94.574 Elements of Computer Systems

SCE 574

106

Software Interrupts

äINT Similar to the Call instruction

äINT nn executes the following sequence of instructions

äPUSH flags,CS, and IP Registers onto Stack

äIP and CS registers are loaded with the contents of the two words at the absolute memory addresses 0:nn*4 and 0:nn*4+2

äProgram control jumps to the address contained in these two absolute memory locations

äAddress locations 0:0 through 0:3FFH are an interrupt vector (table) and contain starting addresses of subroutines you may want to execute

äTo finish an interrupt, use IRET

äIt restores CS, IP and flags in the proper order

äNot RET only restores IP (or IP and CS for FAR call)