Hardware Interaction, DOS Perspective in C programming

Under DOS whenever an external event (like pressing a key or ticking of timer) occurs a signal called hardware interrupt gets generated. For different events there are different interrupts. As a reaction to the occurrence of an interrupt a table called Interrupt Vector Table (IVT) is looked up. IVT is present in memory. It is populated with addresses of different BIOS routines during booting. Depending upon which interrupt has occurred the Microprocessor picks the address of the appropriate BIOS routine from IVT and transfers execution control to it. Once the control reaches the BIOS routine, the code in the BIOS routine interacts with the hardware. Naturally, for different interrupts different BIOS routines are called. Since these routines serve the interrupts they are often called ‘Interrupt Service Routines’ or simply ISRs.

If we want that instead of the default ISR our routine should get called then it is necessary to store the address of this routine in IVT. Once this is done whenever a hardware interrupt occurs our routine’s address from IVT is picked up and the control is transferred to our routine. For example, we may register our ISR in IVT to gain finer control over the way key-hits from the keyboard are tackled. This finer control may involve changing codes of keys or handling hitting of multiple keys simultaneously.
Explicit communication with the hardware can be done in four different ways.

Let us now discuss the pros and cons of using these different methods to interact with the hardware.


(a) Calling DOS Functions

To interact with the hardware a program can call DOS functions. These functions can either directly interact with the hardware or they may call BIOS functions which in turn interact with the hardware. As a result, the programmer is not required to know all the hardware details to be able to interact with it. However, since DOS functions do not have names they have to be called through the mechanism of interrupts. This is difficult since the programmer has to remember interrupt service numbers for calling different DOS functions. Moreover, communication with these functions has to be done using CPU registers. This leads to lot of difficulties since different functions use different registers for communication. So one has to know details of different CPU registers, how to use them, which one to use when, etc.

(b) Calling BIOS Functions

DOS functions can carry out jobs like console I/O, file I/O, printing, etc. For other operations like generating graphics, carrying out serial communication, etc. the program has to call another set of functions called ROM-BIOS functions. Note that there are some functions in ROM-BIOS that do same jobs as equivalent DOS functions. BIOS functions suffer from the same difficulty as DOS functions—they do not have names. Hence they have to be called using interrupts and involve heavy

usage of registers

(c) Calling Library Functions

We can call library functions which in turn can call DOS/BIOS functions to carry out the interaction with hardware. Good examples of these functions are printf( ) / scanf( ) / getch( ) for interaction with console, absread( ) / abswrite( ) for interaction with disk, bioscom( ) for interaction with serial port, etc. But the library doesn’t have a parallel function for every DOS/BIOS function. Hence at some point of time one has to learn how to call DOS/BIOS functions.

(d) Directly interacting with the hardware 

At times the programs are needed to directly interact with the hardware. This has to be done because either there are no library functions or DOS/BIOS functions to do this, or if they are there their reach is limited. For example, while writing good video games one is required to watch the status of multiple keys simultaneously. The library functions as well as the DOS/BIOS functions are unable to do this. At such times we have to interact with the keyboard controller chip directly.

However, direct interaction with the hardware is difficult because one has to have good knowledge of technical details of the chip to be able to do so. Moreover, not every technical detail about how the hardware from a particular manufacturer works is well documented.