In XINU, interrupt servicing is divided into two halves, the low-level routines, and the high-level routines.

• The low-level routines are the ones invoked directly as a result of the interrupt (their addresses are contained in the interrupt dispatch table). These routines are responsible for saving registers, identifying the interrupting device, calling the appropriate high-level routine, restoring the registers, and then calling the RTI instruction. These routines are written in assembly (hopefully the need for this is obvious), and are called interrupt dispatchers (which makes sense as they dispatch or invoke the high-level routines).
• The high-level routines are the ones that do the real work of servicing the device. While the low-level routines are largely device-independent, the high-level routines are not. These routines can usually be written in a high-level language.

Splitting the interrupt handling code into a low-level and high-level halves, while not necessary, is a good idea. It enables us to minimize the use of assembly code (good for obvious reasons), and to separate the device dependant code from the device independent code.