A long time ago, there were the bottom halves, that is, a hardware event was split into two halves: the top half (the hardware interrupt handler) and the bottom half (the software interrupt handler). This is because an interrupt handler must execute as quickly as possible to be ready to serve the next incoming interrupts, so, for instance, the CPU cannot stay for a long time in the interrupt handler's body waiting for the slow peripheral sending or receiving of its data. That's why we used bottom halves; interrupts were split into two parts: the top one, the real hardware interrupt handler, which executes quickly and with disabled interrupts that simply acknowledges the peripheral and then starts a bottom half, executed with enabled interrupts, which can safely complete the sending/receiving job by taking its time.
However, bottom halves were very limiting...