Help with ints

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BOTOKILLER

Joined: 07 Jan 2011
Posts: 154
Location: Ukraine

BOTOKILLER 07 Jun 2011, 17:23

Hi everyone!
I want my OS to do system calls through interrupts - I have no idea how to do it. I looked at OSdev, the article is 'foggy' many things arent explained clearly. Does anybody knows where can I found a good tutorial on that subject???

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NSOS

07 Jun 2011, 17:23

xleelz

Joined: 12 Mar 2011
Posts: 86
Location: In Google Code Server... waiting for someone to download me

xleelz 07 Jun 2011, 17:31

Well, I'm not sure if it helps but the source code to Coty's HeliumOS can be found at http://board.flatassembler.net/topic.php?t=12410

He uses kernel interrupts to print the characters 'A' and 'B' to the screen.

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07 Jun 2011, 17:31

me239

Joined: 06 Jan 2011
Posts: 200

me239 08 Jun 2011, 23:24

Interrupts are terribly easy. Here is a sample code to simulate int 21h, function 02h. Remember, keep your interrupt vectors in a safe place in memory! Otherwise unwanted instructions could be executed and hang the machine.

Code:

call set_interrupt
*YOUR CODE
ivt:
 pushf ; save flags
 cmp ah, 02h ; DOS interrupt function 02h (print character)
 jnz int_ret ; not our function, then leave
 push ax ; save AX
 mov al, dl ; move the character from dl to al
 mov ah, 0eh ; Teletype BIOS function
 int 10h ; print character
 pop ax ; restore AX
int_ret:
 popf ; restore flags
 iret ; interrupt return (basically retf)
set_interrupt:
push es ; save ES
push 0
pop es ; lets zero out es
cli ; disable interrupts
mov word[es:21h*4], ivt ; formula for this is "Interrupt*4 = offset"
mov word[es:21h*4+2], cs ; we must put our segment after the offset
sti ; enable interrupts
pop es ; restore ES
ret ; return to our code

08 Jun 2011, 23:24

BOTOKILLER

Joined: 07 Jan 2011
Posts: 154
Location: Ukraine

BOTOKILLER 09 Jun 2011, 10:30

me239 wrote:

Code:

call set_interrupt
*YOUR CODE
ivt:
 pushf ; save flags
 cmp ah, 02h ; DOS interrupt function 02h (print character)
 jnz int_ret ; not our function, then leave
 push ax ; save AX
 mov al, dl ; move the character from dl to al
 mov ah, 0eh ; Teletype BIOS function
 int 10h ; print character
 pop ax ; restore AX
int_ret:
 popf ; restore flags
 iret ; interrupt return (basically retf)
set_interrupt:
push es ; save ES
push 0
pop es ; lets zero out es
cli ; disable interrupts
mov word[es:21h*4], ivt ; formula for this is "Interrupt*4 = offset"
mov word[es:21h*4+2], cs ; we must put our segment after the offset
sti ; enable interrupts
pop es ; restore ES
ret ; return to our code

I'm developing Pmode OS and Pmode IDT

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NSOS

09 Jun 2011, 10:30

me239

Joined: 06 Jan 2011
Posts: 200

me239 09 Jun 2011, 19:11

BOTOKILLER wrote:

I'm developing Pmode OS and Pmode IDT

I don't know how segmented memory preforms in Pmode (since Interrupt Vectors are in Segment:Offset), but you can skip the BIOS interrupt 10h and just write them to the video buffer.

Code:

ivt:
pushf
cmp ah, 02h
jz int_ret
push ax es
push 0b800h
pop es
mov ah, 07h
stosw
pop es ax
int_ret:
popf
iret

I believe that 16 and 8 bit registers should still work in Pmode.

09 Jun 2011, 19:11

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