MC68030 Datasheet, PDF (138/602 Pages) Motorola, Inc – ENHANCED 32-BIT MICROPROCESSOR

On-Chip Cache Memories
6.1.2.2 READ-MODIFY-WRITE ACCESSES. The read portion of a read-modify-write cycle
is always forced to miss in the data cache. However, if the system allows internal caching
of read-modify-write cycle operands (CIOUT and CIIN both negated), the processor either
uses the data read from memory to update a matching entry in the data cache or creates a
new entry with the read data in the case of no matching entry. The write portion of a read-
modify-write operation also updates a matching entry in the data cache. In the case of a
cache miss on the write, the allocation of a new cache entry for the data being written is
controlled by the WA bit. Table search accesses, however, are completely ignored by the
data cache; it is never updated for a table search access.
6.1.3 Cache Filling
The bus controller can load either cache in either of two ways:
• Single entry mode
• Burst fill mode
In the single entry mode, the bus controller loads a single long-word entry of a cache line.
In the burst fill mode, an entire line (four long words) can be filled. Refer to Section 7 Bus
Operation for detailed information about the bus cycles required for both modes.
6.1.3.1 SINGLE ENTRY MODE. When a cachable access is initiated and a burst mode
operation is not requested by the MC68030 or is not supported by external hardware, the
bus controller transfers a single long word for the corresponding cache entry. An entire long
word is required. If the port size of the responding device is smaller than 32 bits, the
MC68030 executes all bus cycles necessary to fill the long word.
When a device cannot supply its entire port width of data, regardless of the size of the
transfer, the responding device must consistently assert the cache inhibit input (CIIN) signal.
For example, a 32-bit port must always supply 32 bits, even for 8- and 16-bit transfers; a 16-
bit port must supply 16 bits, even for 8-bit transfers. The MC68030 assumes that a 32-bit
termination signal for the bus cycle indicates availability of 32 valid data bits, even if only 16
or 8 bits are requested. Similarly, the processor assumes that a 16-bit termination signal
indicates that all 16 bits are valid. If the device cannot supply its full port width of data, it must
assert CIIN for all bus cycles corresponding to a cache entry.
MOTOROLA
MC68030 USER’S MANUAL
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