AN2491 Freescale Semiconductor / Motorola, AN2491 Datasheet - Page 2

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

Overview
Overview
1 Overview
Simplified (or extended) mnemonics allow an assembly-language programmer to program using more
intuitive mnemonics and symbols than the instructions and syntax defined by the instruction set architecture.
For example, to code the conditional call “branch to an absolute target if CR4 specifies a greater than
condition, setting the LR” without simplified mnemonics, the programmer would write the branch
conditional instruction bc 12,17,target. The simplified mnemonic, branch if greater than, bgt cr4,target,
incorporates the conditions. Not only is it easier to remember the symbols than the numbers when
programming, it is also easier to interpret simplified mnemonics when reading existing code.
Although the original PowerPC architecture documents include a set of simplified mnemonics, these are not
a formal part of the architecture, but rather a recommendation for assemblers that support the instruction set.
Many simplified mnemonics have been added to those originally included in the architecture
documentation. Some assemblers created their own, and others have been added to support extensions to the
instruction set (for example, AltiVec instructions and Book E auxiliary processing units (APUs)). Simplified
mnemonics for new architecturally defined and new implementation-specific special-purpose registers
(SPRs) are described here only in a very general way.
2 Subtract Simplified Mnemonics
This section describes simplified mnemonics for subtract instructions.
2.1 Subtract Immediate
There is no subtract immediate instruction; however, its effect is achieved by negating the immediate
operand of an Add Immediate instruction, addi. Simplified mnemonics include this negation, making the
intent of the computation clearer. These are listed in Table 1.
2.2 Subtract
Subtract from instructions subtract the second operand (rA) from the third (rB). The simplified mnemonics
in Table 2 use the more common order in which the third operand is subtracted from the second.
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rD,rB,rA is not the standard order for the operands. The order of rB and rA is
reversed to show the equivalent behavior of the simplified mnemonic.
Simplified Mnemonic
subc[o][.] rD,rA,rB
sub[o][.] rD,rA,rB
Table 1. Subtract Immediate Simplified Mnemonics
Simplified Mnemonics for PowerPC™ Instructions
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Table 2. Subtract Simplified Mnemonics
Simplified Mnemonic Standard Mnemonic
subic. rD,rA,value
subis rD,rA,value
subic rD,rA,value
subi rD,rA,value
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addic. rD,rA,–value
addis rD,rA,–value
addic rD,rA,–value
addi rD,rA,–value
Standard Mnemonic
subfc[o][.] rD,rB,rA
subf[o][.] rD,rB,rA
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