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Data alignment requirement

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• Review
  • As discussed in the Structure of the computer memory page ( click here):

    Due to the way the computer memory is wired, some combination of adjacent memory cells can be combine together while other combinations are not possible

    Because of this limitation, we must place memory variables that consist of multiple cells (bytes) at certain memory locations

    We call these placement restrictions:

    Memory alignment requirements

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• Memory alignment requirements
  • In many computer system, the following memory alignment requirements are imposed:

    A variable of a data type that take up 4 bytes of memory must be place at a memory address that is divisible by 4 int typed and float typed variables must start at a memory address that is divisible by 4 A variable of a data type that take up 2 bytes of memory must be place at a memory address that is divisible by 2 (i.e.: even address) short typed variables must start at a memory address that is divisible by 2 There are no alignment requirements for byte typed variables. These type of variables can be place anywhere in memory.

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• Assembler directive to help programmer meet the memory alignment requirement
  • The .align directive:

    .align n // If the current memory address is divisible by 2n, // this directive is ignored // Otherwise, the assembler will skip forward to // the next memory address that is divisible by 2n

    Example usage:

    Suppose you want to define 1 byte variable (b = 4) and 1 int varriable (i = 44) The naive solution is: .data b: .byte 4 i: .4byte 44 This will not work !!! Reason: The .data segment starts out as align (address 0 is divisible by every number) After the definition: b: .byte 4 the assembler allocates 1 byte for the variable b. And now the current address = 1 which is not divisible by 4 Therefore, the next variable definition: i: .4byte 44 will put the int variable at an address that is not divisible by 4 !!! Solution: align each variable definition by its requirement alignment So: .data b: .byte 4 // byte typed variable has no alignment requirement .align 2 // Skip to an address that is div by 4 (= 22) first i: .4byte 44

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    I want to show you the effect of misaligned data definition here.

    Although you don't have assembler programming, you can tell that the values used in the variables are 4 and 44.

    In class, I will show you the execution of this program:

    /home/cs255001/demo/asm/2-mov/align.s

    In one case, I will not use the .align directive in the program - and when I run it; you will see that the value 44 will fail to be loaded into the CPU.

    In the second case, I will use the .align directive in the program - and when I run the same program; and you will see that the value 44 will appear in register r2 of the CPU.