Page 31 - ARM Based Microcontroller Projects Using MBED

2.3 EXAMPLES 17

FIG. 2.13 Calling subprogram DISPLAY.

2.2.8 Calling a Subprogram
A subprogram can be called from the main program or from another subprogram. In PDLs,
a subprogram is called by using the keyword CALL, followed by the name of the subprogram.
In flow charts it is common practice to insert vertical lines at the two sides of the box where the
subprogram is called. Fig. 2.13 shows an example of where subprogram DISPLAY is called
from a program.

2.3 EXAMPLES

Some simple examples are given in this section to show how the PDLs and their equivalent
flow charts can be used in program development.

EXAMPLE 2.1
It is required to write a program to convert hexadecimal numbers “A” to “F” into decimals. Show the
algorithm using a PDL and also draw the equivalent flow chart. Assume that the hexadecimal num-
ber to be converted is called HEX_NUM, and the output decimal number is called DEC_NUM.
Solution 2.1
The required PDL is:
BEGIN
IF HEX_NUM ¼ “A” THEN
DEC_NUM ¼ 10
ELSE IF HEX_NUM ¼ “B” THEN
DEC_NUM ¼ 11
ELSE IF HEX_NUM ¼ “C” THEN
DEC_NUM ¼ 12
ELSE IF HEX_NUM ¼ “D” THEN
DEC_NUM ¼ 13