Assembly Language Example Programs
We studied the entire instruction set of 8086/88, pseudo-ops and assembler directives. We have explained the process of entering an assembly language program into a computer and coding it, for example preparing an EXE file from the source code. In this section, we will discuss about some programs which clarify the use of instructions, directives &some other facilities that are available in assembly language programming. After discussion of these programs, it is expected that one should have a clear idea regarding the use of different pseudo-ops and directives, their syntaxes as well under-standing the logic of each program. If someone writes an assembly language program and tries to code it, in the primary attempt, the chances are rare that it will be error free. Errors in programming depend upon the ability of the Programmer. A lot of practice is required to get skills in assembly language programming as comparison to high level languages. So, to get a command on assembly language one should write and execute a number of assembly programs, in addition studying the example programs given in this text.
Before starting the clarification of the written programs, we have to give details one more significant point, that is about the DOS function calls available under INT 21H instruction. DOS is an operating system (OS), which is a program that stands amongst bare computer system hardware and a user. It acts as a user interface that contains the available computer hardware resources. Also, it manages the hardware resources of the computer system. In the Disk OS, the hardware resources of the computer system like keyboard, memory, hard disk, CRT display and floppy disk drives can be handled with the help of the INT 21H instruction. The routines needed to refer to these resources are written as interrupt service routines for 21H interrupt. Under this interrupt, the particular resource is selected depending on the value in register AH. For instance, if AH contains 09H, then CRT display is to be utilized for displaying a message or if, AH register contain OAH, then the keyboard is to be accessed. These interrupts are called 'function calls' and the value that they contain in AH is called 'functions value'. In short, the reason of 'function calls' under INT 21 His to be determined by the 'function value' that is in AH. Some function values also manage the software operations of the machine. The list of the function values available under INT 21 H, their equivalent functions, the needed parameters and the returns are given in tabulated form in the Appendix-B. Make a note of that there are a number of interrupt functions in the DOS, but INT 21H is used more commonly. The readers can find other interrupts of BIOS and DOS from the respective technical references.