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
The Joint Electron Device Engineering Council 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
The specification defines the two common units of information:
- The bit (b) is the smallest unit of information in the binary numeration system and is represented by the digits 0 and 1.
- The byte (B) is a binary character string typically operated upon as one unit. It is usually shorter than a computer word.
Unit prefixes for semiconductor storage capacity
The specification cites three prefixes as follows:
- kilo (K): A multiplier equal to (210). 1024
- 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 specification notes that these prefixes are included in the document only to reflect common usage. It refers to the IEEE/ASTM SI 10-1997 standard as stating, that "this practice frequently leads to confusion and is deprecated". However the JEDEC specification does not explicitly deprecate the common usage. The document further refers 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.
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. Standard 21is published in loose-leaf binder format to accommodate frequent updates.
In the 1980s the configuration and specifications of a family of memories could be specified on a single page. The documentation of modern memory modules, such as the standards for the memory ICs and a reference design of the module requires over one hundred pages. The standards specify the physical and electrical characteristics of the modules, and include the data for computer simulations of the memory module operating in a system.
A memory modules of the type DDR2-SDRAM is available for laptop, desktop, and server computers in a wide selection of capacities and access 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.
The JEDEC terms dictionary includes definitions for prefixes kibi (Ki), mebi (Mi), gibi (Gi) and tebi (Ti) as powers of 2, and kilo, mega, giga and tera as powers of 10. For example,
- 240 tebi Ti tera + binary: (210)4 = 1099511627776 tera: (103)4
The JEDEC DDR3 SDRAM standard JESD-79-3d uses Mb and Gb to specify binary memory capacity: "The purpose of this Standard is to define the minimum set of requirements for JEDEC compliant 512 Mb through 8 Gb for x4, x8, and x16 DDR3 SDRAM devices."
- 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 the megabyte representing 1 024 000 bytes to define the capacity of the 1.44-MB high-density 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" (PDF). JESD 100B.01. p. 8. Retrieved 2009-04-05.
- Ref. ANSI X3.172.
- JEDEC, Double Data Rate (DDR) SDRAM Specification (PDF), archived from the original (pdf) on 2006-10-02, retrieved 2013-08-08
- JEDEC (2007), EP2-2100 DDR2 SDRAM 32b-SO-DIMM Reference Design Specification (PDF), retrieved 2009-04-05
- JEDEC, Bit Wide TTL SRAM (PDF), archived from the original (pdf) on 2003-04-20, retrieved 2013-08-08
- JEDEC, Preliminary publication of JEDEC Semiconductor Memory Standard (PDF), archived from the original (pdf) on 2007-09-26, retrieved 2013-08-08
- JEDEC dictionary entry