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56 SPARC JPS1 Implementation Supplement: Fujitsu SPARC64 V • Release 1.0, 1 July 2002
Description
The memory barrier instruction, MEMBAR, has two complementary functions: to
express order constraints between memory references and to provide explicit control
of memory-reference completion. The membar_mask field in the suggested assembly
language is the concatenation of the cmask and mmask instruction fields.
The mmask field is encoded in bits 3 through 0 of the instruction.
TABLE A-5
specifies
the order constraint that each bit of mmask (selected when set to 1) imposes on
memory references appearing before and after the MEMBAR. From zero to four mask
bits can be selected in the mmask field.
The cmask field is encoded in bits 6 through 4 of the instruction. Bits in the cmask
field, described in
TABLE A-6
, specify additional constraints on the order of memory
references and the processing of instructions. If cmask is zero, then MEMBAR enforces
the partial ordering specified by the mmask field; if cmask is nonzero, then
completion and partial order constraints are applied.
TABLE A-5
Order Constraints Imposed by mmask Bits
Mask Bit Name Description
mmask<3> #StoreStore The effects of all stores appearing before the MEMBAR instruction must be
visible to all processors before the effect of any stores following the MEMBAR.
Equivalent to the deprecated STBAR instruction. Has no effect on SPARC64V
since all stores are performed in program order.
mmask<2> #LoadStore All loads appearing before the MEMBAR instruction must have been performed
before the effects of any stores following the MEMBAR are visible to any other
processor. Has no effect on SPARC64V since all stores are performed in
program order and must occur after performance of any load.
mmask<1> #StoreLoad The effects of all stores appearing before the MEMBAR instruction must be
visible to all processors before loads following the MEMBAR may be performed.
mmask<0> #LoadLoad All loads appearing before the MEMBAR instruction must have been performed
before any loads following the MEMBAR may be performed. Has no effect on
SPARC64 V since all loads are performed after any prior loads.
TABLE A-6
Bits in the cmask Field
Mask Bit Function Name Description
cmask<2> Synchronization
barrier
#Sync All operations (including nonmemory reference operations)
appearing before the MEMBAR must have been performed, and
the effects of any exceptions become visible before any
instruction after the MEMBAR may be initiated.
cmask<1> Memory issue
barrier
#MemIssue All memory reference operations appearing before the MEMBAR
must have been performed before any memory operation after
the MEMBAR may be initiated. Equivalent to #Sync in
SPARC64 V.
cmask<0> Lookaside
barrier
#Lookaside A store appearing before the MEMBAR must complete before
any load following the MEMBAR referencing the same address
can be initiated. Equivalent to #Sync in SPARC64 V.