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MP Configuration Table
Version 1.4 4-17
4.4 Extended MP Configuration Table Entries
A variable number of variable-length entries are located in memory, immediately following entries
in the base section of the MP configuration table described in Section 4.3. These entries compose
the extended section of the MP configuration table. Each entry in the extended section of the table
has three elements:
• ENTRY TYPE
• ENTRY LENGTH
• DATA
ENTRY TYPE and ENTRY LENGTH make up a header that is present in all extended
configuration table entries. The ENTRY TYPE and ENTRY LENGTH fields are eight bit
unsigned values. ENTRY LENGTH specifies the total length of the given configuration entry. The
length of the DATA field is two bytes less than the value specified by ENTRY LENGTH.
This scheme ensures that operating systems can parse all the entries in the extended MP
configuration table area, even if an entry type is unrecognized. Operating systems that find an
unknown entry type when parsing this section of the table should ignore the content and move on
to the next entry, until the offset from the end of the base table reaches the length that is specified
in the EXTENDED TABLE LENGTH field of the configuration table header. The ability to skip
entries with unrecognized type codes beyond those listed in Table 4-13 is essential since it is
anticipated that more types of entries will be added to this list over time.
The total length of the Extended MP configuration table depends upon the configuration of the
system. The entries are sorted on ENTRY TYPE in ascending order. Table 4-13 gives the meaning
of each value of ENTRY TYPE.
Table 4-13. Extended MP Configuration Table Entry Types
Entry Description
Entry Type
Code*
Length
(in bytes) Comments
SYSTEM ADDRESS SPACE
MAPPING
128 20 Entry to declare system visible memory
or I/O space on a bus.
BUS HIERARCHY DESCRIPTOR 129 8 Entry to describe I/O bus interconnection.
COMPATIBILITY BUS ADDRESS
SPACE MODIFIER
130 8 Entry to describe predefined address
ranges that modify the memory or I/O
space visible on a bus in order to support
ISA compatibility.
*
All other type codes are reserved.