The following RISC$-$V procedure is intended to reverse a string. We have deliberately omitted the code that saves and restores registers from the stack. Inspect the code and select which registers, if any, must be saved in the stack.

section $.$text

global strrev

strrevi

$8$ s $*1=$ str

a $*0=$ sz

# t$*$theta $=$ sz $/0\mathrm{.}2$

$1=1$

#Enter stack frame

#What registers should we push to the stack?

#Get the size of the string

mv $$1,$ ao #this is the same as add si$,$ ae$,$ zero

jal strlen

srai to$,$ ab$,1$ # Divide sz by $2$

$11$ tl$,0$ #$1=0$

$[$ matrix L$1$&g& for $*1$oop matrix $]$

bget $1,$ te $,$L$2$ add t$2,$s$1,$t$1||$ str $+1$ subt $3,$ a$*$theta $,$t$1$ forall sz$-1$ a dd$1$ t$3,$t$3,-1$ sz$-1-1$ add t$3,$t$3,$s$1$ forall str $+$ sz $-1-11$bt $*4,$ theta$($t$2)/$str$[1]]1$b t$5,$ theta$($t$3)$ forall str$[$sz $-1-1]$

sbt $*4,$ theta$($t$3)$ ne swap

sbt $*5,$ theta$($t$2)$

addi t $1,$t$1,1$

JL$1$

L$2$:

#Leave stack frame

#what registers should we pop from the stack?

jr rab

which ones from below is correct:

ra

t$0$

No register needs saving

t$4$

t$5$

t$3$

t$1$

t$2$

a$0$

s$1$