Interrupts internal external hardware and software interruptions execution of a program computer virtual internal clock peripheral devices coprocessors CPU integrated circuit assembler INT Instruction DOS BIOS


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Internal Hardware interruptions

Internal interruptions are generated by certain events which come up during the execution of a program. This type of interruptions is managed on their totality by the hardware and it is not possible to modify them.

A clear example of this type of interruptions is the one which actualizes the counter of the computer internal clock, the hardware makes the call to this interruption several times during a second in order to maintain the time up to date.

Though we cannot directly manage this interruption, since we cannot control the time updating by means of software yet it is possible to use its effects on the computer to our benefit. For example to create a virtual clock updated continuously we only have to write a program which reads the actual value of the counter and to translate it into an understandable format for the user.

External Hardware Interruptions

External interruptions are generated by peripheral devices, such as keyboards, printers, communication cards, etc. They are also generated by coprocessors. It is not possible to deactivate external interruptions.

These interruptions are not sent directly to the CPU but they are sent to an integrated circuit whose function is to exclusively handle this type of interruptions

Software Interruptions

Software interruptions can be directly activated by the assembler invoking the number of the desired interruption with the INT Instruction.

The use of interruptions helps us in the creation of programs and by using them our programs gets shorter. It is easier to understand them and they usually have a better performance mostly due to their smaller size. This type of interruptions can be separated in two categories: the operative system DOS interruptions and the BIOS interruptions.

The difference between the two is that the operative system interruptions are easier to use but they are also slower since these interruptions make use of the BIOS to achieve their goal, on the other hand the BIOS interruptions are much faster but they have the disadvantage that since they are part of the hardware, they are very specific and can vary depending even on the manufacturer brand of the circuit.

The election of the type of interruption to use will depend solely on the characteristics you want to give your program.

Since we shall use interrupts for data recovery programming with the help of C language via Interrupt handling with C, we shall discuss only Interrupt 13H, Interrupt 13H Extensions and Interrupt 21H routines specially. It is not so important to discuss all of the other interrupts and their functions because in C language, easier functions are available to perform most of those tasks. However, the knowledge of Interrupt 13H and its Extensions is must, for data recovery programming.

Let us take a brief introduction of Interrupt 20H and Interrupt 21H. The value written in brackets (like 0x20) indicates, how to use

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