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Prestigio Nobile 150
5.2 Intel 82855GME Graphics and Memory Controller Hub (GMCH)
AGP Status Signal Descriptions
Signal Name
Type
O
Status: Provides
GST[2:0]
AGP
information from
the arbiter to an
AGP Master on
what it may do.
ST[2:0] only
have meaning
to the master
when its GNT# is
asserted. When
GNT# is
deasserted
these signals have
no meaning and
must be gnored.
Description
ST[2:0
Meaning
000
Previously requested low priority
read data is being returned to the
master arbiter to an AGP
001
Previously requested high priority
read data is being returned to the
master
010
The master is to provide low priority
write data for a previously queued
write command
011
The master is to provide high
priority write data for a previously
queued write command.
100
Reserved
101
Reserved
110
Reserved
111
The master has been given
permission to start a bus transaction.
The master may queue AGP requests
by asserting PIPE# or start a PCI
transaction by asserting FRAME#
AGP Flow Control Signals
Signal Name
Type
I
Read Buffer Full: Read buffer full indicates if the master is ready to
GRBF#
AGP
accept previously requested low priority read data. When RBF# is
asserted the GMCH is not allowed to initiate the return low priority
read data. That is, the GMCH can finish
returning the data for the request currently being serviced. RBF# is
only sampled at the beginning of a cycle.
If the AGP master is always ready to accept return read data then it is
not required to implement this signal.
During FRAME# Operation: This signal is not used during AGP
FRAME# operation.
I
Write-Buffer Full: indicates if the master is ready to accept Fast
GWBF#
AGP
Write data from the GMCH. When WBF# is asserted the GMCH is
not allowed to drive Fast Write data to the AGP master. WBF# is
only sampled at the beginning of a cycle.
If the AGP master is always ready to accept fast write data then it is
not required to implement this signal.
During FRAME# Operation: This signal is not used during AGP
FRAME# operation.
89
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