STC89LE53RC datasheet(163/165 Pages) STC

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Data memory type values stored in first byte of untyped pointers

Value

Data Memory Type

idata

xdata

pdata

data/bdata

code

FUNCTIONS

In programming the 8051 in assembly, we learnt the advantages of using subroutines to group together common

and frequently used instructions. The same concept appears in 8051 C, but instead of calling them subroutines, we

call them functions. As in conventional C, a function must be declared and defined. A function definition includes

a list of the number and types of inputs, and the type of the output (return type), puls a description of the internal

contents, or what is to be done within that function.

The format of a typical function definition is as follows:

return_type function_name (arguments) [memory] [reentrant] [interrupt] [using]

{

…

}

where

return_type

refers to the data type of the return (output) value

function_name

is any name that you wish to call the function as

arguments

is the list of the type and number of input (argument) values

memory

refers to an explicit memory model (small, compact or large)

reentrant

refers to whether the function is reentrant (recursive)

interrupt

indicates that the function is acctually an ISR

using

explicitly specifies which register bank to use

Consider a typical example, a function to calculate the sum of two numbers:

int sum (int a, int b)

{

return a + b;

}

This function is called sum and takes in two arguments, both of type int. The return type is also int, meaning that

the output (return value) would be an int. Within the body of the function, delimited by braces, we see that the

return value is basically the sum of the two agruments. In our example above, we omitted explicitly specifying the

options: memory, reentrant, interrupt, and using. This means that the arguments passed to the function would be

using the default small memory model, meaning that they would be stored in internal data memory. This function

is also by default non-recursive and a normal function, not an ISR. Meanwhile, the default register bank is bank 0.

Parameter Passing

In 8051 C, parameters are passed to and from functions and used as function arguments (inputs). Nevertheless, the

technical details of where and how these parameters are stored are transparent to the programmer, who does not

need to worry about these techinalities. In 8051 C, parameters are passed through the register or through memory.

Passing parameters through registers is faster and is the default way in which things are done. The registers used

and their purpose are described in more detail below.

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