ARM ARM7TDMI User Manual Page 160

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Instruction Cycle Timings
6-18 Copyright © 1994-2001. All rights reserved. ARM DDI 0029G
6.11 Data swap
This is similar to the load and store register instructions, but the actual swap takes place
in the second and third cycles. In the second cycle, the data is fetched from external
memory. In the third cycle, the contents of the source register are written out to the
external memory. The data read in the second cycle is written into the destination
register during the fourth cycle.
LOCK is driven HIGH during the second and third cycles to indicate that both cycles
must be allowed to complete without interruption.
The data swapped can be a byte or word quantity. Halfword quantities cannot be
specified.
The swap operation can be aborted in either the read or write cycle, and in both cases
the destination register is not affected.
The cycle timings are listed in Table 6-14 where:
s represents the size of the data transfer shown by MAS[1:0] (see Table 6-10 on
page 6-13), s can only represent byte and word transfers. Halfword transfers are
not available.
Note
The data swap operation is not available in Thumb state.
Table 6-14 Data swap instruction cycle operations
Cycle Address MAS [1:0] nRW Data nMREQ SEQ nOPC LOCK
1 pc+8 2 0 (pc+8) 0 0 0 0
2Rn b/w 0(Rn)0 011
3 Rn b/w 1 Rm 1 0 1 1
4 pc+12 2 0 - 0 1 1 0
pc+12
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