MPC8260 PowerQUICC II Family Reference Manual, Rev. 2
Freescale Semiconductor Glossary-3
Critical-data first. An aspect of burst accesses that allow the requested da ta (typically a
word or double word) in a cache block to be transferred first.
DDenormalized number. A nonzero floating-point number whose exponent has a reserved
value, usually the format's minimum, and whose explicit or implicit leading
significand bit is zero.
Direct-mapped cache. A cache in which each main memory address can appear in only
one location within the cache, operates more quickly when the memory request is
a cache hit.
Direct-store. Interface available on processors that implement the PowerPC architecture
only to support direct-store devices from the POWER architecture. When the T bit
of a segment descriptor is set, the descriptor defines the region of memory that is
to be used as a direct-store segment. Note that this facility is being phased out of
the architecture and will not likely be supported in future devices. Therefore,
software should not depend on it and new software should not use it.
EEffective address (EA). The 32- or 64-bit address specified for a load, store, or an
instruction fetch. This address is then submitted to the MMU for translation to
either a physical memory address or an I/O address.
Exception. A condition encountered by the processor that requires special, supervisor-level
processing.
Exception handler. A software routine that executes when an exception is taken.
Normally, the exception handler corrects the condition that caused the exception,
or performs some other meaningful task (that may include aborting the program
that caused the exception). The address for each exception handler is identified by
an exception vector offset defined by the architecture and a prefix selected via the
MSR.
Extended opcode. A secondary opcode field generally located in instruction bits 21–30,
that further defines the instruction type. All PowerPC instructions are one word in
length. The most significant 6 bits of the instruction are the primary opcode,
identifying the type of instruction. See also Primary opcode.
Execution synchronization. A mechanism by which all instructions in execution are
architecturally complete before beginning execution (appearing to begin
execution) of the next instruction. Similar to context synchronization but doesn't
force the contents of the instruction buffers to be deleted and refetched.
Exponent. In the binary representation of a floating-point number, the exponent is the
component that normally signifies the integer power to which the value two is
raised in determining the value of the represented number. See also Biased
exponent.