
42
Am79C978
Figure 12.
Slave Cycle Data Parity Error Response
Master Bus Interface Unit
The master Bus Interface Unit (BIU) controls the acqui-
sition of the PCI bus and all accesses to the initializa-
tion block, descriptor rings, and the receive and
transmit buffer memory. Table 7 shows the usage of
PCI commands by the Am79C978 controller in master
mode.
Bus Acquisition
The microcode will determine when a DMA transfer
should be initiated. The first step in any bus master
transfer is to acquire ownership of the bus. This task is
handled by synchronous logic within the BIU. Bus own-
ership is requested with the REQ signal and ownership
is granted by the arbiter through the GNT signal.
Figure 13 shows the Am79C978 controller bus acquisi-
tion. REQ is asserted and the arbiter returns GNT while
another bus master is transferring data. The
Am79C978 controller waits until the bus is idle
(FRAME and IRDY deasserted) before it starts driving
AD[31:0] and C/BE[3:0] on clock 5. FRAME is asserted
at clock 5 indicating a valid address and command on
AD[31:0] and C/BE[3:0]. The Am79C978 controller
does not use address stepping which is reflected by
FRAME
CLK
AD
IRDY
TRDY
C/
BE
DEVSEL
PAR
ADDR
CMD
PAR
1
2
3
4
5
6
7
8
10
9
DATA
PAR
BE
PERR
22206B-15
Table 7.
Master Commands
C[3:0]
Command
Interrupt
Acknowledge
Special Cycle
I/O Read
I/O Write
Reserved
Reserved
Use
0000
Not used
0001
0010
0011
0100
0101
Not used
Not used
Not used
0110
Memory Read
Read of the initialization
block and descriptor
rings
Read of the transmit
buffer in non-burst mode
Write to the descriptor
rings and to the receive
buffer
0111
Memory Write
1000
1001
Reserved
Reserved
1010
1011
Configuration Read
Configuration Write
Memory Read
Multiple
Dual Address Cycle Not used
Not used
Not used
Read of the transmit
buffer in burst mode
1100
1101
1110
Memory Read Line
Read of the transmit
buffer in burst mode
1111
Memory Write
Invalidate
Not used
Table 7.
Master Commands (Continued)