$(4)$ Why the following context switch code is sufficient so that the old process can

restart later without a problem?

# 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 $4($sesp$),$ seax # put old ptr into eax

popl O$($seax$)$ # save the old IP

movl sesp, $4($seax$)$ # and stack

movl sebx, $8($seax$)$ # and other registers

movl secx$,12($seax$)$

movl sedx, $16($seax$)$

movl sesi, $20($seax$)$

movl sedi, $24($seax$)$

movl sebp, $28($seax$)$

# Load new registers

mov$14($sesp$),$ seax # put new ptr into eax

movl $28($seax$),$ sebp # restore other registers

movl $24($seax$),$ sedi

movl $20($seax$),$ sesi

movl $16($seax$),$ sedx

movl $12($seax$),$ secx

movl $8($seax$),$ sebx

movl $4($seax$),$ sesp # stack is switched here

pushl $0($seax$)$ # return addr put in place

ret # Einally return into new ctxt

Figure $6\mathrm{.}4$: The xv$6$ Context Switch Code