#### 0(%esp) - old registers
#### 16(%esp) - return address
#### 20(%esp) - current thread
#### 24(%esp) - new thread

        .globl thread_switch 
thread_switch:
	# Save caller's register state.
	# Note that the SVR4 ABI allows us to destroy %eax, %ecx, %edx.
	# This stack frame must match the one set up by thread_create().
	pushl %ebx
	pushl %ebp
	pushl %esi
	pushl %edi

	# Get offsetof (struct thread, stack).
	.globl thread_stack_ofs
	mov thread_stack_ofs, %edx

	# Save current stack pointer to old thread's stack, if any.
	movl 20(%esp), %eax
	test %eax, %eax
	jz 1f
	movl %esp, (%eax,%edx,1)
1:

	# Restore stack pointer from new thread's stack.
	movl 24(%esp), %ecx
	movl (%ecx,%edx,1), %esp

	# Restore caller's register state.
	popl %edi
	popl %esi
	popl %ebp
	popl %ebx
        ret