Freescale Semiconductor PowerPC e500 Core - Key Facts about Simplified Branch Mnemonics; Eliminating the BO Operand

Simplified Mnemonics for PowerPC Instructions
PowerPC e500 Core Family Reference Manual, Rev. 1
Freescale Semiconductor C-5
in CR0) is replaced by the eq symbol. Using the simplified mnemonic and the eq operand, the
expression becomes bdnzt eq
,target.
This example tests CR0[EQ]; however, to test the equal condition in CR5 (CR bit 22), the
expression becomes bc 8,22,target. The BI operand of 22 indicates CR[22] (CR5[2], or BI field
0b10110), as shown in Table C-7. This can be expressed as the simplified mnemonic.
bdnzt 4 * cr5 + eq,target.
The notation, 4 * cr5 + eq may at first seem awkward, but it eliminates computing the value of the
CR bit. It can be seen that (4 * 5) + 2 = 22. Note that although 32-bit registers in Book E processors
are numbered 32–63, only values 0–31 are valid (or possible) for BI operands. As shown in
Table C-8, a Book E–compliant processor automatically translates the bit values; specifying a BI
value of 22 selects bit 54 on a Book E processor, or CR5[2] = CR5[EQ].
C.4.1 Key Facts about Simplified Branch Mnemonics
The following key points are helpful in understanding how to use simplified branch mnemonics:
• 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 it can be indicated with an easier to remember formula, 4 * crn + [test bit
symbol], where n indicates the CR field number.
— The condition of the test bit (eq, lt, gt, and so) can be incorporated into the mnemonic,
leaving 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.
C.4.2 Eliminating the BO Operand
The 5-bit BO field, shown in Figure C-2, encodes the following operations in conditional branch
instructions:
• Decrement count register (CTR)
— And test if result is equal to zero
— And test if result is not equal to zero