program on Assembly

Question

can somebody please explain what this program is doing?

.= torg + 1000

main:
        mov pc, sp
        tst –(sp)

        mov #list1, -(sp)
        jsr pc, mystery
        mov r0, res1
        tst (sp)+

        mov #list2, -(sp)
        jsr pc, mystery
        mov r0, res2
        tst (sp)+

        halt


mystery:
        mov r1, -(sp)
        mov r4, -(sp)
        mov r5, -(sp)

        clr r0

        mov 10(sp), r4
        mov r4, r5

loop:
        mov r4, r1
        jsr pc, next
        mov r1, r4
        beq return

        mov r5, r1
        jsr pc, next
        jsr pc, next
        mov r1, r5
        beq return

        cmp r4, r5
        beq setret
        br loop

setret:
        inc r0

return:
        mov (sp)+, r5
        mov (sp)+, r4
        mov (sp)+, r1
        rts pc


next:
        tst r1
        beq abort
        mov (r1), r1
abort:
        rts pc


.= torg + 3000
list1: .word 3006, 3000, 3002, 3004, 0
res1: .word -1

.= torg + 3020
list2: .word 3030, 3026, 0, 3024, 3022
res2: .word -1

I can't understand this snippet, thanks in advance for everyone

mystery:
            mov r1, -(sp)
            mov r4, -(sp)
            mov r5, -(sp)

            clr r0

            mov 10(sp), r4
            mov r4, r5

Answer 1

It appears to be backing up registers 1, 4, and 5 and initializing register 0 (which doesn't need to be backed up). Since @mystery is the destination of a jsr, this is called prologue code. Then, they are initialized for the loop.

The old values are restored at @return.

As for what the whole program does, it appears to find cyclic links in a linked list.

bool is_invalid_list( link_node *l ) {
    while ( l && l->next && l->next->next ) {
        if ( l->next == l->next->next ) return true;
    }
    return false;
}

I don't think this is the simplest or best way to implement this, but not the worst either.