118
Am79C973/Am79C975
P R E L I M I N A R Y
will drive DEVSEL indicating it
will respond to the access.
IOBASE is read and written by
the host. IOBASE is cleared by
H_RESET and is not affected by
S_RESET or by setting the STOP
bit.
4-2
IOSIZE
I/O size requirements. Read as
zeros; write operations have no
effect.
IOSIZE indicates the size of the
I/O
space
the
Am79C975 controller requires.
When the host writes a value of
FFFF FFFFh to the I/O Base Ad-
dress register, it will read back a
value of 0 in bits 4-2. That indi-
cates an Am79C973/Am79C975
I/O space requirement of 32
bytes.
Am79C973/
1
RES
Reserved location. Read as zero;
write operations have no effect.
0
IOSPACE
I/O space indicator. Read as one;
write operations have no effect.
Indicating that this base address
register describes an I/O base
address.
PCI Memory Mapped I/O Base Address Register
Offset 14h
The PCI Memory Mapped I/O Base Address register is
a 32-bit register that determines the location of the
Am79C973/Am79C975 I/O resources in all of memory
space. It is located at offset 14h in the PCI Configuration
Space.
Bit
Name
Description
31-5
MEMBASE Memory mapped I/O base ad-
dress most significant 27 bits.
These bits are written by the host
to specify the location of the
Am79C973/Am79C975 I/O re-
sources in all of memory space.
MEMBASE must be written with a
valid
address
Am79C973/Am79C975 controller
slave memory mapped I/O mode
is turned on by setting the ME-
MEN bit (PCI Command register,
bit 1).
before
the
When
Am79C975 controller is enabled
for memory mapped I/O mode
(MEMEN is set), it monitors the
PCI bus for a valid memory com-
mand. If the value on AD[31:5]
during the address phase of the
cycles matches the value of
MEMBASE,
the
Am79C975 controller will drive
DEVSEL indicating it will respond
to the access.
the
Am79C973/
Am79C973/
MEMBASE is read and written by
the host. MEMBASE is cleared
by H_RESET and is not affected
by S_RESET or by setting the
STOP bit.
4
MEMSIZE
Memory mapped I/O size re-
quirements. Read as zeros; write
operations have no effect.
MEMSIZE indicates the size of
the
memory
Am79C973/Am79C975 controller
requires. When the host writes a
value of FFFF FFFFh to the
Memory Mapped I/O Base Ad-
dress register, it will read back a
value of 0 in bit 4. That indicates
a Am79C973/Am79C975 memo-
ry space requirement of 32 bytes.
space
the
3
PREFETCH Prefetchable. Read as zero; write
operations have no effect. Indi-
cates that memory space con-
trolled by this base address
register is not prefetchable. Data
in the memory mapped I/O space
cannot be prefetched. Because
one of the I/O resources in this
address space is a Reset regis-
ter, the order of the read access-
es is important.
2-1
TYPE
Memory type indicator. Read as
zeros; write operations have no
effect. Indicates that this base ad-
dress register is 32 bits wide and
mapping can be done anywhere
in the 32-bit memory space.
0
MEMSPACE
Memory space indicator. Read
as zero; write operations have no
effect. Indicates that this base ad-
dress register describes a memo-
ry base address.