User guide

Glossary
ARM DDI 0337I Copyright © 2005-2008, 2010 ARM Limited. All rights reserved. Glossary-9
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Thread Control Block A data structure used by an operating system kernel to maintain information specific to a single
thread of execution.
Thumb instruction A halfword that specifies an operation for an ARM processor in Thumb state to perform. Thumb
instructions must be halfword-aligned.
Thumb state A processor that is executing Thumb (16-bit) halfword aligned instructions is operating in
Thumb state.
TPA See Trace Port Analyzer.
TPIU See Trace Port Interface Unit.
Trace Port Analyzer (TPA)
A hardware device that captures trace information output on a trace port. This can be a low-cost
product designed specifically for trace acquisition, or a logic analyzer.
Trace Port Interface Unit (TPIU)
Drains trace data and acts as a bridge between the on-chip trace data and the data stream
captured by a TPA.
Unaligned A data item stored at an address that is not divisible by the number of bytes that defines the data
size is said to be unaligned. For example, a word stored at an address that is not divisible by four.
Wake-up Interrupt
Controller (WIC)
The Wake-up Interrupt Controller provides significantly reduced gate count interrupt detection
and prioritization logic.
Warm reset Also known as a core reset. Initializes the majority of the processor excluding the debug
controller and debug logic. This type of reset is useful if you are using the debugging features
of a processor.
Watchpoint A watchpoint is a mechanism provided by debuggers to halt program execution when the data
contained by a particular memory address is changed. Watchpoints are inserted by the
programmer to enable inspection of register contents, memory locations, and variable values
when memory is written to test that the program is operating correctly. Watchpoints are removed
after the program is successfully tested. See also Breakpoint.
WIC See Wake-up Interrupt Controller.
Word A 32-bit data item.
Word-invariant In a word-invariant system, the address of each byte of memory changes when switching
between little-endian and big-endian operation, in such a way that the byte with address A in
one endianness has address A EOR 3 in the other endianness. As a result, each aligned word of
memory always consists of the same four bytes of memory in the same order, regardless of
endianness. The change of endianness occurs because of the change to the byte addresses, not
because the bytes are rearranged.
The ARM architecture supports word-invariant systems in ARMv3 and later versions. When
word-invariant support is selected, the behavior of load or store instructions that are given
unaligned addresses is instruction-specific, and is in general not the expected behavior for an
unaligned access. It is recommended that word-invariant systems use the endianness that
produces the required byte addresses at all times, apart possibly from very early in their reset
handlers before they have set up the endianness, and that this early part of the reset handler must
use only aligned word memory accesses.
See also Byte-invariant.
Write buffer A pipeline stage for buffering write data to prevent bus stalls from stalling the processor.