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Appendix A: Assembly Language Programming

INTRODUCTION

Assembly language is a computer language lying between the extremes of machine language and high-level

language like Pascal or C use words and statements that are easily understood by humans, although still a long

way from "natural" language.Machine language is the binary language of computers.A machine language program

is a series of binary bytes representing instructions the computer can execute.

Assembly language replaces the binary codes of machine language with easy to remember "mnemonics"that

facilitate programming.For example, an addition instruction in machine language might be represented by the

code "10110011".It might be represented in assembly language by the mnemonic "ADD".Programming with

mnemonics is obviously preferable to programming with binary codes.

Of course, this is not the whole story. Instructions operate on data, and the location of the data is specified by

various "addressing modes" emmbeded in the binary code of the machine language instruction. So, there may be

several variations of the ADD instruction, depending on what is added. The rules for specifying these variations

are central to the theme of assembly language programming.

An assembly language program is not executable by a computer. Once written, the program must undergo

translation to machine language. In the example above, the mnemonic "ADD" must be translated to the binary

code "10110011". Depending on the complexity of the programming environment, this translation may involve

one or more steps before an executable machine language program results. As a minimum, a program called an

"assembler" is required to translate the instruction mnemonics to machine language binary codes. Afurther step

may require a "linker" to combine portions of program from separate files and to set the address in memory at

which th program may execute. We begin with a few definitions.

An assembly language program i a program written using labels, mnemonics, and so on, in which each

statement corresponds to a machine instruction. Assembly language programs, often called source code or

symbolic code, cannot be executed by a computer.

A machine language program is a program containing binary codes that represent instructions to a computer.

Machine language programs, often called object code, are executable by a computer.

A assembler is a program that translate an assembly language program into a machine language program.

The machine language program (object code) may be in "absolute" form or in "relocatable" form. In the latter

case, "linking" is required to set the absolute address for execution.

A linker is a program that combines relocatable object programs (modules) and produces an absolute object

program that is executable by a computer. A linker is sometimes called a "linker/locator" to reflect its separate

functions of combining relocatable modules (linking) and setting the address for execution (locating).

A segment is a unit of code or data memory. A segment may be relocatable or absolute. A relocatable

segment has a name, type, and other attributes that allow the linker to combine it with other paritial segments,

if required, and to correctly locate the segment. An absolute segment has no name and cannot be combined with

other segments.

A module contains one or more segments or partial segments. A module has a name assigned by the user. The

module definitions determine the scope of local symbols. An object file contains one or more modules. A module

may be thought of as a "file" in many instances.

A program consists of a single absolute module, merging all absolute and relocatable segments from all input

modules. A program contains only the binary codes for instructions (with address and data constants) that are

understood by a computer.

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