DOS inturrupt

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GhostXoPCorp

Joined: 13 Dec 2008
Posts: 199
Location: 01F0:0100

GhostXoPCorp

i was wondering on how dos works, i came upon int 21 and how it prints to the screen, does dos use int 10h inside of 21h or do they have a built in driver or one of the io or ms_dos sys file to print to the screen (drivers)

_________________
Oh that divide overflow. Just jumps out of the bushes every time to scare the day lights out of me.

01 Jun 2009, 21:45

comrade

Joined: 16 Jun 2003
Posts: 1137
Location: Russian Federation

comrade

They probably call int 10h inside their int 21h handler. Kinda like routines call other subroutines. Remember software interrupts are similar to a function call mechanism. Except in modern software they also bring an environment change (user-mode to kernel-mode). On DOS they are pretty much like a subroutine call Smile

02 Jun 2009, 00:54

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4633
Location: Argentina

LocoDelAssembly

If Int21 calls Int29 for on-screen printing then:

DOS 2+ - FAST CONSOLE OUTPUT wrote:

AL = character to display

Return:
Nothing BX may be destroyed by some versions of DOS 3.3

Notes: Automatically called when writing to a device with bit 4 of its device driver header set (see also INT 21/AH=52h). COMMAND.COM v3.2 and v3.3 compare the INT 29 vector against the INT 20 vector and assume that ANSI.SYS is installed if the segment is larger. The default handler under DOS 2.x and 3.x simply calls INT 10/AH=0Eh. The default handler under DESQview 2.2 understands the [2J screen-clearing sequence, calls INT 10/AH=0Eh for all others

Ref: http://www.ctyme.com/intr/rb-4124.htm

02 Jun 2009, 02:41

GhostXoPCorp

Joined: 13 Dec 2008
Posts: 199
Location: 01F0:0100

GhostXoPCorp

thanks for the reply, what i thought is that t used its own screen drivers, if so that would of anwered another question, the other question was why is command.com so big? in asm shouldnt it be smaller? or are there things that are background and cant be noticed, maybe its the buffers? one more question (getting annoying, sorry) what is a stacks over flow and i know it has a connection with the keyboard, when key mash i get the little error screen, i promise these are all the questions i have left for dos

02 Jun 2009, 04:05

rCX

Joined: 29 Jul 2007
Posts: 166
Location: Maryland, USA

rCX

Which DOS are you using? In FreeDOS, command.com is 66945bytes.

02 Jun 2009, 16:38

GhostXoPCorp

Joined: 13 Dec 2008
Posts: 199
Location: 01F0:0100

GhostXoPCorp

no, the original ms-dos, but i know why free dos is so big, it doesnt use assembly as a main for programming, but i have seen somewhere some one built an os with more functionallity than dos and much smaller, does dos use assembly, or is there many background things that arents noticed, like for security of anything else?

_________________
Oh that divide overflow. Just jumps out of the bushes every time to scare the day lights out of me.

02 Jun 2009, 16:45

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