Exception - "unanticipated" event caused by the running program.

a) Fault - when restarted, the instruction causing the fault will be redone

e.g., page fault - tried to access a page that was not in main memory

e.g., division by zero - (interrupt generated if the overflow flag is set)

b) Trap - when restarted, the instruction after the one causing the fault is started

e.g., user defined interrupts

c) Aborts - halts program

e.g., hardware error detected

e.g., inconsistent values in system tables encountered

Pentium Registers

General Purpose Registers

Software interrupt - "anticipated" event caused by the running program when it issues an ("int") interrupt instruction

Synchronous event - program "knows" when the interrupt will occur

e.g., "int 3" is for break point interrupt

e.g., requests to the operating system for service - DOS and BIOS

DOS "int 21H" displays a character to screen

register parameters: in AH register put 02H

in DL register put ASCII character to display

Nothing is returned

BIOS "int 16H" keyboard service routines (lower-level functions than DOS)

register parameters: in AH register put 00H for function to read a character

register parameters: in AH register put 01H for function to check keyboard buffer

returns 1 in ZF if character buffer is empty

returns 0 in ZF if character buffer is not empty with ASCII char. put

in AL register and scan codes put in AH register

register parameters: in AH register put 02H for function to check keyboard status

returns status of shift and toggles keys in AL register using bit

assignment (0 - right SHIFT down, 1 - left SHIFT down, 2 - CTRL

down, 3 - ALT down, ... , 7 - INS lock down)

Various ways to interact with I/O devices

Hardware interrupt - "unanticipated" event caused by a hardware device external to the CPU

(asynchronous event - not caused by the running program)

Communication between CPU and device performing hardware interrupt:

1) MNI or INTR pin asserted by device

2) CPU sends interrupt acknowledgement (INTA) signal

3) Interrupting device places interrupt type # on data bus

4) CPU uses interrupt # to invoke corresponding ISR to service interrupt