TSR in C

What does TSR stands for?
 TSR stands for Terminate- and Stay-Resident programs
What’s Special about TSR!!!!!!!!!!
As the name says Terminate-and-Stay-Resident TSR are programs which get loaded in memory and remain or stay there (resident) in memory permanently. They will be removed only when the computer is rebooted or if the TSR is explicitly removed from memory. Until then they will stay (resident) in memory active
Working Technology of TSR
Nothing happens because of TSR occupying place in memory. In other words when TSR is not running it does not affect the running of other DOS programs. It stays in memory in idle state. Only thing it does is it occupies memory space and the occupied TSR memory space cannot be occupied by other programs. So users have to take care of this aspect when writing TSR’s. Since when number of TSR’s is written then each occupies memory space and these spaces cannot be occupied by other programs. So design the TSR’s taking this aspect into consideration.
 Now let us see the working technology of TSR.
 We know that TSR gets loaded into memory and stays there. When number of TSR’s is loaded the TSR gets loaded in the structure of STACK. This is nothing but the TSR gets piled up one above the other with the latest TSR loaded on the top. So one may get amazed how to activate a particular TSR if there are number of TSR available in stack.
 This is done by activating keys which gets associated with TSR. When the particular key associated with TSR gets activated then the corresponding TSR is called which means the program that is associated with that TSR from memory (which is already present in memory) gets activated. Now the next question that pops into one’s mind is how the keys and TSR gets associated. In other words when a key is presses how is the TSR corresponding to it gets activated.
This is done if we understand the simple technology of IVT named as Interrupt Vector Table. Let us see in detail how it works.

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