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Register Description
R
Intel
®
82845 MCH for SDR Datasheet 59
3.5.21 PAM[0:6]—Programmable Attribute Map Registers
(Device 0)
Address Offset: 90–96h (PAM0–PAM6)
Default Value: 00h
Attribute: R/W, RO
Size: 8 bits
The MCH allows programmable memory attributes on 13 Legacy memory segments of various
sizes in the 640 Kbytes to 1 Mbytes 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 processor. Two bits are used to specify memory attributes for each memory
segment. These bits apply to host initiator only access to the PAM areas. The MCH forwards to
system memory for any AGP, PCI or hub interface-initiated accesses to the PAM areas. These
attributes are:
RE - Read Enable. When RE = 1, the host read accesses to the corresponding memory
segment are claimed by the MCH and directed to 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 memory
segment are claimed by the MCH and directed to system memory. Conversely, when
WE = 0, the host write accesses are directed to PCI0.
The RE and WE attributes permit a memory segment to be Read Only, Write Only, Read/Write, or
disabled. For example, if a 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 four bits that control each region have the same encoding and defined in the following
table.
Bits [7, 3]
Reserved
Bits [6, 2]
Reserved
Bits [5, 1]
WE
Bits [4, 0]
RE
Description
X X 0 0 Disabled. System memory is disabled and all
accesses are directed to the hub interface. The MCH
does not respond as a PCI target for any read or write
access to this area.
X X 0 1 Read Only. Reads are forwarded to system memory
and writes are forwarded to the hub interface for
termination. This write protects the corresponding
memory segment. The MCH responds as an AGP or
hub interface target for read accesses but not for any
write accesses.
X X 1 0 Write Only. Writes are forwarded to system memory
and reads are forwarded to the hub interface for
termination. The MCH responds as an AGP or hub
interface target for write accesses but not for any read
accesses.
X X 1 1 Read/Write. This is the normal operating mode of
system memory. Both read and write cycles from the
host are claimed by the MCH and forwarded to
system memory. The MCH responds as an AGP or
hub interface target for both read and write accesses.