CS255 Syllabus

Insert at Start of a Linked List

The List class used in example

We will use the following List class:

public class List { int value; List next; }

Inserting at the start (head) of a linked list

Suppose you have a linked list at the List variable head:

head +--------+ +--------+ -->+--------+ -->+--------+ | |---->| value1 | / | value2 | / | value3 | +--------+ | | / | | / | | +--------+ / +--------+ / +--------+ | next1 |- | next2 |- | null | +--------+ +--------+ +--------+

And you have a linked list element at "elem":

elem +--------+ +--------+ | |---->| valueX | (information has been filled in) +--------+ | | +--------+ | ???? | (don't care, because we know there +--------+ is no "next" element)

To make thing more concrete, here is how the linked list (at head) and the list element may be stored:

Memory: +---------------+ head: | 8000 |-------------------------------+ +---------------+ | | | | ....... | ....... | +---------------+ | 8000 | value1 | Linked list element 1 <-----+ +---------------+ | next1=10000 |-------------------------------------+ +---------------+ | | | | +---------------+ | ....... | ....... | +---------------+ | 9000 | value3 | Linked list element 3 <----+ | +---------------+ | | | next3=0 (null | | | +---------------+ | | ....... | | ....... | | +---------------+ | | 10000 | value2 | Linked list element 2 | <-----+ +---------------+ | | next2=9000 |-----------------------------+ +---------------+ ....... ....... +---------------+ elem: | 20000 |-----------------------------+ +---------------+ | ....... | ....... | +---------------+ | 20000 | valueX | New linked list element <---+ +---------------+ | nextX=???? | +---------------+

Note:

You must use head to access the list elements !!!!

Inserting the element pointed by the List variable elem at the start of the linked means:

To make the element pointed by the List variable elem as the first element in the linked list at head

Graphically:

Before the insertion:

After the insertion:

head +--------+ +--------+ -->+--------+ -->+--------+ | |-+ +->| value1 | / | value2 | / | value3 | +--------+ | | | | / | | / | | | | +--------+ / +--------+ / +--------+ | | | next1 |- | next2 |- | null | | | +--------+ +--------+ +--------+ | | elem | +---------+ +--------+ | +--------+ | | |-+-->| valueX | | +--------+ | | | +--------+ | | nextX |-+ +--------+

This is what will actually happen inside the memory using the concrete example above:

Memory: +---------------+ head: | 20000 |---------------------------------------+ +---------------+ | | | | ....... | ....... | +---------------+ | 8000 | value1 | Linked list element 1 <------+ | +---------------+ | | | next1=10000 |-------------------------------------+ | +---------------+ | | | | | | | | +---------------+ | | | ....... | | | ....... | | | +---------------+ | | | 9000 | value3 | Linked list element 3 <----+ | | | +---------------+ | | | | | next3=0 (null)| | | | | +---------------+ | | | | ....... | | | | ....... | | | | +---------------+ | | | | 10000 | value2 | Linked list element 2 | <-----+ | +---------------+ | | | | next2=9000 |-----------------------------+ | | +---------------+ | | ....... | | ....... | | +---------------+ | | elem: | 20000 |-----------------------------+ | | +---------------+ | | | ....... | | | ....... | | | +---------------+ | | | 20000 | valueX | new linked list element <---+---------+ +---------------+ | | nextX=8000 |----------------------------------+ +---------------+

Review: insert at start of list in Java

Main method:

List head; // Assume head has a list already List elem; main( ) { ptr = new List( ); // Make a new list element // Effect: // 1. reserve 8 bytes of memory // to store a List object // 2. return the address of the // location of the reserved memory ptr.value = 1234; // Assignment some value head = Insert(head, ptr); // Insert into list }

The insert method:

static List Insert(List head, List newelem) { newelem.next = head; return(newelem); }

Example Program: (Demo above code)
- Prog file: click here
How to run the program:

Allocating memory for a List element

The effect of the new operator:

new List( ) will:

Simulating the new operator:

I have provided a function (called malloc) in the cs255 assembler library to simulate the new operator

The malloc( ) function:

Input parameter: D0 = number of bytes that need to be allocated (i.e., reserved) Output parameter: A0 = location (address) of the start of reserved memory

The main function in M68000 code

Main( ) in Java

ptr = new List(); // Make a new list element // Effect: // 1. reserve 8 bytes of memory // to store a List oject // 2. return the address of the // location of the reserved memory ptr.value = 1234; // Assignment some value head = Insert(head, ptr); // Insert into list

Main( ) in M68000 code:

**** ptr = new List(); move.l #8, d0 ; 8 byte used to store a List object jsr malloc ; allocate (8 bytes) of memory move.l a0, ptr ; ptr = address of the allocated memory **** ptr.value = 1234; move.l #1234, (a0) **** head = Insert(head, ptr); move.l head, -(a7) ; pass head on stack move.l ptr, -(a7) ; pass ptr on stack bsr InsertList ; call InsertList adda.l #8, a7 ; clean up parameters move.l d0, head ; head = return value

The insert at head method in M68000

The Insert() function in Java:

static List Insert(List head, List newelem) { newelem.next = head; return(newelem); }

The Insert() function in M68000 code:

Stack Frame: (for your reference) +--------------+ <------ a6 | saved a6 | 0(a6) +--------------+ | return addr | 4(a6) +--------------+ | newelem | 8(a6) +--------------+ | head | 12(a6) +--------------+
InsertList: * Prelude move.l a6, -(a7) * Save a6 move.l a7, a6 * Set up my own a6 suba.l #0, a7 * No local variable needed.... * nexelem.next = head; move.l 12(a6),d0 * d0 = head movea.l 8(a6), a0 * a0 = newelem move.l d0, 4(a0) * newelem.next = head * return(newelem); move.l 8(a6), d0 * Put newelem in return location * Postlude movea.l a6, a7 * De-allocate the local variables movea.l (a7)+, a6 * Restore a6 for caller rts

Example Program: (Demo above code)
- Prog file: list1-stack.s in /home/cs255000/demo/asm/list1-stack.s