Interrupts

The async ones are outside the processor meanwhile the sync ones happen side by side with some activity within the processor.
There are 2 types of interrupts. 1. Software 2. Hardware

the instruction used for interruption is INT and the routines which are triggered in response to this are called interrupt service routines (ISR) or interrupt handler.
The INT instruction takes byte argument ranging from 0 to 255.

INT 0 is divide by zero interrupt.

The correlation between interrupt number and interrupt handler uses a table called interrupt vector table.
Each entry in this table is 4 bytes long consisting of segment and offset values with offset being the first 2 bytes (little endian).
The whole entry is called a vector.

The IRET instruction is used to return to the caller.

The operations done by int are following: - sp <- sp - 2 - [sp] <- flag - sp <- sp - 2 - IF <- 0 - TF <- 0 - [SP] <- CS - sp <- sp - 2 - [sp] <- IP - IP <- [0:N * 4] - CS <- [0:N * 4 + 2]

The operations performed by INT are following - IP <- [SP] - SP <- SP + 2 - CS <- [SP] - SP <- SP + 2 - flag <- [SP] - SP <- SP + 2

The starting 256 * 4 bytes = KB space is reserved for Instruction Vector Table (IVT).

INT 0 - Divide by zero
INT 1 - Trap, single step interrupt
INT 2 - Non Maskable interrupt
INT 3 - Debug interrupt
INT 4 - Arithmetic Overflow, change of sign bits

Hooking an Interrupt

; Hooking divide by zero interrupt
[org 0x0100]
jmp start

message: db 'You divided something by zero.', 0

;;;;; COPY LINES 028-050 FROM EXAMPLE 7.4 (strlen) ;;;;;
;;;;; COPY LINES 005-024 FROM EXAMPLE 7.1 (clrscr) ;;;;;
;;;;; COPY LINES 050-090 FROM EXAMPLE 7.4 (printstr) ;;;;;

; Divide by zero interrupt handler
myisrfor0: 
    push ax ; Push all regs
    push bx
    push cx
    push dx
    push si
    push di
    push bp
    push ds
    push es

    push cs
    pop ds ; Point ds to our data segment

    call clrscr ; Clear the screen

    mov ax, 30
    push ax ; Push x position

    mov ax, 20
    push ax ; Push y position

    mov ax, 0x71 ; White on blue attribute
    push ax ; Push attribute

    mov ax, message
    push ax ; Push offset of message

    call printstr ; Print message

    pop es
    pop ds
    pop bp
    pop di
    pop si
    pop dx
    pop cx
    pop bx
    pop ax

    iret ; Return from interrupt

; Subroutine to generate a divide by zero interrupt
genint0: 
    mov ax, 0x8432 ; Load a big number in ax
    mov bl, 2 ; Use a very small divisor
    div bl ; Interrupt 0 will be generated
    ret

start: 
    xor ax, ax
    mov es, ax ; Load zero in es
    mov word [es:0*4], myisrfor0 ; Store offset at n*4
    mov [es:0*4+2], cs ; Store segment at n*4+2

    call genint0 ; Generate interrupt 0

    mov ax, 0x4c00 ; Terminate program
    int 0x21

BIOS and DOS Interrupts

A software that is burnt into the ROM is called firmware.
The interface to the hardware is called BIOS (Basic Input Output services)
BootStrap means to load the OS.

The video services are accessible through INT 10.
The keyboard services are accessible through INT 16.
The parallel ports through INT 17.
The DOS services are available through INT 21.

There can be different services related to video.
To access each individual service, we do something like INT 10 Service 1 etc.
And these service number are usually given in AH register.
The sub service numbers are stored in AL or BL registers.

; Print string using BIOS service
[org 0x0100]
jmp start

message: db 'Hello World'

start:
    mov ah, 0x13      ; Service 13 - print string
    mov al, 1         ; Subservice 01 – update cursor
    mov bh, 0         ; Output on page 0
    mov bl, 7         ; Normal attribute
    mov dx, 0x0A03    ; Row 10, column 3
    mov cx, 11        ; Length of string

    push cs
    pop es            ; Segment of string

    mov bp, message   ; Offset of string
    int 0x10          ; Call BIOS video service

    mov ax, 0x4c00    ; Terminate program
    int 0x21