System Integration Module
MOTOROLA
MC68307 USER’S MANUAL
5-31
not available externally on the MC68307, they are still available internally for comparison
purposes.
111 = Not supported; reserved. Chip select does not activate if this value is used
110 = Value may be used
000 = Value may be used
After system reset, the FCx field in BR3–BR0 defaults to supervisor program space
(FC=110) to select a ROM device containing the reset vector. Because of the priority
mechanism and the enable (EN) bit, only the CS0 line is active after a system reset.
NOTE
The FCx bits can be masked and ignored by the chip select logic
using the compare function code bit (CFC) in the option register
(ORx).
BA23–BA13—Base Address
These bits are used to set the starting address of a particular address space. The address
compare logic uses only A23–A13 to cause an address match within its block size.
After system reset, the base address defaults to zero to select a ROM device on which
the reset vector resides. All base address values default to zero on system reset, but, be-
cause of the priority mechanism, only CS0 is active.
NOTE
All address bits can be masked and ignored by the chip select
logic through the base address mask in the option register
(ORx).
RW—Read/Write
This bit, in conjunction with the mask read/write bit (MRW) in the option register (ORx),
allows a chip select to assert for only read cycles, only write cycles, or both types of cycle.
0 = The chip select line is asserted for read operations only.
1 = The chip select line is asserted for write operations only.
After system reset, this bit defaults to zero (read-only operation).
NOTE
This bit can be masked and ignored by the read-write compare
logic, as determined by the mask read/write bit (MRW) in the op-
tion register (ORx). The line is then asserted for both read and
write cycles.
On write protect violation cycles (RW=0 and MRW=1), a bus error exception (BERR) is
generated if the write protect violation enable bit (WPVE) in the SCR is set, and the WPV
status bit in the SCR is set so that the exception handler can determine the reason for Bus
Error.