Chapter 1. Overview
1-27
PowerPC Registers and Programming Model
The following tables summarize the PowerPC registers implemented in the MPC7400;
Table 1-1 describes registers (excluding SPRs) deTned by the PowerPC architecture.
Table 1-1. PowerPC Architecture-Defined Registers on the MPC7400
(Excluding SPRs)
The OEA deTnes numerous special-purpose registers that serve a variety of functions, such
as providing controls, indicating status, conTguring the processor, and performing special
operations. During normal execution, a program can access the registers, shown in
Figure 1-5, depending on the programs access privilege (supervisor or user, determined by
the privilege-level (PR) bit in the MSR). GPRs and FPRs are accessed through operands
that are part of the instructions. Access to registers can be explicit (that is, through the use
of speciTc instructions for that purpose such as Move to Special-Purpose Register (
and Move from Special-Purpose Register (
mfspr
the execution of an instruction. Some registers can be accessed both explicitly and
implicitly.
mtspr
)
) instructions) or implicit, as the part of
In the MPC7400, all SPRs are 32 bits wide. Table 1-2 describes the architecture-deTned
SPRs implemented by the MPC7400.
The
Programming Environments Manual
these registers in detail, including bit descriptions. Section 2.1.1, òRegister Set,ó describes
how these registers are implemented in the MPC7400. In particular, this section describes
which features the PowerPC architecture deTnes as optional are implemented on the
MPC7400.
describes
Register
Level
Function
CR
User
The condition register (CR) consists of eight four-bit Telds that reect the results of certain
operations, such as move, integer and oating-point compare, arithmetic, and logical
instructions, and provide a mechanism for testing and branching.
FPRs
User
The 32 oating-point registers (FPRs) serve as the data source or destination for
oating-point instructions. These 64-bit registers can hold either single- or double-precision
oating-point values.
FPSCR
User
The oating-point status and control register (FPSCR) contains the oating-point exception
signal bits, exception summary bits, exception enable bits, and rounding control bits
needed for compliance with the IEEE-754 standard.
GPRs
User
The 32 GPRs serve as the data source or destination for integer instructions.
MSR
Supervisor
The machine state register (MSR) deTnes the processor state. Its contents are saved when
an exception is taken and restored when exception handling completes. The MPC7400
implements MSR[POW], (deTned by the architecture as optional), which is used to enable
the power management feature. The MPC7400-speciTc MSR[PM] bit is used to mark a
process for the performance monitor.
SR0DSR15
Supervisor
The sixteen 32-bit segment registers (SRs) deTne the 4-Gbyte space as sixteen 256-Mbyte
segments. The MPC7400 implements segment registers as two arraysa main array for
data accesses and a shadow array for instruction accesses; see Figure 1-1. Loading a
segment entry with the Move to Segment Register
mfsr
instruction reads the master register, shown as part of the data MMU in Figure 1-1.
(
mtsr
)
instruction loads both arrays. The