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Matt Bishop Office: 2209 Watershed Sciences Phone: +1 (530) 752-8060 Email: mabishop@ucdavis.edu |
Version of April 1, 2022 at 1:00AM
/* ----------------------------------------------------------------------
* saving/loading registers
*
* The stack contains three items upon entry to this routine:
*
* SP+4 => address of 9 word save area with new registers + PS
* SP+2 => address of 9 word save area for old registers + PS
* SP => return address
*
* The saved state consists of: the values of R0-R5 upon entry, SP+2, PC
* equal to the return address, and the PS (i.e., the PC and SP are saved
* as if the calling process had returned to its caller.
*----------------------------------------------------------------------
*/
.globl _ctxsw /* declare name global */
_ctxsw: /* entry point to proc. */
mov r0,*2(sp) /* save old R0 in old register area */
mov 2(sp),r0 /* get address of old register area in R0 */
add $2,r0 /* increment to saved pos. of R1 */
mov r1,(r0)+ /* save R1-R5 in successive locations of old */
mov r2,(r0)+ /* process register save are */
mov r3,(r0)+
mov r4,(r0)+
mov r5,(r0)+
add $2,sp /* move SP beyond the return address, as if a */
/* return had occurred */
mov sp,(r0)+ /* save stack pointer */
mov -(sp),(r0)+ /* save caller’s return address as PC */
mfps (r0) /* save processor status beyond registers */
mov 4(sp),r0 /* get address of start of new register area */
/* ready to load registers for the new process */
/* and abandon the old stack */
mov 2(r0),r1 /* load R1-R5 and SP from the new area */
mov 4(r0),r2
mov 6(r0),r3
mov 8.(r0),r4 /* dot following a number makes it decimal; */
mov 10.(r0),r5 /* otherwise it is octal */
mov 12.(r0),sp /* have switched stacks now */
mov 16.(r0),-(sp) /* push new process PS on new process stack */
mov 14.(r0),-(sp) /* push new process PC on new process stack */
mov (r0),r0 /* load R0 from new area */
rtt /* load PC, PS, and reset SP all at once */
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ECS 235B, Foundations of Computer and Information Security Version of April 1, 2022 at 1:00AM
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