![](https://csdnimg.cn/release/download_crawler_static/57546992/bg5.jpg)
JEDEC Standard No. 21-C, Release 30
Page 4.1.2.L-6 – 5
5 Parsing the SPD (Cont’d)
The SPD revision stored in Byte 1 applies to all information for the module, including base information, module specific information,
and hybrid information. Each SPD revision exactly defines how many bytes are valid in all other SPD blocks. The number of
supported bytes (or bits) may increase from one SPD revision to another within each block as indicated by the “additions” nibble of
the SPD revision. For example, an SPD revision 1.3 has more bytes or bits defined than SPD revision 1.2.
This progression of SPD contents is important for the BIOS to DIMM compatibility model. An older system may have a BIOS that
only understands SPD revision 1.2 encoding, so if a module is installed that contains revision 1.3 information, the system can
accurately interpret all of the historical revision 1.2 information retained in the module that is the subset of the revision 1.3
specification. Similarly, if a module with SPD revision 1.1 information is installed, that same BIOS can interpret all information that
was current at the time that SPD 1.1 was defined. Therefore, BIOSes must maintain a knowledge of the active information for each
historical SPD revision in order to support older modules.
Step 3: Parse Byte 3 - Determine module type and appropriate overlays.
Key byte 3 for module type determines the subsequent use of overlay information. Byte 3 bits 3~0 define the host to module interface
style: unbuffered, registered, or load reduced. Standard module types are defined in Section 9.
Key byte 3 also determines the presence of hybrid module types. Byte 3, bit 7 asserts if the module is hybrid, and Byte 3, bits 6~4
define the type of hybrid module. A hybrid module may appear to the system as a superset of any of the base module types. For
example, an NVDIMM may present a UDIMM, RDIMM, or LRDIMM compatible interface to the system. Because of this unique
second overlay for hybrid information on top of the base module type, the SPD revision for Hybrid modules must include all relevant
information for any of the base module types if applicable. Refer to SPD revision progression in Section 6 for more information.
If Byte 3, bits 7~4 indicate the presence of a Hybrid module type, the BIOS must parse two more overlays. Section 10, Hybrid
Module Parameters (Bytes 192~255) and Section 11, Hybrid Modules Extended Function Parameters (Bytes 256~319). As with the
module specific parameters, both of the Hybrid Module parameter sets may increase from SPD revision to revision.
Step 4: Parse Bytes 0-127 - Make base configuration settings to memory interface based on these bytes.
All module types are required to read and interpret this block of data to set up the DRAM type, maximum operating frequency, the
number of row, column, bank bits, write recovery time, etc. While these bytes primarily describe the timing of the DRAMs, timing
represents the capabilities of the module and it may be necessary to downgrade the timing based on other factors including layout or
support components on the module, such as registers.
Step 5: Parse Bytes 128-191 - Configure Standard module memory interface.
These bytes will be referenced and used by the system as needed based on the Standard module type that was determined after Byte 3
was parsed as indicated in Step 2. Bytes 128~191 are coded differently for each standard module type as defined in Section 9.
Step 6: Parse Bytes 192-255 (If Hybrid) - Read Hybrid Module parameters settings.
These bytes will be referenced and used by the system as needed based on the Hybrid module type that was determined after Byte 3
was parsed as indicated in Step 2. Function Format Interface Descriptors defined in bytes 204~205 through 218~219 will be read and
utilized as indicated in Step 7. Bytes 192~255 are coded differently for each Hybrid module type and defined in Section 10.
Step 7: Parse Bytes 256-319 (If Hybrid Extended Functions are specified).
These bytes will be referenced and used by the system as needed based on the Function Format interface descriptors read previously
during Step 6 from SPD bytes 204~205 through 218~219. Support up to 8 functions per Hybrid module can be defined within Bytes
256~319 and are coded based on Section 11.
For NVDIMM hybrid modules, parsing the two data blocks of hybrid memory parameters in SPD bytes 192~255 and the extended
function parameters in SPD bytes 256~319 is intimately related. Each function descriptor contains a 4-bit field in bits 13~10 that
creates an address pointer into the extended function parameter block. The formula is: 256 + 4 * function descriptor bits 13~10.
Downloaded by xiao yangyang (1191339283@qq.com) on Dec 6, 2021, 5:30 pm PST
teahc
评论0
最新资源