Indeed, hooking the timer (or keyboard) interrupt was also the way to do "multitasking" (TSRs) in x86 real mode under MS-DOS. The much-maligned x86 segment reg

Indeed, hooking the timer (or keyboard) interrupt was also the way to do "multitasking" (TSRs) in x86 real mode under MS-DOS. The much-maligned x86 segment registers do come in handy here: they provide hardware-accelerated position independent memory addressing! Most DOS utility programs were small enough to fit in a single 64K segment, and provided they never touched the segment registers, these programs were trivially relocatable to any memory address.