How to Develop A Program

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This section presents an overview of the Ax51 macro assembler, Lx51

linker/locater and how it is used.

What is an Assembler?

An assembler is a software tool designed to simplify the task of writing computer

programs. It translates symbolic code into executable object code. This object

code may then be programmed into a microcontroller and executed. Assembly

language programs translate directly into CPU instructions which instruct the

processor what operations to perform. Therefore, to effectively write assembly

programs, you should be familiar with both the microcomputer architecture and

the assembly language.

Assembly language operation codes (mnemonics) are easily remembered (MOV

for move instructions, ADD for addition, and so on). You can also symbolically

express addresses and values referenced in the operand field of instructions.

Since you assign these names, you can make them as meaningful as the

mnemonics for the instructions. For example, if your program must manipulate a

date as data, you can assign it the symbolic name DATE. If your program

contains a set of instructions used as a timing loop (a set of instructions executed

repeatedly until a specific amount of time has passed), you can name the

instruction group TIMER_LOOP.

An assembly program has three constituent parts:

Machine instructions

Assembler directives

Assembler controls

Keil Software — A51/AX51/A251 Macro Assembler and Utilities

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Assembler Directives” on page 99 for details on all of the assembler directives.

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Assembler controls set the assembly modes and direct the assembly flow.

“Chapter 7. Invocation and Controls” on page 195 contains a comprehensive

guide to all the assembler controls.

Modular Programming

Many programs are too long or complex to write as a single unit. Programming

becomes much simpler when the code is divided into small functional units.

Modular programs are usually easier to code, debug, and change than monolithic

programs.

The modular approach to programming is similar to the design of hardware that

contains numerous circuits. The device or program is logically divided into

“black boxes” with specific inputs and outputs. Once the interfaces between the

units have been defined, the detailed design of each unit can proceed separately.

The benefits of modular programming are:

Efficient Program Development: programs can be developed more quickly

with the modular approach since small subprograms are easier to understand,

design, and test than large programs. With the module inputs and outputs

defined, the programmer can supply the needed input and verify the correctness

of the module by examining the output. The separate modules are then linked

and located by the linker into an absolute executable single program module.

Finally, the complete module is tested.

Multiple Use of Subprograms: code written for one program is often useful in

others. Modular programming allows these sections to be saved for future use.

Because the code is relocatable, saved modules can be linked to any program

which fulfills their input and output requirements. With monolithic

programming, such sections of code are buried inside the program and are not so

available for use by other programs.

Ease of Debugging and Modifying: modular programs are generally easier to

debug than monolithic programs. Because of the well defined module interfaces

of the program, problems can be isolated to specific modules. Once the faulty