix to units of semiconductor storage capacity" as follows:

kilo (K): A multiplier equal to 1024 (2¹⁰).
mega (M): A multiplier equal to 1 048 576 (2²⁰ or K², where K = 1024).
giga (G): A multiplier equal to 1 073 741 824 (2³⁰ or K³, where K = 1024).

It notes that these prefixes are used in their decimal sense for serial communication data rates measured in bits:

Contrast with the SI prefix kilo (k) equal to 10³, as in a 1 kbit/s data transfer rate, which is equal to 1000 bits per second.

It also defines:

bit (b): In the binary numeration system, either of the digits 0 or 1. (Ref. ANSI X3.172.)
byte (B): A binary character string operated upon as a unit and usually shorter than a computer word. (Ref. ANSI X3.172.) NOTE A byte is usually eight bits.

The rationale for including these definitions is explained and contrasted to the IEC standard prefixes by this footnote in the standard:

The definitions of kilo, giga, and mega based on powers of two are included only to reflect common usage. IEEE/ASTM SI 10-1997 states "This practice frequently leads to confusion and is deprecated." Further confusion results from the popular use of a "megabyte" consisting of 1 024 000 bytes to define the capacity of the familiar "1.44-MB" diskette. An alternative system is found in Amendment 2 to IEC 60027-2: Letter symbols to be used in electrical technology – Part 2

All JEDEC standards avoid the use of the terms megabit, megabyte, gigabyte, etc, and refer to memory capacity as a number followed by the units. (64Mb, 256MB or 1GB.)

The IEC uses the prefix "kibi-" to mean 1024, similarly "mebi-", "gibi-" and "tebi-" for its powers, noting further: "IEC suggests that, in English, the first syllable of the name of the binary-multiplier prefix should be pronounced in the same way as the first syllable of the name of the corresponding SI prefix and that the second syllable should be pronounced as 'bee'."