xv6/trapasm.S

#define SEG_KCODE 1  // kernel code
#define SEG_KDATA 2  // kernel data+stack
#define SEG_KCPU  3  // kernel per-cpu data

  # vectors.S sends all traps here.
.globl alltraps
alltraps:
  # Build trap frame.
  pushl %ds
  pushl %es
  pushl %fs
  pushl %gs
  pushal
  
  # Set up data and per-cpu segments.
  # Can find out KDATA from %ss.
  # Assume that KCPU is KDATA+1.
  movw $(SEG_KDATA<<3), %ax
  movw %ss, %ax
  movw %ax, %ds
  movw %ax, %es
  movw $(SEG_KCPU<<3), %ax
  movw %ax, %fs
  movw %ax, %gs

  # Call trap(tf), where tf=%esp
  pushl %esp
  call trap
  addl $4, %esp

  # Return falls through to trapret...
.globl trapret
trapret:
  popal
  popl %gs
  popl %fs
  popl %es
  popl %ds
  addl $0x8, %esp  # trapno and errcode
  iret

  # A forked process switches to user mode by calling
  # forkret1(tf), where tf is the trap frame to use.
.globl forkret1
forkret1:
  movl 4(%esp), %esp
  jmp trapret