As x86 computers shifted from 32-bit to 64-bit, they also shifted from using 8259-style Programmable Interrupt Controllers with 8 interrupt lines. (Or two multiplexed PICs for a total of 15 interrupt lines.) Then, if you were to install 32-bit Windows for an operating system, Windows would implement 32 software IRQLs (Interrupt Request Levels) with IRQLs 3 through 26 (or so) being reserved for devices.
Then the x64 platform came along. You need a machine with an APIC, which has 256 interrupt lines, in order to install 64-bit Windows on it. However, 64-bit Windows only implements 16 IRQLs.
So my question is, does anyone know why 64-bit Windows would implement fewer IRQLs than its 32-bit counterpart even though it has many more hardware interrupt lines at its disposal?