org     100h
        jmp     demo
        ;; Sudan function. BC=N, DE=X, HL=Y; output in HL.
sudan:  mov     a,b             ; N=0?
        ora     c
        jz      sdnbse
        mov     a,h             ; Y=0?
        ora     l
        jz      sdnbse
        push    b               ; Save N and Y (we don't need X)
        push    h
        dcx     h
        call    sudan           ; Calculate result of inner call
        xchg                    ; Set X = result of inner call
        pop     h               ; Get Y
        dad     d               ; Set Y = Y + result of inner call
        pop     b               ; Get N
        dcx     b               ; N = N-1
        jmp     sudan           ; Calculate result of outer call
sdnbse: dad     d               ; Return X+Y (base case)
        ret

        ;; Output routine (show 'sudan(N,X,Y) = value'), using CP/M call
show:   push    h!      push    d!      push    b!      ; Copies of args
        push    h!      push    d!      push    b!      ; For output
        lxi     d,sdt!  call    pstr                    ; Print call
        pop     h!      call    prhl
        lxi     d,sdc!  call    pstr
        pop     h!      call    prhl
        lxi     d,sdc!  call    pstr
        pop     h!      call    prhl
        lxi     d,sdi!  call    pstr
        pop     b!      pop     d!      pop     h!      ; Restore args
        call    sudan!  call    prhl                    ; Find and print result
        lxi     d,sdnl! jmp     pstr
prhl:   lxi     d,numbuf!       push    d!      lxi     b,-10
prdgt:  lxi     d,-1
pdiv:   inx     d!              dad     b!      jc      pdiv
        mvi     a,'0'+10!       add     l!      pop     h
        dcx     h!              mov     m,a!    push    h
        xchg!   mov     a,h!    ora     l!      jnz     prdgt
        pop     d
pstr:   mvi     c,9!    jmp     5

        ;; Set up big system stack (using CP/M)
demo:   lhld    6
        sphl
        ;; Show a couple of values
        lxi     b,0!    lxi     d,0!    lxi     h,0!    call    show
        lxi     b,1!    lxi     d,1!    lxi     h,1!    call    show
        lxi     b,2!    lxi     d,1!    lxi     h,1!    call    show
        lxi     b,3!    lxi     d,1!    lxi     h,1!    call    show
        lxi     b,2!    lxi     d,2!    lxi     h,1!    call    show
        rst     0
sdt:    db      'sudan($'
sdc:    db      ', $'
sdi:    db      ') = $'
        db      '.....'
numbuf: db      '$'
sdnl:   db      13,10,'$'