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Cortex-A8 Technical Reference Manual - ARM Information Center

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If software running on the processor has control over an external device that drives the<br />

authentication signals, it must make the change using a safe sequence:<br />

Debug<br />

1. Execute an implementation-specific sequence of instructions to change the signal value.<br />

For example, this might be a single STR instruction that writes certain value to a control<br />

register in a system peripheral.<br />

2. If step 1 involves any memory operation, issue a Data Synchronization Barrier (DSB).<br />

3. Poll the DSCR or Authentication Status Register to check whether the processor has<br />

already detected the changed value of these signals. This is required because the processor<br />

might not see the signal change until several cycles after the DSB completes.<br />

4. Issue an Instruction Synchronization Barrier (ISB) exception entry or exception return.<br />

The software cannot perform debug or analysis operations that depend on the new value of the<br />

authentication signals until this procedure is complete. The same rules apply when the debugger<br />

has control of the processor through the ITR while in debug state.<br />

The relevant combinations of the DBGEN, NIDEN, SPIDEN, and SPNIDEN values can be<br />

determined by polling DSCR[17:16], DSCR[15:14], or the Authentication Status Register.<br />

<strong>ARM</strong> DDI 0344K Copyright © 2006-2010 <strong>ARM</strong> Limited. All rights reserved. 12-68<br />

ID060510 Non-Confidential

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