System information

4 Graphic Card Implementation
write to
wake up (interrupt)
PCI Target Control Request Buffer
BAR Switch
Driver
read from
signal end of processing
acknowledge
write to
write to
Time Window
Processing
VGA
Driver
send to
Figure 4.11: Timing of the access to the Request Buffer.
the location of the CHARM Register. It interfaces with the AHB Bus. The register entry
which is used to acknowledge the interrupt is called VGA_ACK_REGISTER. Table E lists
all entries of the CHARM Register.
VGA Interrupt and Acknowledge Dependency
The acknowledge and interrupt signals are used to synchronize the Request Buffer access.
Figure 4.12 illustrates the access rights to the Request Buffer. The state "PCI" symbolizes
that the PCI target control unit has exclusive access to the buffer. But the other two
states permits only the processing VGA software to access the buffer. The two values under
the state name represent the VGA interrupt line and the content of acknowledge register.
Furthermore, the edges are labeled with an "INT_REQ" and an "ACK" value. Thereby,
"INT_REQ" represents the wish of the PCI Target Control to activate the interrupt. The
"ACK" represents the content of the acknowledge register. The acknowledge register is set
by the BAR Switch driver. Hence, the VGA interrupt port is set or reset by the interrupt
signal of the target control unit and by the acknowledge register.
Summarily, the interrupt is set by the target control unit and reset by the BAR Switch
driver.
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