AM79C978AVCW datasheet(35/256 Pages) AMD

35 / 256 page

Am79C978A

Figure 6.

Slave Write Using Memory Command

Expansion ROM Transfers

The host must initialize the Expansion ROM Base Ad-

dress register at offset 30H in the PCI configuration

space with a valid address before enabling access to

the device. The Am79C978A controller will not react to

any access to the Expansion ROM until both MEMEN

(PCI Command register, bit 1) and ROMEN (PCI Ex-

pansion ROM Base Address register, bit 0) are set to 1.

After the Expansion ROM is enabled, the Am79C978A

controller will assert DEVSEL on all memory read ac-

cesses with an address between ROMBASE and

ROMBASE + 1M - 4. The Am79C978A controller

aliases all accesses to the Expansion ROM of the com-

mand types Memory Read Multiple and Memory Read

Line to the basic Memory Read command. Eight-bit,

16-bit, and 32-bit read transfers are supported.

Since setting MEMEN also enables memory mapped

access to the I/O resources, attention must be given the

PCI Memory Mapped I/O Base Address register before

enabling access to the Expansion ROM. The host must

set the PCI Memory Mapped I/O Base Address register

to a value that prevents the Am79C978A controller from

claiming any memory cycles not intended for it.

The Am79C978A controller will always read four bytes

for every host Expansion ROM read access. TRDY will

not be asserted until all four bytes are loaded into an in-

ternal scratch register. The cycle TRDY is asserted de-

pends on the programming of the Expansion ROM

interface timing. Figure 7 assumes that ROMTMG

(BCR18, bits 15-12) is at its default value.

Note: The Expansion ROM should be read only during

PCI configuration time for the PCI system.

When the host tries to write to the Expansion ROM, the

Am79C978A controller will claim the cycle by asserting

DEVSEL. TRDY will be asserted one clock cycle later.

The write operation will have no effect. Writes to the

Expansion ROM are done through the BCR30 Expan-

sion Bus Data Port. See the Expansion Bus Interface

section for more details.

During the boot procedure, the system will try to find an

Expansion ROM. A PCI system assumes that an Expan-

sion ROM is present when it reads the ROM signature

55H (byte 0) and AAH (byte 1).

Slave Cycle Termination

There are three scenarios besides normal completion

of a transaction where the Am79C978A controller is the

target of a slave cycle and it will terminate the access.

FRAME

CLK

IRDY

TRDY

C/BE

DEVSEL

STOP

PAR

ADDR

0111

PAR

2345678

DATA

PAR

22399A-9

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