ARM R4F Computer Hardware User Manual


 
Level Two Interface
ARM DDI 0363E Copyright © 2009 ARM Limited. All rights reserved. 9-18
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Example 9-1 Write merging
MOV r0, #0x4000
STRH r1, [r0, #0x18]; Store a halfword at 0x4018
STR r2, [r0, #0xC] ; Store a word at 0x400C
STMIA r0, {r4-r7} ; Store four words at 0x4000
STRB r3, [r0, #0x1D]; Store a byte at 0x401D
If the memory at address
0x4000
is marked as Strongly Ordered or Device type memory, the AXI
transactions shown in Table 9-23 are generated.
In the example above, each store instruction produces an AXI burst of the same size as the data
written by the instruction.
Table 9-24 shows a possible resulting transaction if the same memory is marked as
Non-cacheable Normal, or Cacheable write-through.
In this example:
The store buffer has merged the
STRB
and
STRH
writes into one buffer entry, and therefore
a single AXI transfer, the fourth in the burst.
The writes, which occupy three buffer entries, have been merged into a single AXI burst
of four transfers.
The write generated by the
STR
instruction has not occurred, because it was overwritten by
the
STM
instruction.
The write transfers have occurred out of order with respect to the original program order.
Table 9-23 AXI transactions for Strongly Ordered or Device type memory
AWADDRM AWBURSTM AWSIZEM AWLENM WSTRBM
0x4018
Incr 16-bit 1 data transfer 0b00000011
0x400C
Incr 32-bit 1 data transfer 0b11110000
0x4000
Incr 32-bit 4 data transfers 0b00001111
0b11110000
0b00001111
0b11110000
0x401D
Incr 8-bit 1 data transfer 0b00100000
Table 9-24 AXI transactions for Non-cacheable Normal or Cacheable write-through
memory
AWADDRM AWBURSTM AWSIZEM AWLENM WSTRBM
0x4000
Incr 64-bit 4 data transfers 0b11111111
0b11111111
0b00000000
0b00100011