ref: a7a07b2d43529b60a239bbb2c7acabcbdbb812e5
dir: /sys/src/9/kw/lproc.s/
#include "mem.h"
#include "arm.h"
/*
* This is the first jump from kernel to user mode.
* Fake a return from interrupt.
*
* Enter with R0 containing the user stack pointer.
* UTZERO + 0x20 is always the entry point.
*
*/
TEXT touser(SB), 1, $-4
/* store the user stack pointer into the USR_r13 */
MOVM.DB.W [R0], (R13)
/* avoid the ambiguity described in notes/movm.w. */
// MOVM.S.IA.W (R13), [R13]
MOVM.S (R13), [R13]
ADD $4, R13
/* set up a PSR for user level */
MOVW $(PsrMusr), R0
MOVW R0, SPSR
/* save the PC on the stack */
MOVW $(UTZERO+0x20), R0
MOVM.DB.W [R0], (R13)
/*
* note that 5a's RFE is not the v6 arch. instruction (0xe89d0a00,
* I think), which loads CPSR from the word after the PC at (R13),
* but rather the pre-v6 simulation `MOVM.IA.S.W (R13), [R15]'
* (0xe8fd8000 since MOVM is LDM in this case), which loads CPSR
* not from memory but from SPSR due to `.S'.
*/
RFE
/*
* here to jump to a newly forked process
*/
TEXT forkret(SB), 1, $-4
ADD $(4*15), R13 /* make r13 point to ureg->type */
MOVW 8(R13), R14 /* restore link */
MOVW 4(R13), R0 /* restore SPSR */
MOVW R0, SPSR /* ... */
MOVM.DB.S (R13), [R0-R14] /* restore registers */
ADD $8, R13 /* pop past ureg->{type+psr} */
RFE /* MOVM.IA.S.W (R13), [R15] */