MOTOROLA
Chapter 7. PCI Bus Interface
7-27
However, if the PCI-to-ISA bridge runs a memory transaction that does not use a full 32-bit
address, access to system memory is impossible. Assertion of ISA_MASTER indicates that
an ISA master is requesting access to system memory.
The ISA_MASTER signal should be asserted coincident with the PCI-to-ISA bridge
receiving a PCI bus grant. When the MPC106 detects ISA_MASTER asserted during the
address phase, the MPC106 automatically asserts DEVSEL to claim the transaction
regardless of the address in AD[31–0]. Due to the automatic assertion of DEVSEL when
ISA_MASTER is detected, bus contention may occur if the current transaction is not truly
intended for the MPC106 (system memory access).
If the PCI-to-ISA bridge can generate a full 32-bit address, the ISA_MASTER signal is
unnecessary and may be tied to V
DD
(high).
7.7.2 FLSHREQ and MEMACK
The FLSHREQ signal allows a PCI agent to request that the MPC106 flush its internal
buffers. The MEMACK allows the MPC106 to acknowledge that it has flushed its internal
buffers.
If a master on the PCI bus asserts FLSHREQ, the MPC106 stops accepting new
transactions from the 60x bus (except snoop copy-back operations), completes all
outstanding transactions, and then asserts MEMACK. The MPC106 holds MEMACK
asserted until two cycles after the master negates FLSHREQ. When FLSHREQ is negated,
the master must wait until after MEMACK is negated before it can reassert FLSHREQ.
7.8 Emulation Support
When the ESCR1[EMULATION_MODE_EN] bit is set, the MPC106 operates in an
emulation mode that is fully compliant with the PC emulation option described in the
PowerPC Microprocessor Common Hardware Reference Platform: A System Architecture.
The features described in the following sections are provided by the MPC106 when
operating in PC emulation mode.
7.8.1 PCI Address Decoding
When the MPC106 is operating in PC emulation mode, the MPC106 compares the
addresses of PCI memory transactions to the ESCR1[TOP_OF_MEM] bits, and
simultaneously tests whether the address falls within the PCI compatibility hole
(configured by setting ESCR1[PCI_COMPATIBILITY_HOLE] while in PC emulation
mode). If the address is not at the top of memory and does not fall within the PCI
compatibility hole, the MPC106 claims the transaction by asserting DEVSEL. If the
address is above the block address in ESCR1[TOP_OF_MEM], or if the address falls inside
the PCI compatibility hole (with ESCR1[PCI_COMPATIBILITY_HOLE] set to 1) the
MPC106 ignores the PCI transaction.