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Inserting a list element at the start of a linked list

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• Review: Java statements to insert at the start of a linked list
  • Suppose we have an existing linked list at head and a list element at ptr:

    The Java statements (that you learned in CS171) to insert the list element at the start of the list are:

    /* ======================================================== Statements to insert list element "ptr" at start of list ======================================================== */ ptr.next = head; // Make ptr.next points to first element of list head = ptr; // Make head points to the new list element

    The result of these statements is:

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/asm/4-linked-list/ListInsert.java

    How to run the program:

    To compile:   javac ListInsert.java             To run:          java ListInsert

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• Assembler code to insert a list element at the start of a list
  • When you write the Java statements in (ARM) assembler, you will see clearly how an list element is inserted in a list:

    The references (= addresses) in certian field are updated to make the new order chaining of list elements !!!

    I will show you the assembler program below.

    But before I do so, I want to make sure that you realize:

    When you write an assembler program, you will make copies of a variable in the registers in the CPU Warning: We you update a register, you will not update the memory variable !!! (You only update a copy, not the original !!!) You must update the program variables that are stored in the memory !!!

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  • I will skip the linked list definition, because I explained that already.

    I will assume that the list has been defined and I will only show you the assembler code to insert a new list element into an existing list.

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  • The assembler program that accomplishes the statements:

    ptr.next = head; head = ptr;

    is as follows:

    main: /* ------------------------------------- ptr.next = head; -------------------------------------- */ // Get the value in the RHS movw r0, #:lower16:head movt r0, #:upper16:head // r0 = addr(head) ldr r0, [r0] // r0 = head // Get the ADDRESS of the LHS movw r1, #:lower16:ptr movt r1, #:upper16:ptr // r1 = addr(ptr) ldr r1, [r1] // r1 = ptr // Note: Addr(ptr.next) = r1+4, use base+offset mode !! // Store the RHS in the addr of LHS str r0, [r1, #4] // Update variable in memory !! /* ------------------------------------- head = ptr; -------------------------------------- */ // Get the value in the RHS movw r0, #:lower16:ptr movt r0, #:upper16:ptr // r0 = addr(ptr) ldr r0, [r0] // r0 = ptr // Get the ADDRESS of the LHS movw r1, #:lower16:head movt r1, #:upper16:head // r1 = addr(head) // Store the RHS in the addr of LHS str r0, [r1] // Update variable in memory !!

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/asm/4-linked-list/list-insert.s

    How to run the program:

    To compile:   as255 list-insert              To run: use EGTAPI