flat assembler - INT 2FH AX=4810H

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geekbasic@gmx.com

Joined: 25 Oct 2022
Posts: 71
Location: Arizona

geekbasic@gmx.com 22 Jan 2023, 03:10

INT 2FH AX=4810H - Read Command Line

ah=48h
al=10h

I want to be able to read and print the command line to a string in a COM program.

This is essentially what I am attempting. It's not working.

Code:

use16
org 256

call cmdline

mov dx,cmdlineparams
call strecho

int 32

cmdline:

        mov dx,cmdlineparams
        mov ax,18448
        int 47

ret

strecho:

        mov si,dx
        nextchar:
        mov al,byte [ds:si]
        cmp al,0
        jz echoed
        inc si
        call strcharecho
        jmp nextchar
        echoed:

ret

strcharecho:

        mov ah,14
        int 16

ret

cmdlineparams db "",0
rb 1023

I'm reading around about this having something do with the command line being in PSP, but I am not sure what to do.

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22 Jan 2023, 03:10

macomics

Joined: 26 Jan 2021
Posts: 928
Location: Russia

macomics 22 Jan 2023, 08:31

You don't need functions to access the command line. It is written in the PSP of the program at the relative address 128 (80h) and has the following format.

Code:

cmd_length db ? ; PSP+80h
cmd_string db 7Fh dup (?) ; PSP+81h

cmd_line.asm

Code:

format binary as "COM"
org 256
use16

label CMD_LENGTH byte at 128
label CMD_STRING byte at 129

; fwrite as is
    mov dx, CMD_STRING ; ds:dx = PSP+81h
    movzx cx, [CMD_LENGTH] ; cx = byte ptr PSP+80h
    mov bx, 1 ; stdout (CON)
    mov ah, 64 ; fWrite
    int 33

; move
    cld
    lea di, [buffer]
    lea si, [CMD_STRING]
    movzx cx, [CMD_LENGTH]
    rep movs byte [di], [si]
    mov al, '$' ; EOLN
    stos byte [di]

; print
    lea dx, [buffer]
    mov ah, 9 ; Print line
    int 33

; wait key
    mov ah, 7 ; read key (without echo)
    int 33

; exit
    mov ax, 76 * 256 + 0 ; exit to DOS (code = 0)
    int 33

buffer rb 128

Description:
Filesize:	1.28 KB
Viewed:	8091 Time(s)

22 Jan 2023, 08:31

DimonSoft

Joined: 03 Mar 2010
Posts: 1228
Location: Belarus

DimonSoft 22 Jan 2023, 20:41

May I also add my 5 pennies of offtopic? I find it quite an arguable style to use decimal value representation for stuff that is almost everywhere written/described/explained in hexadecimal. Besides making the code look different from what a debugger would show by default (which makes it harder to the author (and those who try to help them) to debug), it might also force people make mistakes. Say, it’s quite easy to mix int 16 and int 16h. The former is the BIOS video stuff interrupt, the latter is for keyboard input stuff. single letter of difference makes it easy to miss it and treat one thing as another.

Programming large projects is already hard enough. Why make it even harder for ourselves?

22 Jan 2023, 20:41

sinsi

Joined: 10 Aug 2007
Posts: 789
Location: Adelaide

sinsi 23 Jan 2023, 05:04

Isn't INT 2Fh 4810h a DosKey interrupt and nothing to do with a command line?

DimonSoft, agree 100% - it's weird seeing INT 33 everywhere (that's the mouse interrupt? 33H) for DOS INT 21H calls Smile

Just to confuse things even more, MASM had .radix which treated numbers differently, .radix 10 (the default) and .radix 16 (so you didn't have to use 'H')

23 Jan 2023, 05:04

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20303
Location: In your JS exploiting you and your system

revolution 23 Jan 2023, 05:23

I prefer a prefix instead of a postfix.

