al_fixed man page
al_fixed — Allegro 5 API
#include <allegro5/allegro.h> typedef int32_t al_fixed;
A fixed point number.
Allegro provides some routines for working with fixed point numbers, and defines the type
al_fixed to be a signed 32-bit integer. The high word is used for the integer part and the low word for the fraction, giving a range of -32768 to 32767 and an accuracy of about four or five decimal places. Fixed point numbers can be assigned, compared, added, subtracted, negated and shifted (for multiplying or dividing by powers of two) using the normal integer operators, but you should take care to use the appropriate conversion routines when mixing fixed point with integer or floating point values. Writing
fixed_point_1 + fixed_point_2 is OK, but
fixed_point + integer is not.
The only advantage of fixed point math routines is that you don't require a floating point coprocessor to use them. This was great in the time period of i386 and i486 machines, but stopped being so useful with the coming of the Pentium class of processors. From Pentium onwards, CPUs have increased their strength in floating point operations, equaling or even surpassing integer math performance. However, many embedded processors have no FPUs so fixed point maths can still be useful there.
Depending on the type of operations your program may need, using floating point types may be faster than fixed types if you are targeting a specific machine class.