Page 336 - Programming Microcontrollers in C

Table Look-Up 321

Each table entry contains four bytes. The first byte is the inde­
pendent variable value, often designated as x. The second byte
contains the dependent variable value, usually called y, and the final
two bytes contain a floating point format of the slope. The slope
must be floating point. The third byte contains the integer position of
the slope, and the fourth byte contains the fractional value of the
slope. With the binary point placed between the third and fourth bytes
of the table entry, we can accomplish some interesting multiply op­
erations. Prior to looking at the coding of this problem, let us complete
the table and convert the slopes into hexadecimal format.

5
20 5 0x00 0x80
40 15 0x00 0xe0
80 50 0x01 0xc1
120 120 0x02 0x00
180 240

Table 6-5: Look-Up Table with Hexadecimal Slopes

How does one build a table of this nature in C? Of course, it is an
array of structures, or it could be a structure that contains an array of
structures. The latter approach would be

struct entry
{
char x;
char y;
int slope;
};

struct lut
{
char entries;
struct entry data[5];
} _lut = {
5,
20, 5, 0x0080,
40, 15, 0x00e0,