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MOTOROLA M68040 USER’S MANUAL 4- 11
4.6 MEMORY ACCESSES FOR CACHE MAINTENANCE
The cache controller in each memory unit performs all maintenance activities that supply
data from the cache to the execution units. The activities include requesting accesses to
the bus interface unit for reading new cache lines and writing dirty cache lines to memory.
The following paragraphs describe the memory accesses resulting from cache fill
operations (by both caches) and push operations (by the data cache). Refer to Section 7
Bus Operation for detailed information about the bus cycles required.
4.6.1 Cache Filling
When a new cache line is required, the cache controller requests a line read from the bus
controller. The bus controller requests a burst read transfer by indicating a line access
with the size signals (SIZ1, SIZ0) and indicates which line in the set is being loaded with
the transfer line number signals (TLN1, TLN0). TLN1 and TLN0 are undefined for the
instruction cache. These pins indicate the appropriate line numbers for data cache
transfers only. Table 4-2 lists the definition of the TLNx encoding.
Table 4-2. TLNx Encoding
TLN1 TLN0 Line
0 0 Zero
0 1 One
1 0 Two
1 1 Three
The responding device sequentially supplies four long words of data and can assert the
transfer cache inhibit signal (TCI) if the line is not cachable. If the responding device does
not support the burst mode, it should assert the TBI signal for the first long word of the line
access. The bus controller responds by terminating the line access and completes the
remainder of the line read as three, sequential, long-word reads.
Bus controller line accesses implicitly request burst mode operations from external
memory. To operate in the burst mode, the device or external hardware must be able to
increment the low-order address bits as described in Section 7 Bus Operation. The
device indicates its ability to support the burst access by acknowledging the initial long-
word transfer with transfer acknowledge (TA) asserted and TBI negated. This procedure
causes the processor to continue to drive the address and bus control signals and to latch
a new data value for the cache line at the completion of each subsequent cycle (as
defined by TA) for a total of four cycles. The bursting mechanism requires addresses to
wrap around so that the entire four long words in the cache line are filled in a single
operation.
When a cache line read is initiated, the first cycle attempts to load the line entry
corresponding to the instruction half-line or data item requested by the IU. Subsequent
transfers are for the remaining entries in the cache line. In the case of a misaligned
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