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Register Description
D15343-003 61
4.8.17 PAM(6:0) – Programmable Attribute Map Register (Device #0)
The GMCH allows programmable DDR SDRAM attributes on 13 Legacy system memory
segments of various sizes in the 640 kB -1 MB address range. Seven Programmable Attribute Map
(PAM) registers are used to support these features. Cacheability of these areas is controlled via the
MTRR registers in the P6 processor. Two bits are used to specify system memory attributes for
each system memory segment. These bits apply to both Host and Hub interface initiator accesses to
the PAM areas. These attributes are:
• RE - Read Enable. When RE = 1, the CPU Read accesses to the corresponding system
memory segment are claimed by the GMCH and directed to main system memory. Conversely,
when RE = 0, the Host Read accesses are directed to PCI0.
• WE - Write Enable. When WE = 1, the Host Write accesses to the corresponding system
memory segment are claimed by the GMCH and directed to main system memory. Conversely,
when WE = 0, the Host Write accesses are directed to PCI0.
The RE and WE attributes permit a system memory segment to be Read Only, Write Only, Read/
Write, or Disabled. For example, if a system memory segment has RE = 1 and WE = 0, the segment
is Read Only.
Each PAM register controls two regions, typically 16 kB in size. Each of these regions has a 4-bit
field. The 4 bits that control each region have the same encoding and are defined in the following
table.
Table 20. Attribute Bit Assignment
Address Offset:
Default Value:
Access:
Size:
59-5Fh
00h Each
Read/Write
4 bits/register, 14 registers
Bits [7, 3]
Reserved
Bits [6, 2]
Reserved
Bits [5, 1]
WE
Bits [4, 0]
RE
Description
XX00Disabled. DDR SDRAM is disabled and all accesses
are directed to Hub interface. The GMCH does not
respond as a Hub interface target for any Read or
Write access to this area.
XX01Read Only. Reads are forwarded to DDR SDRAM
and Writes are forwarded to Hub interface for
termination. This Write protects the corresponding
DDR SDRAM segment. The GMCH will respond as a
Hub interface target for Read accesses but not for
any Write accesses.
XX10Write Only. Writes are forwarded to DDR SDRAM
and Reads are forwarded to the Hub interface for
termination. The GMCH will respond as a Hub
interface target for Write accesses but not for any
Read accesses.
XX11Read/Write. This is the normal operating mode of
main system memory. Both Read and Write cycles
from the host are claimed by the GMCH and
forwarded to DDR SDRAM. The GMCH will respond
as a Hub interface target for both Read and Write
accesses.