AN2491 Freescale Semiconductor / Motorola, AN2491 Datasheet - Page 5

• Manufacturer Part Number: AN2491
• Description: Simplified Mnemonics for PowerPC Instructions
• Manufacturer: Freescale Semiconductor / Motorola

4.1 Key Facts about Simplified Branch Mnemonics
The following key points are helpful in understanding how to use simplified branch mnemonics:
4.2 Eliminating the BO Operand
The 5-bit BO field, shown in Figure 2, encodes the following operations in conditional branch instructions:
BO bits can be interpreted individually as described in Table 5.
MOTOROLA
BO Bit
0
1
2
3
4
•
•
•
•
•
•
All simplified branch mnemonics eliminate the BO operand, so if any operand is present in a branch
simplified mnemonic, it is the BI operand (or a reduced form of it).
If the CR is not involved in the branch, the BI operand can be deleted
If the CR is involved in the branch, the BI operand can be treated in the following ways:
— It can be specified as a numeric value, just as it is in the architecturally defined instruction, or
— The condition of the test bit (eq, lt, gt, and so) can be incorporated into the mnemonic, leaving
Decrement count register (CTR)
— And test if result is equal to zero
— And test if result is not equal to zero
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 4.3, “Incorporating the BO Branch Prediction.”
If set, ignore the CR bit comparison.
If set, the CR bit comparison is against true; if not set the CR bit comparison is against false.
If set, the CTR is not decremented.
If BO[2] is set, this bit determines whether the CTR comparison is for equal to zero or not equal to zero.
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 4.3,
“Incorporating the BO Branch Prediction.”
it can be indicated with an easier to remember formula, 4 * crn + [test bit symbol], where n
indicates the CR field number.
the need for an operand that defines only the CR field.
– If the test bit is in CR0, no operand is needed.
– If the test bit is in CR1–CR7, the BI operand can be replaced with a crS operand (that is,
cr1, cr2, cr3, and so forth.
Figure 2. BO Field (Bits 6–10 of the Instruction Encoding)
Simplified Mnemonics for PowerPC™ Instructions
Freescale Semiconductor, Inc.
For More Information On This Product,
Table 5. BO Bit Encodings
Go to: www.freescale.com
0
1
Description
2
3
4
Branch Instruction Simplified Mnemonics
5