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PCI Functional Description
PCI Addressing
SYM53C875/875E Data Manual
3-1
Chapter 3
PCI Functional Description
PCI Addressing
T here are three types of PCI-defined address
space:
I
Configuration space
I
Memory space
I
I/O space
Configuration space is a contiguous 256 x 8-bit set
of addresses dedicated to each “slot” or “stub” on
the bus. Decoding C_BE/(3-0) determines if a PCI
cycle is intended to access configuration register
space. T he IDSEL bus signal is a “chip select” that
allows access to the configuration register space
only. A configuration read/write cycle without
IDSEL will be ignored. T he eight lower order
addresses are used to select a specific 8-bit register.
AD(10-8) are decoded as well, but they must be
zero or the SYM53C875 will not respond. Accord-
ing to the PCI specification, AD(10-8) are to be
used for multifunction devices. T he host processor
uses the PCI configuration space to initialize the
SYM53C875.
T he lower 128 bytes of the SYM53C875 configu-
ration space holds system parameters while the
upper 128 bytes map into the SYM53C875 oper-
ating registers. For all PCI cycles except configura-
tion cycles, the SYM53C875 registers are located
on the 256-byte block boundary defined by the
base address assigned through the configured reg-
ister. T he SYM53C875 operating registers are
available in both the upper and lower 128-byte
portions of the 256-byte space selected.
At initialization time, each PCI device is assigned a
base address (in the case of the SYM53C875, the
upper 24 bits of the address are selected) for mem-
ory accesses and I/O accesses. On every access, the
SYM53C875 compares its assigned base addresses
with the value on the Address/Data bus during the
PCI address phase. If there is a match of the upper
24 bits, the access is for the SYM53C875 and the
low order eight bits define the register to be
accessed. A decode of C_BE/ (3-0) determines
which registers and what type of access is to be
performed.
PCI defines memory space as a contiguous 32-bit
memory address that is shared by all system
resources, including the SYM53C875. Base
Address Register One determines which 256-byte
memory area this device will occupy.
PCI defines I/O space as a contiguous 32-bit I/O
address that is shared by all system resources,
including the SYM53C875. Base Address Register
Zero determines which 256-byte I/O area this
device will occupy.
PCI Bus Commands and
Functions Supported
Bus commands indicate to the target the type of
transaction the master is requesting. Bus com-
mands are encoded on the C_BE/(3-0) lines dur-
ing the address phase. PCI bus command
encoding and types appear in Table 3-1.
T he I/O Read command is used to read data from
an agent mapped in I/O address space. All 32
address bits are decoded.
T he I/O Write command is used to write data to an
agent when mapped in I/O address space. All 32
address bits are decoded.