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March 2002 Release
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Virtex-II Pro™ Platform FPGA Documentation
Appendix C:
Simplified Mnemonics
R
If the likely outcome (branch or fall through) of a conditional-branch instruction is known,
a suffix can be added to the mnemonic that tells the assembler how to set the
y
bit, as
follows:
•
+
indicates that the branch should be predicted taken.
•
−
indicates that the branch should be predicted not taken.
The suffix can be added to any branch-conditional mnemonic, including simplified
mnemonics. For example, “
blt+
target” indicates the
branch to target if CR0 is less than
instruction should be predicted taken.
For relative and absolute branches, the default value of the
y
bit depends on whether the
displacement field is negative or non-negative. With these instructions, the prediction
override has the following effect:
•
For negative displacement fields:
-
A “
+
” suffix clears the
y
bit to 0.
-
A “
−
” suffix sets the
y
bit to 1.
•
For non-negative displacement fields:
-
A “
+
” suffix sets the
y
bit to 1.
-
A “
−
” suffix clears the
y
bit to 0.
For branches to an address in the LR or CTR, the prediction override has the following
effect:
•
A “
+
” suffix sets the
y
bit to 1.
•
A “
−
” suffix clears the
y
bit to 0.
Compare Instructions
The PowerPC compare instructions include an L opcode field that specifies whether the
comparison is performed on a word or doubleword operand. In 32-bit implementations
like the PPC405, only word comparisons are supported. Simplified mnemonics are shown
in
that dispense with the need to encode the L field in the instruction syntax.
The
crf
D field can be omitted if the comparison result is placed into the CR0 field.
Otherwise, the target CR field must be specified as the first operand.
CR-Logical Instructions
The condition register logical instructions, are used to set, clear, copy, or invert a specific
condition register bit. The simplified mnemonics in
provide a shorthand for
several common operations. The variables
bx
and
by
are used to specify individual CR bits.
Table C-13:
Simplified Mnemonics for Compare Instructions
Operation
Simplified Mnemonic
Equivalent Mnemonic
Compare Word Immediate
cmpwi
crf
D,
r
A, SIMM
cmpi
crf
D, 0,
r
A, SIMM
Compare Word
cmpw
crf
D,
r
A,
r
B
cmp
crf
D, 0,
r
A,
r
B
Compare Logical Word Immediate
cmplwi
crf
D,
r
A, UIMM
cmpli
crf
D, 0,
r
A, UIMM
Compare Logical Word
cmplw
crf
D,
r
A,
r
B
cmpl
crf
D, 0,
r
A,
r
B
Table C-14:
Simplified Mnemonics for CR-Logical Instructions
Operation
Simplified Mnemonic
Equivalent Mnemonic
Condition Register Set
crset
bx
creqv
bx
,
bx
,
bx
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