ARM Architecture Reference Manual ARMv7-A and ARMv7-R edition

Application Level Programmers’ Model
Jazelle exception conditions
These are conditions that cause Jazelle hardware acceleration to exit into a software handler,
as described in Jazelle exception conditions on page A2-79.
Debug events These are conditions that cause a debug system to take action. Most aspects of debug events
are not visible to application-level code, and are described in Chapter C3 Debug Events.
Aspects that are visible to application-level code include:
The BKPT instruction causes a BKPT Instruction debug event to occur, see BKPT
Instruction debug events on page C3-20.
The DBG instruction provides a hint to the debug system.
Checks These are provided in the ThumbEE extension. A check causes an unconditional branch to
a specific handler entry point. The base address of the ThumbEE check handlers is held in
the TEEHBR, see ThumbEE Handler Base Register (TEEHBR) on page A2-71.
A2.11.1 The Yield instruction
In a Symmetric Multi-Threading (SMT) design, a thread can use a Yield instruction to give a hint to the
processor that it is running on. The Yield hint indicates that whatever the thread is currently doing is of low
importance, and so could yield. For example, the thread might be sitting in a spin-lock. Similar behavior
might be used to modify the arbitration priority of the snoop bus in a multiprocessor (MP) system. Defining
such an instruction permits binary compatibility between SMT and SMP systems.
ARMv7 defines a YIELD instruction as a specific NOP-hint instruction, see YIELD on page A8-812.
The YIELD instruction has no effect in a single-threaded system, but developers of such systems can use the
instruction to flag its intended use on migration to a multiprocessor or multithreading system. Operating
systems can use YIELD in places where a yield hint is wanted, knowing that it will be treated as a NOP if there
is no implementation benefit.
A2-82 Copyright © 1996-1998, 2000, 2004-2008 ARM Limited. All rights reserved. ARM DDI 0406B