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Annotation of xinu/sys/.ctxsw.s, revision 1.1

1.1     ! root        1: |* ctxsw.s - ctxsw
        !             2: 
        !             3: |*-----------------------------------------------------------------------------
        !             4: |* ctxsw  --   actually perform context switch, saving and loading registers
        !             5: |*             This routine is executed in kernel mode.
        !             6: |*
        !             7: |*             Upon entry, d0-d7, a0-a6, the ssp, the sr, and the pc are
        !             8: |*             saved.  The usp is ignored.
        !             9: |*
        !            10: |*             Upon exit, all registers saved are restored, and the value
        !            11: |*             of the supervisor stack pointer is copied into the user 
        !            12: |*             stack pointer.
        !            13: |*-----------------------------------------------------------------------------
        !            14: 
        !            15:        .data
        !            16:        .text
        !            17:        .globl  _ctxsw
        !            18: _ctxsw:
        !            19:        movl    a0,     sp@(-4)         ; Save A0 onto old stack        
        !            20:        movl    sp@(4), a0              ; Move address of old area into A0 
        !            21:        moveml  #0xffff,a0@             ; Save all registers            
        !            22:        movl    sp@(-4),a0@(32)         ; Put original A0 in old savearea       
        !            23:        addl    #64,    a0              ; Skip past registers d0-7,a0-7
        !            24:        movw    #0,     a0@+            ; Pad SR savearea, since SR is a word
        !            25:        movw    sr,     a0@+            ; Save SR in old savearea       
        !            26:        movl    sp@,    a0@+            ; Save PC in old savearea       
        !            27: 
        !            28: |Save image:d0,d1,d2,d3,d4,d5,d6,d7,a0,a1,a2,a3,a4,a5,a6,ssp,0x00, sr, pc
        !            29: |            0  4  8 12 16 20 24 28 32 36 40 44 48 52 56  60  64   66  68 
        !            30: 
        !            31:        movl    sp@(8), a0              ; Move address of new area into A0
        !            32:        moveml  a0@,#0x7fff             ; Restore all regs (even A0) except SP
        !            33:        movl    sp@(8), a0              ; Move address of new area into A0 again
        !            34:        movl    a0@(60),sp              ; Put  SSP into both kernel stack
        !            35:        movl    sp,     usp             ;      and            user  stack
        !            36:        movw    a0@(66),sr              ; Restore SR from new area      
        !            37:        movl    a0@(68),sp@             ; Move PC onto current stack    
        !            38:        movl    a0@(32),a0              ; Restore A0 from new area      
        !            39:        rts
        !            40:        .data

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