
RCPU
REFERENCE MANUAL
OVERVIEW
MOTOROLA
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1-13
The time base (TB) can be read at the user privilege level. A separate SPR
number is provided for writing to the time base. Writes to the time base can
occur only at the supervisor privilege level.
NOTE
While these registers are defined as SPRs and can be accessed by
using the
mtspr
and
mfspr
instructions, they (except for the time
base) are typically accessed implicitly.
1.3.1.8 Supervisor-Level SPRs
The processor also contains SPRs that can be accessed only by supervisor-level
software. These registers consist of the following:
The data access exception (DAE)/source instruction service register (DSISR)
defines the cause of data access and alignment exceptions.
The data address register (DAR) holds the address of an access after an
alignment or data access exception.
Decrementer register (DEC) is a 32-bit decrementing counter that provides a
mechanism for causing a decrementer exception after a programmable delay.
The DEC frequency is provided as a subdivision of the processor clock fre-
quency.
The machine status save/restore register 0 (SRR0) is used by the processor
to save the address of the instruction that caused the exception, and the ad-
dress to return to when a return from interrupt (
rfi
) instruction is executed.
The machine status save/restore register 1 (SRR1) is used to save machine
status on exceptions and to restore machine status when an
rfi
instruction is
executed.
General SPRs, SPRG[0:3], are provided for operating system use.
The processor version register (PVR) is a read-only register that identifies the
version (model) and revision level of the PowerPC processor.
The time base (TB) can be written to only at the supervisor privilege level.
Separate SPR numbers are provided for reading and writing to the time base.
The following supervisor-level SPRs are implementation-specific to the RCPU:
The EIE, EID, and NRI are provided to facilitate exception processing.
Cache control SPRs allow system software to control the operation of the in-
struction cache.
Development support SPRs allow development-system software control over
the on-chip development support.
The floating-point exception cause register (FPECR) is a 32-bit internal status
and control register used to assist the software emulation of floating-point op-
erations.
1.3.2 Instruction Set and Addressing Modes
The following subsections describe the PowerPC instruction set and addressing
modes and summarize the instructions implemented in the RCPU.
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.