2-4
MPC7400 RISC Microprocessor Users Manual
The MPC7400 Processor Register Set
The PowerPC UISA registers are user-level. General-purpose registers (GPRs) and
oating-point registers (FPRs) are accessed through instruction operands. Access to
registers can be explicit (by using instructions for that purpose such as Move to
Special-Purpose Register (
mtspr
) and Move from Special-Purpose Register (
mfspr
)
instructions) or implicit as part of the execution of an instruction. Some registers are
accessed both explicitly and implicitly.
Implementation Notes
The MPC7400 fully decodes the SPR Teld of the instruction. If
the SPR speciTed is undeTned, the illegal instruction program exception occurs. The
PowerPCs user-level registers are described as follows:
¥
User-level registers
(UISA)The user-level registers can be accessed by all
software with either user or supervisor privileges. They include the following:
General-purpose registers (GPRs). The thirty-two GPRs (GPR0DGPR31) serve
as data source or destination registers for integer instructions and provide data
for generating addresses. See òGeneral Purpose Registers (GPRs),ó in Chapter 2,
òPowerPC Register Set,ó of
The Programming Environments Manual
for more
information.
Floating-point registers (FPRs). The thirty-two FPRs (FPR0DFPR31) serve as
the data source or destination for all oating-point instructions. See
òFloating-Point Registers (FPRs),ó in Chapter 2, òPowerPC Register Set,ó of
The
Programming Environments Manual
.
Vector registers (VRs). The thirty-two VRs (VR0DVR31) serve as the data
source or destination for all AltiVec instructions. See Section 7.1.1.4, òVector
Status and Control Register (VSCR).ó
Condition register (CR). The 32-bit CR consists of eight 4-bit Telds, CR0DCR7,
that reect results of certain arithmetic operations and provide a mechanism for
testing and branching. See òCondition Register (CR),ó in Chapter 2, òPowerPC
Register Set,ó of
The Programming Environments Manual
.
Floating-point status and control register (FPSCR). The FPSCR contains all
oating-point exception signal bits, exception summary bits, exception enable
bits, and rounding control bits needed for compliance with the IEEE 754
standard. See òFloating-Point Status and Control Register (FPSCR),ó in
Chapter 2, òPowerPC Register Set,ó of
The Programming Environments Manual
.
Vector status and control register (VSCR). A 32-bit vector register that is read
and written in a manner similar to the FPSCR. See Section 7.1.1.4, òVector
Status and Control Register (VSCR).ó
The remaining user-level registers are SPRs. Note that the PowerPC architecture
provides a separate mechanism for accessing SPRs (the
mtspr
and
mfspr
instructions). These instructions are commonly used to explicitly access certain
registers, while other SPRs can be more typically accessed as the side effect of
executing other instructions.
XER register. The XER indicates overow and carries for integer operations. See