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The assembler push and pop instructions

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• What happens with the Program Stack when we push a value onto the Program Stack
  • Suppose the Program Stack is as follows before we push a new value:

    The stack pointer SP points at the top of the program stack

    All memory cells that lies below the memory location pointed to by SP are used (= reserved/allocated)

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  • The state the Program Stack will be like this after we push a new value X:

    The stack pointer will move up to indicate that the space is reserved (for the variable x).

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• Academic reminder
  • For simplicity, we will only store int typed values on the program stack

    (The ARM processor can only push the value in a whole register (= 32 bits) on the stack.

    If you push a data that consists of less than 32 bits, then you (as an assembler programmer) need to manage that yourself.

    To avoid this tedious task, I will only push int typed (= 32 bits) on the stack....)

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• ARM (assembler) instruction to push a value on the Program Stack
  • Syntax:

    push {regName} // Push the value in "regName" onto the program stack

    Note:

    The push instruction in ARM can push multiple registers with a single push instruction, but you must specify the list of registers in a specific order. To minimize the amount of knowledge that you need to learn (in order to write assembler program), I will only show you how to push 1 register)

    Example:

    main: mov r0, #4 push {r0} // Push value in r0 on stack mov r0, #9 push {r0} // Push value in r0 on stack

    Result after the first push instruction (snapshot from EGTAPI's stack area)

    Notice that the stack pointer register SP (= 0x7affc) is pointing to the top of the program stack

    Result after the second push instruction (snapshot from EGTAPI's stack area)

    Notice that the stack pointer register SP (= 0x7aff8) is updated and still points to the top of the program stack

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/asm/8-sub/push.s

    How to run the program:

    To compile:   as255 push              To run: use EGTAPI

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• Popping values off the Program Stack
  • ARM (assembler) instruction to pop an int typed value from the (top of the) program stack:

    pop {regName} // Pop the int typed value from the top // of the program stack and store it in register "regName"

    Example:

    main: mov r0, #4 push {r0} mov r0, #9 push {r0} pop {r1} // r1 = 9 pop {r2} // r2 = 4

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/asm/8-sub/push.s

    How to run the program:

    To compile:   as255 push              To run: use EGTAPI

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• De-allocating variables from the Program Stack
  • When memory used to stored values of a variable is no longer needed, the memory that was reserved for the variable must be de-allocated (= un-reserved)

    Suppose the following is the state of the Program Stack after we have reserved some memory for the variable x:

    If we want to de-allocate the memory space used for variable x and discard the value in the de-alocated variable, the resulting stack will look like this:

    In this example, we assume that the value in the variable x is no longer needed nor used in future computation.

    I.e.: we discard the variable stored in the variable x

    If we only want to de-allocate some space on the syatem stack, there is a more efficient way to do this task (rather than using the pop instruction !)

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  • In order to de-allocate the space used by some variables that are stored on the program stack, we can simple increase the stack pointer (which will change the stack top that marks the boundary of the reserved memory area !!!)

    Example: to de-allocate one int typed variable from the (top of the) program stack, we use:

    add sp, sp, #4 // Move program stacktop down 4 bytes // This will remove the reservation of 4 bytes // that was used to stored the int typed variable

    Example: to de-allocate two int typed variable from the (top of the) program stack, we use:

    add sp, sp, #8 // Move program stacktop down 8 bytes // This will remove the reservation of 8 bytes // that was used to stored TWO int typed variable

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• Functions of the program stack pointer register SP
  • The stack pointer SP has 2 important functions:

    SP indicate the top of the stack; as such: SP points to the location in the stack that the push and pop operations will operate Another function of SP is: SP marks the memory locations that are reserved (= currently used !!!) Specifically: +----------+ | | | | AVAILABLE !!!! (not used) | | SP -->+----------+ | | ^ | | | USED !!!! | | | | Program | | (= reserved !!!!) | stack | | | | | | | v +----------+

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  • Therefore:

    When you perform a push operation (= increase the stack size), you will: create one or more variables (on the stack !!!) When you perform a pop operation (= decrease the stack size), you will:          destroy one or more variables (on the stack !!!)