2. DAT2343 Accessing Services Through Interrupts © Alan T. Pinck / Algonquin College; 2003

3. Operating System Services A large part of the code which makes up an Operating System is composed of subroutines or functions which perform common tasks required by most application programs. Having these “service” subroutines in the OS means that application programs do not need to recreate them for each new application.

4. Hardware Interrupts as a Method of Accessing OS Services

5. Hardware Interrupts and the Instruction Processing Cycle Copy instruction at address given by Instruction Pointer into Instruction Register Increment Instruction Pointer by length of instruction in Instruction Register Execute instruction in Instruction Register IF a hardware interrupt has occurred Save value in Instruction Pointer (as if a CALL had occurred) and the value of the Flags, then reset Instruction Pointer to address from Interrupt Vector Table for this hardware device Repeat from first step

6. Hardware Interrupts: IRQ numbers Each device which can generate a request for a service using a hardware interrupt must have its own “interrupt request number” (IRQ). The IRQ number acts as an index into the Interrupt Vector Table to determine the address of the Service Routine to be processed in response to a request from the specific device.

7. Software Interrupts as Replacements for Calls to OS Subroutines The Operating System includes service functions for application software as well as for hardware. If a CALL instruction were use to access these services, each service would need to always be at the same fixed memory address (regardless of the version of the operating system or the computer’s configuration). Instead, these software support services are normally accessed, through the Interrupt Vector Table, using an Interrupt instruction (instead of a hardware interrupt signal).

8. End of Lecture