
RCPU
REFERENCE MANUAL
REGISTERS
MOTOROLA
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2-1
SECTION 2
REGISTERS
This section describes the RCPU register organization as defined by the three lev-
els of the PowerPC architecture: the user instruction set architecture (UISA), the
virtual environment architecture (VEA), and the operating environment architecture
(OEA), as well as the RCPU’s implementation-specific registers.
2.1 Programming Models
The processor operates at one of two privilege levels: supervisor level (typically
used by the operating environment) or user level (used by the application soft-
ware). This division allows the operating system to control the application environ-
ment, protecting operating-system and critical machine resources. Instructions that
control the state of the processor and supervisor registers can be executed only
when the processor is operating at the supervisor level.
Supervisor-level access is provided through the processor’s exception mecha-
nism. That is, when an exception is taken, either due to an error or problem that
needs to be serviced or deliberately through the use of a trap instruction, the pro-
cessor begins operating in supervisor mode. The level of access is indicated by the
privilege-level (PR) bit in the machine state register (MSR).
Figure 2-1
shows the user-level and supervisor-level RCPU programming models
and also illustrates the three levels of the PowerPC architecture. The numbers to
the left of the SPRs indicate the decimal number that is used in the syntax of the
instruction operands to access the register.
NOTE
Registers such as the general-purpose registers (GPRs) and float-
ing-point registers (FPRs) are accessed through operands that are
part of the instructions. Access to registers can be explicit (that is,
through the use of specific instructions for that purpose such as move
to special-purpose register (
mtspr
) and move from special-purpose
register (
mfspr
) instructions) or implicitly as the part of the execution
of an instruction. Some registers are accessed both explicitly and
implicitly.
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