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Cortex-A8 Technical Reference Manual - ARM Information Center

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Table 12-29 shows how the bit values correspond with the OS Save and Restore Register<br />

functions.<br />

Bits Field Function<br />

[31:0] OS save and<br />

restore<br />

Debug<br />

Table 12-29 OS Save and Restore Register bit functions<br />

OS save and restore. A sequence of reads from this register returns the contents of all the<br />

registers that can be saved. A sequence of writes restores the saved values. The OS must initiate<br />

the sequence by writing a 0xC5ACCE55 key to the OSLAR to set the internal pointer to the starting<br />

value. This is followed by a read from the OSSRR, and then followed by a series of reads or<br />

writes. The first OSSRR read returns the length of the rest of the sequence, that is, the number<br />

of registers to be saved or restored.<br />

These registers are saved and restored in the following order:<br />

1. WCR1<br />

2. WCR0<br />

3. WVR1<br />

4. WVR0<br />

5. BCR5<br />

6. BCR4<br />

7. BCR3<br />

8. BCR2<br />

9. BCR1<br />

10. BCR0<br />

11. BVR5<br />

12. BVR4<br />

13. BVR3<br />

14. BVR2<br />

15. BVR1<br />

16. BVR0<br />

17. DTRTX<br />

18. DSCR<br />

19. DTRRX<br />

20. DSCCR<br />

21. VCR<br />

22. WFAR.<br />

Note<br />

• If the OS issues a write to the OSSRR after the sequence has been initialized by writing<br />

the key to the OSLAR, the behavior is Unpredictable.<br />

• Subsequent accesses after reading the length of the sequence must be either all reads or<br />

all writes. If the OS mixes reads and writes, the result is Unpredictable. Additionally, if<br />

the OS performs more accesses than registers are in the sequence, the result is also<br />

Unpredictable.<br />

• This process restores only writable bits. Readable bits such as flags that reflect the<br />

processor state, are not updated. This means that, after the restore sequence, the readable<br />

bits indicate the current state of the processor rather than the state of the processor at the<br />

time the OS saved them. The only exceptions to this rule are the DSCR[30:29] and<br />

DSCR[27:26] bits, these can be restored.<br />

• DTRRX writes and DTRTX reads through the OSSRR do not cause the APB interface to<br />

stall regardless of the value of the DSCR[22:21] field.<br />

<strong>ARM</strong> DDI 0344K Copyright © 2006-2010 <strong>ARM</strong> Limited. All rights reserved. 12-35<br />

ID060510 Non-Confidential

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