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PCI Functional Description
PCI Addressing
SYM53C825A/53C825AE 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 SYM53C825A will not respond.
According to the PCI specification, AD(10-8) are
to be used for multifunction devices. T he host pro-
cessor uses the PCI configuration space to initial-
ize the SYM53C825A.
T he lower 128 bytes of the SYM53C825A config-
uration space holds system parameters while the
upper 128 bytes map into the SYM53C825A
operating registers. For all PCI cycles except con-
figuration cycles, the SYM53C825A registers are
located on the 256-byte block boundary defined by
the base address assigned through the configured
register. T he SYM53C825A 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 SYM53C825A, the
upper 24 bits of the address are selected) for mem-
ory accesses and I/O accesses. On every access, the
SYM53C825A 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
SYM53C825A 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 SYM53C825A. 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 SYM53C825A. Base Address Regis-
ter 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.