Interrupt Service Routines

The interrupt service routine (ISR) is the software module that is executed when the hardware requests an interrupt. From the last section, we see that there may be one large ISR that handles all requests (polled interrupts), or many small ISR's specific for each potential source of interrupt (vectored interrupts). The design of the interrupt service routine requires careful consideration of many factors that will be discussed in this chapter. When an interrupt is requested (and the device is armed and the I bit is one), the microcomputer will service an interrupt:
1) the execution of the main program is suspended (the current instruction is finished),

2) the interrupt service routine, or background thread is executed,

3) the main program is resumed when the interrupt service routine executes iret .



When the microcomputer accepts an interrupt request, it will automatically save the execution state of the main thread by pushing all its registers on the stack. After the ISR provides the necessary service it will execute a iret instruction. This instruction pulls the registers from the stack, which returns control to the main program. Execution of the main program will then continue with the exact stack and register values that existed before the interrupt. Although interrupt handlers can allocate, access then deallocate local variables, parameter passing between threads must be implemented using global memory variables. Global variables are also equired if an interrupt thread wishes to pass information to itself, e.g., from one interrupt instance to another. The execution of the main program is called the foreground thread, and the executions of interrupt service routines are called background threads.