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exec.S
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exec.S
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# exec.S - memory operation & instruction used for the context switch between
# - different processes.
#define ASM 1
#include "syscall.h"
#include "paging.h"
#include "signal.h"
.globl user_exec
.globl switch_pd
.globl back_to_parent
.globl from_exception
.globl shell_ss
.globl shell_bp
shell_ss:
.long 0
shell_bp:
.long 0
# --------------------------------------------------------------------
# function name : switch_pd
# input : pd pointer
# output : none
# description : set the CR3 to points to next process's pd
# ----------------------------------------------------------
switch_pd:
pushl %ebp
movl %esp,%ebp
pushl %ebx # callee saved regs
pushl %esi
movl 8(%ebp),%esi # get pd pointer
movl %CR3,%ebx
andl $0x00000FFF,%ebx
orl %esi,%ebx
movl %ebx,%CR3 # update CR3
popl %esi
popl %ebx
leave
ret
# --------------------------------------------------------------------
# function name : user_exec
# input : pcb[k]->mm.user_stack, entry_point, pcb[k]->parent
# output : none
# description : execute the 2nd process using iret. Never return back
# --------------------------------------------------------------------
user_exec:
movl 12(%esp),%esi
movl (%esi),%edi
cmpl $0,%edi # check if parent is the 0th process
je now_do # if true, go now_do
movl %esp,4(%esi) # save esp in pcb
movl %ebp,8(%esi) # save ebp in pcb
now_do:
movl 4(%esp),%esi # prepare stack for iret
movl 8(%esp),%edi
pushfl
popl %ecx # push eflags
orl $0x4000,%ecx
pushl $0x002B # push segment selector
pushl %esi
pushl %ecx
pushl $0x0023
pushl %edi
movw $0x002B,%si
movw %si,%ds
movw $0x0030,%si
iret # iret to the new program
# ----------------------------------------------------------------------------
# function name : back_to_parent
# input : current, int status
# output : none
# description : return to the parent program if calls halt or program finishes
# ----------------------------------------------------------------------------
back_to_parent:
movl 4(%esp),%esi #restore the parent esp and eip
movb 8(%esp),%bl
movzbl %bl,%eax
movl 4(%esi),%esp
movl 8(%esi),%ebp
movl $0, user_div_zero_handler # clear all flags before return
movl $0, user_segfault_handler
movl $0, user_alarm_handler
ret # return to the parent program
# ---------------------------------------------------------------------------
# function name : from_exception
# input : task_struct* current
# output : 256
# description : return from a exception to parent process
# ---------------------------------------------------------------------------
from_exception:
movl 4(%esp),%eax
movl 4(%eax),%esp #restore the parent esp and eip
movl 8(%eax),%ebp
movl $256,%eax
movl $0, user_div_zero_handler # clear all flags before return
movl $0, user_segfault_handler
movl $0, user_alarm_handler
ret