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Notice that both the assembly and C language solutions for the above example require almost the same number

of lines. However, the difference lies in the readability of these programs. The C version seems more human than

assembly, and is hence more readable. This often helps facilitate the human programmer's efforts to write even

very complex programs. The assembly language version is more closely related to the machine code, and though

less readable, often results in more compact machine code. As with this example, the resultant machine code from

the assembly version takes 83 bytes while that of the C version requires 149 bytes, an increase of 79.5%!

The human programmer's choice of either high-level C language or assembly language for talking to the

8051, whose language is machine language, presents an interesting picture, as shown in following figure.

Human language

Eg. English, Malay, Chinese

Machine language

Eg. 10011101 0101010101

Complier

Assembler

C (high-level) language

Eg. for (x=0; x<9; x++)...

Assembly language

Eg. MOV, ADD, SUB

Conversion between human, high-level, assembly, and machine language

8051 C COMPILERS

We saw in the above figure that a complier is needed to convert programs written in 8051 C language into

machine language, just as an assembler is needed in the case of programs written in assembly language. A

complier basically acts just like an assembler, except that it is more complex since the difference between C and

machine language is far greater than that between assembly and machine language. Hence the complier faces a

greater task to bridge that difference.

Currently, there exist various 8051 C complier, which offer almost similar functions. All our examples

and programs have been compiled and tested with Keil's μ Vision 2 IDE by Keil Software, an integrated 8051

program development envrionment that includes its C51 cross compiler for C. A cross compiler is a compiler that

normally runs on a platform such as IBM compatible PCs but is meant to compile programs into codes to be run

on other platforms such as the 8051.

DATA TYPES

8051 C is very much like the conventional C language, except that several extensions and adaptations have been

made to make it suitable for the 8051 programming environment. The first concern for the 8051 C programmer is

the data types. Recall that a data type is something we use to store data. Readers will be familiar with the basic C

data types such as int, char, and float, which are used to create variables to store integers, characters, or floating-

points. In 8051 C, all the basic C data types are supported, plus a few additional data types meant to be used

specifically with the 8051.

The following table gives a list of the common data types used in 8051 C. The ones in bold are the specific

8051 extensions. The data type bit can be used to declare variables that reside in the 8051's bit-addressable

locations (namely byte locations 20H to 2FH or bit locations 00H to 7FH). Obviously, these bit variables can only

store bit values of either 0 or 1. As an example, the following C statement:

bit flag = 0;

declares a bit variable called flag and initializes it to 0.

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