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Assembly Language

high level language support

summary

    This web page examines high level language support instructions in assembly language. Specific examples of instructions from various processors are used to illustrate the general nature of assembly language.

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assembly/high level language interface

    Because so many search requests are asking about how to interface between high level languages and assembly language, I have added this brief discussion.

    Unfortunately, I can only provide you with some general guidelines rather than specific details. You will need to do further research on your own. If your high level compiler allows you to see an assembly language representation of its output, you should be able to figure out the details on your own.

    Each processor has its own conventions on subroutine and function linkage. Each high level language also has its own conventions regarding subroutine and function linkage. Some operating systems add their own conventions to the mix. As these three sets of conventions are often at odds, you will need to do your own research to determine how any conflicts in conventions are resolved on your system (again, if your compiler produces an assembly language representation, you can use these listings and carefully crafted test code to determine your local rules).

    C treats all subroutines as functions. Unless otherwise declared, each function returns an integer (which is usually the default size of a word on the processor). Parameters are passed by value by pushing the data onto the stack in right to left order (based on the function declaration). Objects less than four bytes (such as boolean, integer, and character) are sign-extended to four bytes. C arrays are passed by pointer. Sometimes C will break the rule and instead pass a pointer for a large data item (such as C arrays), so you may want to examine sample object code to see how various large data structures are handled. Depending on the processor, C function results are returned on the top of the stack (typically the space for the function return value is allocated on the stack after all of the parameters have been passed) or in a register (usually the default scratch register for that processor). The calling routine is responsible for removal of parameters.

    Pascal has both subroutines and functions. For subroutines, there is no space allocated on the stack for a return value, while for functions the space for the return value is allocated on the stack after all of the parameters are passed. Parameters are pushed onto the stack in left to right order (based on the subroutine or function declaration). Parameters that are 32 bits or smaller are passed by value (the actual data is placed on the stack), while parameters that are greater than 32 bits are passed by reference (a pointer to the data is placed on the stack). Parameters of variable length are always passed by reference (a pointer) regardless of actual size (this applies in particular to pascal strings). If a data type is not an exact multiple of a byte (such as a bit string or set or certain kinds of ennumerated data), then the data element will be rounded up to the nearest byte, usually with zero padding in the high order bits (check your compiler output). Booleans are passed as a single byte (0 or 1), but boolean function results are returned as a pair of bytes with the boolean in the high byte. VARs are passed as four byte pointers. On some processors, data types that are not exact multiples of 16 or 32 bits are zero padded (high order bytes) to a 16 bit or 32 bit size (again, check your compiler output). The called routine is normally responsible for removal of parameters (other than a function return parameter).

high level language support

    Many processors have instructions designed to support constructs common in high level languages. Ironically, a few high level language constructs have been based on specific hardware instructions on specific processors. One famous example is the computed GOTO (three possible branches based on whether the tested value is positive, zero, or negative), which is based on a hardware instruction in an early IBM processor (and if anyone can loan or give me a data book on the processor, I sure would appreciate it).

    Most modern processors have some kind of loop instructions. These are some variation on the theme of testing for a condition and/or making a count with a short branch back to complete a loop if the exit condition fails.

    Many modern processors have some kind of hardware support for temporary data storage (for the temporary variables used in subroutines and functions), combining special hardware instructions with argument and/or frame and/or stack pointers.

    Bounds check instructions are used to check if an array reference is out of bounds.

    Digital Equipment Corporation’s “VAX Architecture Reference Manual” gives the following example of the CASE instruction using the VAX PASCAL compiler (quoted under the fair use doctrine):

    case i of
        32:         x := sin(x);
        33:         x := cos(x);
        34:         x := exp(x);
        35:         x := ln(x);
        36, 37:     x := arctanh(x);
        otherwise   x := reserved
    end

          casel   i, #32, <#37-32>
    1$:   .word   sin - 1$      ; Selector is 32.
          .word   cos - 1$      ; Selector is 33.
          .word   exp - 1$      ; Selector is 34.
          .word   ln - 1$       ; Selector is 35.
          .word   arctanh - 1$  ; Selector is 36.
          .word   arctanh - 1$  ; Selector is 37.
    otherwise:
          movl    reserved, x   ; Selector is less than.
                                ; 32 or greater than 37

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    Created: February 21, 2001 (from machcode.htm)

    Last Updated: December 1, 2010


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