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Chapter 3 EC™ Interface
3.5.1.1 No Merge
In No Merge mode writes to a different word within the same line are accumulated in the buffer. A burst write transaction
occurs when the buffer becomes full (4 words). Stores to the same word cause the previous word to be driven onto the
bus.
3.5.1.2 Full Merge
In Full Merge mode all combinations of writes to the buffer are allowed.
3.5.1.3 SysAD Valid Merge
In SysAD Valid Merge mode only valid SysAD byte enable patterns to be stored into the write buffer. When the byte
enable pattern of a write to the buffer is merged with the pattern for the previous write, the resulting pattern must be a
valid SysAD pattern in order for the merge to occur. If the resulting pattern is not a valid SysAD pattern the previous
write is driven onto the bus before the current write is written to the buffer. Table 3-3 shows the valid SysAD patterns.
Table 3-3 Valid SysAD Byte Enable Patterns
EB_BE[3:0]
0001
0010
0100
1000
0011
1100
0111
1110
1111
Table 3-4 shows a typical code sequence of three store byte instructions and the behavior of each merge mode.
Table 3-4 MergeMode Example
Instruction
EB_BE[3:0] (No Merge)
EB_BE[3:0] (Full Merge)
EB_BE[3:0]
(SysAD Merge)
SB 0
0001
merge
0001
SB 1
0010 (0001 written out)
merge (0011)
merge (0011)
SB 2
1000 (0010 written out)
merge (1011)
1011 = invalid merge pattern
0011 written out, then
1000 written to buffer
SYNC
flush buffer (1000 written out)
flush buffer (1011 written out)
flush buffer (1000 written out)
In Table 3-4 above, the following sequence occurs in No Merge mode:
• The SB 0 instruction causes a value of 0001 on EB_BE[3:0] to be written to the write buffer.
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MIPS32 4K™ Processor Core Family Integrator’s Manual, Revision 1.07