
4-26
MC68330 USER'S MANUAL
MOTOROLA
U = Unaffected by reset
Supervisor Only
BA31–BA8 — Base Address Bits 31–8
The base address field, the upper 24 bits of each base address register, selects
the starting address for the chip select. The corresponding bits in AM31 – AM8 in
the address mask register define the size of the block specified by the chip select.
The base address field (and the function code field) is compared to the address
on the address bus to determine if a chip select should be generated.
FC3–FC0 — Function Code Bits 3–0
The value programmed in this field causes a chip select to be asserted for a
certain address space type. There are eight address spaces specified as either
user or supervisor, program or data, and CPU. These bits should be used to allow
access to one type of address space in the user program. If access to more than
one type of address space is desired, the function code mask bits should be used
in addition to the function code bits. To prevent access to CPU space, set the NCS
bit.
NOTE:
Since FC3 is not implemented in the MC68330, the
programmer must set FC3 to zero in this register.
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 allowed
0=Either read or write allowed
FTE — Fast-Termination Enable
This bit causes the cycle to terminate early with an internal
DSACKx, giving a fast
two-clock external access. When clear, all external cycles are at least three clocks.
If fast termination is enabled, the DD bits of the corresponding address mask
register are overridden (see Section 3 Bus Operation).
1=Fast-termination cycle enabled (termination determined by PS bits)
0=Fast-termination cycle disabled (termination determined by DD and PS bits)
NCS — No CPU Space
This bit specifies whether or not a chip select will assert on a CPU space access
cycle. If both supervisor data and program accesses are desired, while ignoring
CPU space accesses, then this bit should be set. The NCS bit is cleared at reset.
1=Suppress the chip select when accessing CPU space
0=Asserts the chip select on CPU space accesses
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.
1=Contents valid
0=Contents not valid