UM0434 datasheet(388/390 Pages) STMICROELECTRONICS

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UM0434

Glossary

389/391

Underflow. A condition that occurs during arithmetic operations when the result cannot be

represented accurately in the destination register. For example, underflow can happen if two

floating-point fractions are multiplied and the result requires a smaller

exponent and/or

mantissa than the single-precision format can provide. In other words, the result is too small

to be represented accurately.

User mode. The operating state of a processor used typically by application software. In

user mode, software can access only certain control registers and can access only user

memory space. No privileged operations can be performed. Also referred to as problem

state.

VEA (virtual environment architecture). The level of the

architecture that describes the

memory model for an environment in which multiple devices can access memory, defines

aspects of the cache model, defines cache control instructions, and defines the time-base

facility from a user-level perspective.

Implementations that conform to the PowerPC VEA

also adhere to the UISA, but may not necessarily adhere to the OEA.

Virtual address. An intermediate address used in the translation of an

effective address to

a physical address.

Virtual memory. The address space created using the memory management facilities of

the processor. Program access to

virtual memory is possible only when it coincides with

physical memory.

Way. A location in the cache that holds a cache block, its tags and status bits.

Word. A 32-bit data element.

Write-back. A cache memory update policy in which processor write cycles are directly

written only to the cache. External memory is updated only indirectly, for example, when a

modified cache block is

cast out to make room for newer data.

Write-through. A cache memory update policy in which all processor write cycles are

written to both the cache and memory.

UM0434 Datasheet(PDF) 388 Page - STMicroelectronics