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parisc: document the shadow registers

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Signed-off-by: Helge Deller <[email protected]>
Cc: <[email protected]> # 3.10
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Helge Deller 2013-06-29 13:24:16 +02:00
commit a83f58bcb2
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Documentation/parisc/registers
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@ -77,6 +77,14 @@ PSW default E value 0
Shadow Registers used by interruption handler code
TOC enable bit 1
=========================================================================
The PA-RISC architecture defines 7 registers as "shadow registers".
Those are used in RETURN FROM INTERRUPTION AND RESTORE instruction to reduce
the state save and restore time by eliminating the need for general register
(GR) saves and restores in interruption handlers.
Shadow registers are the GRs 1, 8, 9, 16, 17, 24, and 25.
=========================================================================
Register usage notes, originally from John Marvin, with some additional
notes from Randolph Chung.