
MOTOROLA
Chapter 2. Register Model
2-1
Chapter 2
Register Model
This chapter describes the PowerPC register model and specific implementation on the G2
and G2_LE core.
2.1
Register Set
This section describes the register organization in the G2 core as defined by the three levels
of the PowerPC architecture—user instruction set architecture (UISA), virtual environment
architecture (VEA), and operating environment architecture (OEA), as well as the core
implementation-specific registers. Full descriptions of the basic register set defined by the
PowerPC architecture are provided in Chapter 2, “Register Set,” in the
Programming
Environments Manual
.
The PowerPC architecture defines register-to-register operations for all computational
instructions. Source data for these instructions is accessed from the on-chip registers or is
provided as an immediate value embedded in the opcode. The three-register instruction
format allows specification of a target register distinct from the two source registers, thus
preserving the original data for use by other instructions and reducing the number of
instructions required for certain operations. Data is transferred between memory and
registers with explicit load and store instructions only.
Note that there may be registers common to other processors of this family that are not
implemented in the G2 core. When the core detects special-purpose register (SPR)
encodings other than those defined in this document, it either takes an exception or it treats
the instruction as a no-op. (Note that exceptions are referred to as interrupts in the
architecture specification.) Conversely, some SPRs in the G2 core may not be implemented
in other processors or may not be implemented in the same way.
2.1.1
PowerPC Register Set
The UISA registers, shown in Figure 2-1, can be accessed by either user- or
supervisor-level instructions (the architecture specification refers to user- and
supervisor-level as problem state and privileged state, respectively). The general-purpose
registers (GPRs) and floating-point registers (FPRs) are accessed through instruction
operands. Access to registers can be explicit (that is, through the use of specific instructions
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