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The writing of TSR is just a matter of stealing interrupts, reassigning the address in the IVT to point to the function we want to execute and keeping the program resident in memory.

The following C functions will be used to do the above.

getvect()
setvect()
keep()

The following example is a TSR which makes caps lock permanently high.

#include 

void interrupt(*old)();
void interrupt new();
char for *scr=(char far *)0x417;

main()
{
	old=getvect(0x08);
	setvect(0x08,new);
	keep(0,100);
}

void interrupt new()
{
	*scr=64;
	(*old)();
}