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MC68349 USER'S MANUAL
MOTOROLA
register define the size of the block for the chip select. The base address field (and the
base function code field) is compared to the address on the address bus to determine if
a chip select should be generated.
BFC3–BFC0—Base Function Code Bits 3–0
The value programmed into this field causes a chip select to be asserted for a certain
address space type. There are nine function code address spaces (see Section 3 Bus
Operation) specified as either user or supervisor, program or data, CPU, and DMA.
These bits should be used to allow access to one type of address space. If access to
more than one type of address space is desired, the FCMx bits should be used in
addition to the BFCx bits. To prevent access to CPU space, set the NCS bit.
WP—Write Protect
This bit can restrict write accesses to the address range in a base address register. An
attempt to write to the range of addresses specified in a base address register that has
this bit set returns BERR .
1 = Only read accesses are allowed.
0 = Either read or write accesses are allowed.
EDS—Extended Delay Select
This bit is used along with the DDx bits of the corresponding address mask register to
specify the number (zero to six) of wait sates or a fast termination (two-clock external
access). See table 4-11 for the encodings of the EDS bit and DDx bits.
NCS—No CPU Space
This bit specifies whether or not a chip select will assert on a CPU space access cycle
(FC3–FC0 = $7 or $F). If both supervisor data and program accesses are desired, while
ignoring CPU space accesses, then this bit should be set.
1 = Suppress the chip select on a CPU space access.
0 = Assert the chip select on a CPU space access.
V—Valid Bit
This bit indicates that the contents of its base address register and address mask
register pair are valid. The programmed chip selects do not assert until the V-bit is set.
A reset clears the V-bit in each base address register, but does not change any other
bits in the base address and address mask registers.
1 = Contents are valid.
0 = Contents are not valid.