Category Archives: Interrupt

Old Knowledge of x86 Architecture : “8086 Interrupt Mechanism”

By : Antonius (@sw0rdm4n)
http:///www.ringlayer.net

Interrupt is a mechanism that allow hardware or software to suspend normal execution on microprocessor in order to switch to interrupt service routine for hardware / software. Interrupt can also described as asynchronous electrical signal that sent to a microprocessor in order to stop current execution and switch to the execution signaled (depends on priority). Whether an interrupt is prioritized or not depends on the interrupt flag register which controlled by priority / programmable interrupt controller (PIC).
There are 5 type of interrupts:

  • hardware interrupt, this is external interrupt caused by hardware; for example when pressing keyboard.
  • Non-maskable interrupt (NMI), the interrupt that can not be ignored by microprocessor.
  • Software interrupt, this is maskable interrupt that comes from a software; this interrupt comes when an assembly routine execute int instruction
  • Internal interrupt, this interrupt is a result of processor state violation, for example : divide by zero error
  • Reset, this interrupt will reset cpu state

Interrupt Vector Table (IVT) on 8086

Interrupt vector table on 8086 is a vector that consists of 256 total interrupts placed at first 1 kb of memory from 0000h to 03ffh, where each vector consists of segment and offset as a lookup or jump table to memory address of bios interrupt service routine (f000h to ffffh) or dos interrupt service routine address, the call to interrupt service routine is similar to far procedure call.
The size for each interrupt vector is 4 bytes (2 word in 16 bit), where 2 bytes (1 word) for segment and 2 bytes for offset of interrupt service routine address.
So it takes 1024 bytes (1 kb) memory for interrupt vector table. On 8086 with dos operating system, interrupt vector table at 00h-1fh (int num 0-31) consists of lookup / jump table
address to hardware or bios interrupt handler routine, meanwhile 20h-ffh (int num 32-255) consist of jump table address to dos interrupt handler routine.
For example int 13h that located on ivt at 0000:004c contains address of Bios ROM Interrupt Service Routine, what it records is segment F000h, and offset 1140h, each bytes of that address will be placed little endian. The lower the interrupt number on interrupt vector table means the more priority needed for an interrupt.

Interrupt Vector Table (taken from somewhere on internet)

ivt

Below is example of bios interrupt declaration from bios source code at 8086tiny bios source code:

; Interrupt vector table - to copy to 0:0
int_table	dw int0
          	dw 0xf000
          	dw int1
          	dw 0xf000
          	dw int2
          	dw 0xf000
          	dw int3
          	dw 0xf000
          	dw int4
          	dw 0xf000
          	dw int5
          	dw 0xf000
          	dw int6
          	dw 0xf000
          	dw int7
          	dw 0xf000
          	dw int8
          	dw 0xf000
          	dw int9
          	dw 0xf000
          	dw inta
          	dw 0xf000
          	dw intb
          	dw 0xf000
          	dw intc
          	dw 0xf000
          	dw intd
          	dw 0xf000
          	dw inte
          	dw 0xf000
          	dw intf
          	dw 0xf000
          	dw int10
          	dw 0xf000
          	dw int11
          	dw 0xf000
          	dw int12
          	dw 0xf000
          	dw int13
          	dw 0xf000
          	dw int14
          	dw 0xf000
          	dw int15
          	dw 0xf000
          	dw int16
          	dw 0xf000
          	dw int17
          	dw 0xf000
          	dw int18
          	dw 0xf000
          	dw int19
          	dw 0xf000
          	dw int1a
          	dw 0xf000
          	dw int1b
          	dw 0xf000
          	dw int1c
          	dw 0xf000
          	dw int1d
          	dw 0xf000
          	dw int1e

On boot, interrupt vector table initialized by bios on rom, then interrupt vector table loaded to RAM.

Microprocessor get correponding entry on interrupt vector table by multiplying interrupt number with 4h. For example if int 16h called : 16 * 4 = 58h. Corresponding logical address that contains address of rom bios routine for int num 4h will be on 0000h:0058h.

