March 2002 Release
Virtex-II Pro™ Platform FPGA Documentation
1-800-255-7778
Processor Operating Modes
R
All context-synchronizing instructions are execution synchronizing. However, unlike a
context-synchronizing operation, there is no guarantee that subsequent instructions
execute in the context established by an execution-synchronizing instruction. The new
context becomes effective sometime after the execution-synchronizing instruction
completes and before or during a subsequent context-synchronizing operation.
Storage Synchronization
The PowerPC architecture specifies a weakly consistent memory model for shared-
memory multiprocessor systems. With this model, the order that the processor performs
memory accesses, the order that those accesses complete in memory, and the order that
those accesses are viewed as occurring by another processor can all differ. The PowerPC
architecture supports storage-synchronizing operations that provide a capability for
enforcing memory-access ordering, allowing programs to share memory. Support is also
provided to allow programs executing on a processor to share memory with some other
mechanism that can access memory, such as an I/O device.
Device control registers (DCRs) are treated as memory-mapped registers from a
synchronization standpoint. Storage-synchronization operations must be used to enforce
synchronization of DCR reads and writes.
Processor Operating Modes
The PowerPC architecture defines two levels of privilege, each with an associated
processor operating mode:
•
Privileged mode
•
User mode
The processor operating mode is controlled by the privilege-level field in the machine-state
register (MSR[PR]). When MSR[PR] = 0, the processor operates in privileged mode. When
MSR[PR] = 1, the processor operates in user mode. MSR[PR] = 0 following reset, placing
the processor in privileged mode. See
for more
information on this register.
Attempting to execute a privileged instruction when in user mode causes a privileged-
instruction program exception (see
Throughout this book, the terms
privileged
and
system
are used interchangeably to refer to
software that operates under the privileged-programming model. Likewise, the terms
user
and
application
are used to refer to software that operates under the user-programming
model. Registers and instructions are defined as either privileged or user, indicating which
of the two programming models they belong to. User registers and user instructions
belong to both the user-programming and privileged-programming models.
Privileged Mode
Privileged mode
allows programs to access all registers and execute all instructions
supported by the processor. The
privileged-programming model
comprises the entire register
set and instruction set supported by the PPC405. Operating systems are typically the only
software that runs in privileged mode.
The registers available only in privileged mode are shown in
. Refer to
the corresponding section describing each register for more information. The instructions
available only in privileged mode are shown in
. The operation of each
instruction is described in
Privileged mode is sometimes referred to as
supervisor state
.
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