allocate-registers jump, compare
allocate-registers zero, one, two, nil
allocate-registers free
allocate-registers lst, end, cell, element

;; Initialize constant registers
    li zero, 0  ; zero is useful for copying one register's contents to another
    li one, 1   ; one is useful for allocating a memory location
    li two, 2   ; two is useful for allocating two consecutive memory locations
    li nil, -1  ; nil represents an empty list

;; Initialize the free register to point to the beginning of memory
    li free, 0  ; free points to the next available memory location

;; Read a list into memory: lst points to the start; end points to the end

    ; Allocate a memory location for a pointer to the start of the list
    add lst, free, zero
    add free, free, one

    ; The just-allocate memory location is the initial end of the list
    add end, lst, zero

read-element:
    ; Read a number to find out if there is another element in the list
    ; (0 means there is no more element in the list)
    read element
    li jump, read-end
    jeqz element, jump

    ; Read the next element in the list
    read element

    ; Allocate two consecutive memory locations for the new element and for
    ; a pointer to the rest of the list
    add cell, free, zero
    add free, free, two
    st element, cell
    st cell, end
    add end, cell, one

    ; Loop
    li jump, read-element
    j jump

read-end:
    ; Terminate the list: the rest of the list is the empty list
    st nil, end
    ld lst, lst

;; Print the list starting at lst

print:
    ; Stop if the list is empty
    seq compare, lst, nil
    li jump, print-1
    jeqz compare, jump
    halt

print-1:
    ; Print the current element
    ld element, lst
    write element

    ; Advance lst to the rest of the list
    add lst, lst, one
    ld lst, lst

    ; Continue with the rest of the list
    li jump, print
    j jump

test empty-list
    >>> 0

test singleton-list
    >>> 1
    >>> 3
    >>> 0
    3

test non-empty-list
    >>> 1
    >>> 3
    >>> 100
    >>> 10
    >>> -1
    >>> -2
    >>> 0
    3
    10
    -2