说明：  PIC16F877单片机运算子程序，包含 PIC16F877汇编语言程序主体框架、 四则运算子程序、 3字节浮点四则运算子程序、定点数与浮点数转换程序
(PIC16F877 microcontroller computing subroutine that contains the main framework of the PIC16F877 assembly language program, arithmetic subroutine, the 3-byte floating-point arithmetic routines, fixed-point and floating-point conversion process)