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368
March 2002 Release
1-800-255-7778
Virtex-II Pro™ Platform FPGA Documentation
Chapter 3:
User Programming Model
R
The 5-bit BI opcode field in branch-conditional instructions specifies which of the 32 bits in
the CR are used in the branch-condition test. For example, if BI
=
0b01010, CR
10
is used in
the test.
In some encodings of the BO field, certain BO bits are ignored. Ignored bits can be assigned
a meaning in future extensions of the PowerPC architecture and should be cleared to 0.
Valid BO field encodings are shown in
. In this table,
z
indicates the ignored bits
that should be cleared to 0. The
y
bit (BO[4]) specifies the branch-prediction behavior for
the instruction as described in
Specifying Branch-Prediction Behavior
Branch Instructions
The following sections describe the branch instructions defined by the PowerPC
architecture. A number of simplified mnemonics are defined for the branch instructions.
See
for more information.
Branch Unconditional
lists the PowerPC
unconditional branch
instructions. These branches specify a 26-
bit signed displacement to the branch-target address by appending the 24-bit LI instruction
field with 0b00. The displacement value gives unconditional branches the ability to cover
an address range of ±32 MB.
BO[2]
CTR Test Control
0—Decrement CTR by one, and test whether CTR satisfies the condition specified by
BO[3].
1—Do not change or test CTR.
BO[3]
CTR Test Value
0—Test for CTR
≠
0.
1—Test for CTR
=
0.
BO[4]
Branch Prediction Reversal
0—Apply standard branch prediction.
1—Reverse the standard branch prediction.
Table 3-5:
Valid BO Opcode-Field Encoding
BO[0:4]
Description
0000
y
Decrement the CTR. Branch if the decremented CTR
≠
0 and CR[BI]
=
0.
0001
y
Decrement the CTR. Branch if the decremented CTR
=
0 and CR[BI]
=
0.
001
zy
Branch if CR[BI]
=
0.
0100
y
Decrement the CTR. Branch if the decremented CTR
≠
0 and CR[BI]
=
1.
0101
y
Decrement the CTR. Branch if the decremented CTR
=
0 and CR[BI]
=
1.
011
zy
Branch if CR[BI]
=
1.
1
z
00
y
Decrement the CTR. Branch if the decremented CTR
≠
0.
1
z
01
y
Decrement the CTR. Branch if the decremented CTR
=
0.
1
z
1
zz
Branch always.
Table 3-4:
BO Field Bit Definitions
(Continued)
BO Bit
Description
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