That's what makes TSRs possible. A TSR is essentially just an ordinary program, the only difference is that when it exits, it tells the OS to leave the process

That's what makes TSRs possible. A TSR is essentially just an ordinary program, the only difference is that when it exits, it tells the OS to leave the process in memory rather than unloading it. Yeah, I remember programming a bunch of those in TurboPascal. There were compiler directives to limit the heap size and some inline assembly was needed to save/restore the stack inside the interrupt handler, but once it worked, the rest was like a normal program. It was very useful to have a calculator, taking notes, etc. Now we take multitasking for granted, of course.