Interrupt Mechanism

For an example, an interrupt signal to processor may be signaled from a keyboard press. The interrupt signal then will be send via system bus to priority / programmable logic controller
(example of commonly used pic on 8086 architecture is 8259a PIC).
PIC will determine whether imr (interrupt mask register) is masked or not, if imr sets to 1,irq will not be send to processor otherwise if imr sets to 0 ,
PIC will determine the priority of interrupt by checking interrupt service register (ISR) , where the lower interrupt number the higher the priority, once sequences completed, irq will be send to processor.,
If interrupt flag on microprocessor sets to 0, processor will ignored the incoming interrupt signal. But if interrupt flag on microprocessor sets to 1, once interupt signal received by processor, microprocessor will stop current execution.
Microprocessor then will saves flag registers, then push address of next execution counter (cs:ip) on the stack for later return. Microprocessor then will send ack to PIC, the PIC then send interrupt number to processor, This number then will be multiplied by 4 as offset address of the interrupt vector table.
Microprocessor then will do a lookup to find a corresponding segment (cs) address and offset (ip) address of isr address from interrupt vector table, once found, processor will sets interrupt flag to 0 (disable interrupt) then both segment and offset address of interrupt service routine which taken from interrupt vector table will be put on cs:ip, where cs is the  segment address of interrupt service routine (ISR) and ip (instruction pointer) will be offset address of interrupt service routine, then processor begins executing routines from interrupt service routine (similar to far call).
After interrupt service routine / interrupt handler routine executed, processor will pop back cs and ip then pop back flag register on the stack, then micropocessor will return to address that previously saved on the stack which now on cs:ip.

Another example of mechanism from the software point of view is when an assembly program execute int 13h function 1h (get recent disk status operation). The processor will record flag register and program counter for return address onto the stack, where this will be used for iret instruction in order to go back to next software routine after interrupt routine instruction completed.
The next execution of processor will lookup corresponding entry for interrupt number 13h on interrupt vector table.
As already mentioned before offset address on interrupt vector table is interrupt number multiplied with 4 : 13h * 4h = 4ch = 0000:004c (0000:004c on ivt contains logical address of interrupt 13h ISR address).
Once the entry found, where it contains of 1 word segment (cs) and 1 word offset (ip) of bios rom routine address for interrupt 13h disk service it will be loaded to cs and ip.
Based on the calculation cs will be and ip will be taken from rom bios address that recorder on interrupt vector table address at 0000:004c.
Then the execution will be driven (far call) to absolute address of bios rom routine that handle int 13h function 1h, for example here is a sample of bios routine that handle int 13h function 0h for disk reset (8086tiny bios source code):

int13:
	cmp	ah, 0x00 ; Reset disk
	je	int13_reset_disk
	cmp	ah, 0x01 ; Get last status
	je	int13_last_status
-------cutted-----------
int13_last_status:
	mov	ah, [cs:disk_laststatus]
	je	ls_no_error 
	stc ; set carry flag
	iret
    ls_no_error:
	clc
	iret

Once an assembly program call int 13h , microprocessor will save flag register and program counter (address of current execution), microprocessor then do a lookup to find corresponding entry for “int 13h” on interrupt vector table.
Once corresponding entry found, it will jump to corresponding int 13h bios label (far call), since we use function 1h the next instruction will jump to int13_last_status label.
On int13_last_status label we see instruction : “mov ah, [cs:disk_laststatus]” this will move byte at address that pointed by cs:disk_laststatus. Absolute address of cs:disk_laststatus contains byte of bios disk error code, if it’s 0 means no error, rather than 0
means there’s an error of recent disk operation.
If ah = 0 then instruction clear carry flag before iret will be executed, but if error found, the next instruction will set carry flag before iret.
Once iret executed the instruction will then be returned back to next routine of assembly program that calls int 13h.

