title: Xv6 context switch
body: <p>This code is the code given for context switching in xv6. Although I am able to understand that the overall code saves the registers from the old context on to the PCB, I am just not able to wrap my head around how each line works.</p>
  
  <pre><code>void swtch(struct context **old, struct context *new);
  #
  # Save current register context in old
  # and then load register context from new.
  .globl swtch
  swtch:
  # Save old registers
  movl 4(%esp), %eax # put old ptr into eax
  popl 0(%eax) # save the old IP
  movl %esp, 4(%eax) # and stack
  movl %ebx, 8(%eax) # and other registers
  movl %ecx, 12(%eax)
  movl %edx, 16(%eax)
  movl %esi, 20(%eax)
  movl %edi, 24(%eax)
  movl %ebp, 28(%eax)
  
  # Load new registers
  movl 4(%esp), %eax #put new ptr into eax
  movl 28(%eax), %ebp #restore other registers
  movl 24(%eax), %edi
  movl 20(%eax), %esi
  movl 16(%eax), %edx
  movl 12(%eax), %ecx
  movl 8(%eax), %ebx
  movl 4(%eax), %esp #stack is switched here
  pushl 0(%eax) #return addr put in place
  ret #finally return into new ctxt
  </code></pre>
  
  <p>Can someone help me out with how the CPU registers are saved onto the PCB?</p>