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Add a comment about why at is turned off in the exception handler.

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Only k0 and k1 may be touched until we save registers somewhere.

MFC After: 2 days
This commit is contained in:
Warner Losh 2016-01-26 18:39:23 +00:00
parent 4456b8f5cf
commit c8719df5f9
1 changed files with 6 additions and 0 deletions
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6
sys/mips/mips/exception.S
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@ -177,6 +177,12 @@ MipsDoTLBMiss:
VECTOR(MipsException, unknown) VECTOR(MipsException, unknown)
/* /*
* Find out what mode we came from and jump to the proper handler. * Find out what mode we came from and jump to the proper handler.
*
* Note: at turned off here because we cannot trash the at register
* in this exception code. Only k0 and k1 may be modified before
* we save registers. This is true of all functions called through
* the pointer magic: Mips{User,Kern}Intr, Mips{User,Kern}GenException
* and MipsTLBInvalidException
*/ */
.set noat .set noat
mfc0 k0, MIPS_COP_0_STATUS # Get the status register mfc0 k0, MIPS_COP_0_STATUS # Get the status register