Another example is when int 21h function 2a being called, when microprocessor determine that the interrupt is prioritize rather than current execution, microprocessor will save flag register, program counter onto stack. Once more, microprocessor will have a lookup on interrupt vector table. As previous, logical address of int 21h on interrupt vector table can be found on interrupt vector table offset 0084h. We got IVT offset by multiplying interrupt number with 4. For interrupt 21h, the calculation to get interrupt vector address is : “21h * 4h = 84h = 0000h:0084h”. So entry for int 21h ISR address from interrupt vector table is located at logical address 0000:00084h.
Microprocessor will do a far call to interrupt service routine address, where it the address is recorded at interrupt vector table logical address at 0000h:0084h. For example here on ms dos 2:

        procedure   $GET_DATE,NEAR   ;System call 42
ASSUME  DS:NOTHING,ES:NOTHING
; Inputs:
;       None
; Function:
;       Return current date
; Returns:
;       Date in CX:DX

        PUSH    SS
        POP     DS
ASSUME  DS:DOSGROUP
        invoke  READTIME        ;Check for rollover to next day
        MOV     AX,[YEAR]
        MOV     BX,WORD PTR [DAY]
        invoke  get_user_stack           ;Get pointer to user registers
ASSUME  DS:NOTHING
        MOV     [SI.user_DX],BX  ;DH=month, DL=day
        ADD     AX,1980         ;Put bias back
        MOV     [SI.user_CX],AX  ;CX=year
        MOV     AL,BYTE PTR [WEEKDAY]
        RET
$GET_DATE ENDP

Once return :

	AL = day of the week (0=Sunday)
	CX = year (1980-2099)
	DH = month (1-12)
	DL = day (1-31)

After interrupt execution completed (right after iret), the processor will back to next  instruction which recorded on stack before by pop it back to cs and ip (previously, the next instruction is suspended when microprocessor receives irq from pic).

Some Bios (Basic Input Output System) Interrupts

Below is some examples of bios interrupt and it’s usage. We do not provide complete list here, since the purpose just for understanding bios interrupt usage.

int 10h

int 10h handling routine was provided by bios, this interrupt is used for video mode operations.
Example : int 10h function 00h.

This is for setting video mode.
requirement :

ah = video mode
al = 00h

Example usage of int 10h function 00h:

;10_0.asm
;int 10h function 00h demo
;this is for setting video mode
;made by Antonius (sw0rdm4n)
;http://www.ringlayer.net
;compile with tasm 2.0 and tlink 3.0
;tasm 10_0.asm
;tlink /t 10_0.obj
.model tiny
.data
	strx db 'h4x0r$'
  
.code
org 100h
start:
	mov al, 00h; set video mode  40 x 25 resolution  
	call _uber
	
	mov al, 02h; set video mode  80 x 25 resolution
	call _uber
	
	mov al, 06h; set video mode  640 x 200  resolution
	call _uber

	mov al, 13h; set video mode  vga 640 x 480 resolution
	call _uber

	int 20h

_uber proc near
	call _setvideo
	call _printf 
	call _wait
	retn
_uber endp	
_printf proc near
	mov dx, offset strx
	mov ah, 09h
	int 21h
	retn
_printf endp
_wait proc near
	mov ah,00h
	int 16h
	retn
_wait endp
_setvideo proc near
	mov ah,00h
	int 10h
	retn
_setvideo endp
end start

int 12h

int 12h used to get memory size. This interrupt will returns the contents of the word at segment 0040h and offset 0013h into ax register.

Example code:

;memory.asm
;get memory size
;returns the contents of the word at segment 0040h and offset 0013h into ax register
;made by Antonius (@sw0rdm4n)
;http://www.ringlayer.net
.model tiny
.data
	_mem dw 00
.code
org 100h
start:
	xor ax,ax
	int 12h
	nop
	int 20h
end start

Assemble : tasm memory.asm , link : tlink /t memory.obj, then debug “memory.com”

memory

we can see right after iret from bios routine executed, register ax will contains 1 word: 0280. Dump memory content
at 0040:0013 :

memory2

We can see it contains bytes in little endian order : 80 02, in non little endian = 0280h , in decimal = 640. So we got 640kb memory size.

