JEDEC memory standards
The JEDEC memory standards are the specifications for semiconductor memory circuits and similar storage devices promulgated by the Joint Electron Device Engineering Council (JEDEC) Solid State Technology Association, a semiconductor trade and engineering standardization organization.
JEDEC Standard 100B.01 specifies common terms, units, and other definitions in use in the semiconductor industry. JESC21-C specifies semiconductor memories from the 256 bit static RAM to the latest DDR3 SDRAM modules. In August 2011, JEDEC announced that its DDR4 standard was expected to be published in mid-2012.
JEDEC standardization goals
JEDEC characterizes its standardization efforts as follows:
JEDEC standards and publications are designed to serve the public interest through eliminating misunderstandings between manufacturers and purchasers, facilitating interchangeability and improvement of products, and assisting the purchaser in selecting and obtaining with minimum delay the proper product for use by those other than JEDEC members, whether the standard is to be used either domestically or internationally.
JEDEC Standard 100B.01
JEDEC Standard 100B.01 is entitled Terms, Definitions, and Letter Symbols for Microcomputers, Microprocessors, and Memory Integrated Circuits. The purpose of the standard is to promote the uniform use of symbols, abbreviations, terms, and definitions throughout the semiconductor industry.
Units of information storage
The specification defines the two common units of information storage:
- 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.
Redefinition of some standard SI prefixes
The specification contains redefinitions of some commonly used SI prefixes: kilo, mega, and giga, but not tera or any larger prefix. The specification notes that these prefixes are included in the document "only to reflect common usage" (at the time of writing it). It refers to the IEEE/ASTM SI 10-1997 standard as stating, that "this practice frequently leads to confusion and is deprecated". The document notes that these prefixes are used in their decimal sense for serial communication data rates measured in bits.
The specification redefines these prefixes as follows:
- kilo (K): A multiplier equal to 1,024 (210).
- mega (M): A multiplier equal to 1048576 (220 or K2, where K = 1024).
- giga (G): A multiplier equal to 1073741824 (230 or K3, where K = 1024).
The document further refers the reader to the description of the IEC binary prefixes in Amendment 2 of IEC 60027-2, "Letter symbols to be used in electrical technology", for an alternate system of prefixes[notes 1] and includes a table of the IEC prefixes in the note. However the JEDEC specification does not explicitly include the IEC prefixes in the list of "General terms and definitions".
All JEDEC standards avoid the use of the terms megabit, megabyte and gigabyte and refer to memory capacity as a number followed by the units. (64Mb, 256MB, 1GB.)
The standard JESD21-C: Configurations for Solid State Memories is maintained by JEDEC committee JC41. This committee consists of members from manufacturers of microprocessors, memory ICs, memory modules, and other components, as well as component integrators, such as video card and personal computer makers. 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 and a reference design of the module. 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". For example:
1GB 2Rx4 PC2-3200P-333-11-D2 is a 1 GB 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.
- Quote from JEDEC Standard 100B.01, page 8:
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.
- EE Times. "Jedec readies DDR4 memory spec." August 22, 2011. Retrieved August 25, 2011.
- JEDEC Solid State Technology Association (December 2002). "Terms, Definitions, and Letter Symbols for Microcomputers, Microprocessors, and Memory Integrated Circuits". JESD 100B.01. p. 8. Retrieved 2009-04-05.
- JEDEC, Double Data Rate (DDR) SDRAM Specification (pdf), archived from the original on 2006-10-02, retrieved 2013-08-08
- JEDEC (2007), EP2-2100 DDR2 SDRAM 32b-SO-DIMM Reference Design Specification, retrieved 2009-04-05
- JEDEC, Bit Wide TTL SRAM (pdf), archived from the original on 2003-04-20, retrieved 2013-08-08
- JEDEC, Preliminary publication of JEDEC Semiconductor Memory Standard (pdf), archived from the original on 2007-09-26, retrieved 2013-08-08