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Developer’s Manual March, 2003 10-7
Intel
®
80200 Processor based on Intel
®
XScale
™
Microarchitecture
External Bus
10.2.3 Critical Word First
The CWF signal is only used during read bursts of eight words (Len = 6). CWF needs to be driven
at the same time as DValid of the first data cycle of the transaction. This bit indicates to the
requesting master what order the data is returning in. The Intel
®
80200 processor uses this sort of
transaction to fill a cache line.
There are two acceptable return orders that may be signalled with CWF:
• (CWF = 0) starting with the words 0 and 1 (64-bit bus) or word 0 (32-bit bus) and sequentially
returning the data
• (CWF = 1) returning the word or pair of words that contain the byte indicated by the request
address on the first cycle, sequentially returning the next highest memory location word or pair
of words until the end of the eight word aligned block, then returning the lowest word or pair
of words and incrementing back up the word or pair of words just below the word or pair of
words pointed to by the request address.
The allowable orders are spelled out below. In this description, A,B,C, and D refer to pairs of 32-bit
words in the aligned 8-word block (A=0,1 B=2,3 C=4,5 D=6,7). In the 32-bit bus section, a-h are
referring to the eight words in the aligned 8-word block. Note that the Intel
®
80200 processor will
align all 8-word burst transactions on a 64-bit address. Specifically, if the instruction or data
address requested by the program is odd word aligned, the Intel
®
80200 processor will emit a
64-bit aligned address by setting A[2] = 0x0.
1. The Intel® 80200 processor will never generate an 8-word burst with A[2] = 0x1. The addresses for these requests will be 64-bit
aligned.
The timing of write transactions on the data bus is similar to the timing of reads. After a write
request has gone out, two cycles before the chipset or memory is ready to receive the data from the
Intel
®
80200 processor the DValid is asserted. Two cycles later the Intel
®
80200 processor drives
the data onto the data bus. For a write whose data all fits within one aligned 64-bit memory block
(for a 64-bit bus) or a 32-bit aligned memory block (for a 32-bit bus) there is a single data cycle.
For a burst write, there can be up to two (64-bit bus) or four (32-bit bus) data cycles, for a
maximum write burst of four 32-bit words.
Table 10-4. Return Order for 8-Word Burst, 64-bit Data Bus
CWF A[4:3] Return Order
0XXABCD
100ABCD
1 01 BCDA
1 10 CDAB
111DABC
Table 10-5. Return Order for 8-Word Burst, 32-bit Data Bus
CWF A[4:2]
1
Return Order
0 XXX abcdefgh
1 000 abcdefgh
1 010 cdefghab
1 100 efghabcd
1 110 ghabcdef