Some DOS Interrupts

Dos interrupt is interrupt routines provided by dos, on interrupt vector table 20h-3fh is dos vector interrupt. Below is some of dos interrupt (we do not give complete lists)

int 21h

int 21h is dos function codes, provided by dos operating system.
example : “int 21h function 35h“, this interrupt function is used to get address of interrupt service routine where it’s recorded at interrupt vector table.

requirements:
ah = 35h
al = interrupt number 

After int 21h function 35h executed, es (extra segment 16 bit) register will be segment address of ISR and bx (base register 16 bit) register will be offset address of ISR.

Example code that uses int 21h function 35h:

;getivt.asm
;simple routine to get interrupt vector table content
;segment of isr will be saved to es register
;offset of isr will be saved to bx register
;compile : 
;tasm getivt.asm
;tlink /t getivt.obj
;programmer : Antonius (sw0rdm4n)
;http://www.ringlayer.net 
.model tiny
.data
	_segment dw 0000h
	_offset dw 0000h
.code
	org 100h
start:
	mov bl,13h ; get bios rom routine address for int 13h
	call _get_ivt
	nop	
	nop
	call _cleanres
	mov bl, 21h ; get os routine address for int 21h
	call _get_ivt
	nop
	nop	
	call _cleanres
	mov bl, 10h ; get bios rom routine address for int 10h
	call _get_ivt
	nop
	nop
	call _cleanres
	int 20h; ret to dos
_cleanres proc near
	xor bx, bx
	ret
_cleanres endp
_get_ivt proc near
	mov ah,35h
	mov al, bl
	int 21h
	retn
_get_ivt endp	
end start	

Let’s see what above routines executed in background. Compile using tasm 2.0 and tlink 3.0:

get_interrupt_vector_table_content

Debug it using debug.exe : “debug getivt.com” then stepping by type : “t” then enter until the first int 21h executed:

getivt1

We can see that bx contains offset of address of int 13h routine provided from rom bios (on RAM) : 1140h. Meanwhile es contains segment address of bios routine that serve int 13h : f000h.
So bios routine that provides ISR for interrupt 13h starts from address f000h:1140h. On interrupt vector table int 13h can be lookup on 0000:004c (13h * 4h = 4ch).
To dump memory content at 0000:004c we type: “d 0000:004c l 4” means we wish to dump 4 bytes from memory at segment 0000h offset 004ch.

getivt2

We can see on 0000:004c contains 1 word for segment and 1 word for offset address of interrupt service routine for int 13h, which encoded using little endian order : 1 word (2 bytes) for offset : 40 11 and 1 word (2 bytes) for segment : 00 f0. Where if encoded to non little endian offset = 1140h and if segment encoded to non little endian = f000h. We’ve cross checked that isr address of 13h handle routine mapped at address f000:1140.

int 21h function 2ah

This interrupt used to get system date, once iret of isr executed, cx will contains year, dh contains month,dl contains day. This routine mostly abused for time bomb virus in past time.

Example code:

;date.asm
;once iret of isr executed, 
;cx will contains year
;dh contains month
;dl contains day
;coded 8 Sept 2014
;by sw0rdm4n
.model tiny
.data
	_timebomb_year dw 07deh
	_timebomb_month db 09h
	_timebomb_day db 08h
	_timebomb_msg db "AlexanderPD timebomb start - I'm sorry sir !", 13,10, '$'
.code
org 100h
start:
	mov ah, 2ah
	int 21h
_timing_bomb_check:
	cmp cx, _timebomb_year
	je _check_month	
	int 20h
_check_month:
	cmp dh,_timebomb_month
	je _check_day
	int 20h
_check_day:
	cmp dl, _timebomb_day
	je _timebomb_joke
	int 20h
_timebomb_joke:
	call _setvid
	mov ah, 09h
	mov dx, offset _timebomb_msg
	int 21h
	int 20h
_setvid proc near
	mov al, 0h
	mov ah,00h
	int 10h
	ret
_setvid endp
end start	

Every time this code executed on 8 Sept 2014, a joke message will be displayed:

date

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