Xv6上下文切换

Question

此代码是xv6中为上下文切换提供的代码。虽然我能够理解整个代码将寄存器从旧的上下文保存到PCB上，但我无法理解每条线的工作原理。

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

有人可以帮我解决CPU寄存器如何保存到PCB上的问题吗？