Fiddling with IRQ settings was because the IBM PC design didn't allow sharing of interrupts (I think because they didn't expect it to be as popular as it was).

Fiddling with IRQ settings was because the IBM PC design didn't allow sharing of interrupts (I think because they didn't expect it to be as popular as it was). Squeezing out memory was due to two design flaws---one being IBM assigning the address space above 640K for peripherals and ROMS (MS-DOS never had a 640K limit) and two being MS-DOS required memory to be contiguous.