Freescale Semiconductor PowerPC e500 Core - C-2 BO Field (Bits 6-10 of the Instruction Encoding)

PowerPC e500 Core Family Reference Manual, Rev. 1
C-6 Freescale Semiconductor
Simplified Mnemonics for PowerPC Instructions
• Test condition register (CR)
— Test condition true
— Test condition false
• Branch prediction (taken, fall through). If the prediction bit, y, is needed, it is signified by
appending a plus or minus sign as described in Section C.4.3, “Incorporating the BO
Branch Prediction.”
BO bits can be interpreted individually as described in Table C-5.
Thus, a BO encoding of 10100 (decimal 20) means ignore the CR bit comparison and do not
decrement the CTR—in other words, branch unconditionally. Encodings for the BO operand are
shown in Table C-6. A z bit indicates that the bit is ignored. However, these bits should be cleared,
as they may be assigned a meaning in a future version of the architecture.
As shown in Table C-6, the ‘c’ in the standard mnemonic is replaced with the operations otherwise
specified in the BO field, (d for decrement, z for zero, nz for non-zero, t for true, and f for false).
01234
Figure C-2. BO Field (Bits 6–10 of the Instruction Encoding)
Table C-5. BO Bit Encodings
BO Bit Description
0 If set, ignore the CR bit comparison.
1 If set, the CR bit comparison is against true, if not set the CR bit comparison is against false
2 If set, the CTR is not decremented.
3 If BO[2] is set, this bit determines whether the CTR comparison is for equal to zero or not equal to zero.
4 The
y bit. If set, reverse the static prediction. Use of the this bit is optional and independent from the
interpretation of the rest of the BO operand. Because simplified branch mnemonics eliminate the BO operand,
this bit is programmed by adding a plus or minus sign to the simplified mnemonic, as described in
Section C.4.3, “Incorporating the BO Branch Prediction.”
Table C-6. BO Operand Encodings
BO Field
Value
1
(Decimal)
Description Symbol
0000
y 0 Decrement the CTR, then branch if the decremented CTR ≠ 0; condition is FALSE. dnzf
0001
y 2 Decrement the CTR, then branch if the decremented CTR = 0; condition is FALSE. dzf
001
zy 4 Branch if the condition is FALSE.
2
Note that ‘false’ and ‘four’ both start with ‘f’. f
0100
y 8 Decrement the CTR, then branch if the decremented CTR ≠ 0; condition is TRUE. dnzt
0101
y 10 Decrement the CTR, then branch if the decremented CTR = 0; condition is TRUE. dzt