Code:

int 0x21 ; good
int 21h ; bad (IMO)

Tell me how to parse it before I start parsing, not after I have already passed it to find the postfix. Smile

But best is to use a symbolic constant:

Code:

DOS_INTERRUPT = 0x21 ; best
;...
int DOS_INTERRUPT

23 Jan 2023, 05:23

sinsi

Joined: 10 Aug 2007
Posts: 789
Location: Adelaide

sinsi 23 Jan 2023, 06:58

0x21 begone, demon of C
DOS_INTERRUPT waaaay too much typing
Smile

23 Jan 2023, 06:58

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20303
Location: In your JS exploiting you and your system

revolution 23 Jan 2023, 07:17

Copy + Paste

23 Jan 2023, 07:17

sinsi

Joined: 10 Aug 2007
Posts: 789
Location: Adelaide

sinsi 23 Jan 2023, 08:50

edlin didn't have such fancy stuff

23 Jan 2023, 08:50

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20303
Location: In your JS exploiting you and your system

revolution 23 Jan 2023, 08:56

Code:

DOS = 0x21
;...
int DOS

Same character count as int 21h, but better. Razz

23 Jan 2023, 08:56

macomics

Joined: 26 Jan 2021
Posts: 928
Location: Russia

macomics 23 Jan 2023, 10:14

Ahh

Code:

format binary as "COM"
org 256
use16

label CMD_LENGTH byte at 128
label CMD_STRING byte at 129

DOS equ int 0x21

; fwrite as is
    mov dx, CMD_STRING ; ds:dx = PSP+81h
    movzx cx, [CMD_LENGTH] ; cx = byte ptr PSP+80h
    mov bx, 1 ; stdout (CON)
    mov ah, 64 ; fWrite
    DOS

; move
    cld
    lea di, [buffer]
    lea si, [CMD_STRING]
    movzx cx, [CMD_LENGTH]
    rep movs byte [di], [si]
    mov al, '$' ; EOLN
    stos byte [di]

; print
    lea dx, [buffer]
    mov ah, 9 ; Print line
    DOS

; wait key
    mov ah, 7 ; read key (without echo)
    DOS

; exit
    mov ax, 76 * 256 + 0 ; exit to DOS (code = 0)
    DOS

buffer rb 128

So very shortly

23 Jan 2023, 10:14

DimonSoft

Joined: 03 Mar 2010
Posts: 1228
Location: Belarus

DimonSoft 24 Jan 2023, 14:20

revolution wrote:

I prefer a prefix instead of a postfix.
Code:
int 0x21 ; good
int 21h ; bad (IMO)     
Tell me how to parse it before I start parsing, not after I have already passed it to find the postfix.

I prefer being paid for my code then, instead of having zeroes all over:

Code:

int $21

Well, in fact I just hate the C ecosystem and its artefacts, so 21h looks like a neutral choice to avoid holy wars.

revolution wrote:

Code:

DOS = 0x21 ; best
;...
int DOS

Very confusing for Spanish-speaking people Smile

24 Jan 2023, 14:20

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20303
Location: In your JS exploiting you and your system

revolution 24 Jan 2023, 14:47

I don't care who designs the mousetrap. If it is better then I will use it.

Even if Satan herself designed the best mousetrap, I would use it simply because it is the best. Twisted Evil

24 Jan 2023, 14:47

DimonSoft

Joined: 03 Mar 2010
Posts: 1228
Location: Belarus

DimonSoft 26 Jan 2023, 08:12

Do you have an option to avoid mousetraps?

26 Jan 2023, 08:12

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20303
Location: In your JS exploiting you and your system

revolution 26 Jan 2023, 08:20

DimonSoft wrote:

Do you have an option to avoid mousetraps?

The context was entering hex numbers. So, yes, you can avoid using any hex numbers. Convert everything to decimal forever.

Code:

mov eax,805306371 ; the bit mask, it is so much better in decimal

26 Jan 2023, 08:20

al_Fazline

Joined: 24 Oct 2018
Posts: 54

al_Fazline 26 Jan 2023, 09:21

I'd rather have everybody switch to hex everywhere, why not? Though then, preferably there would be completely new digits for A-F so it can't be confused with written words.

Or a 12-base system would be superior to decimal but unfortunately it didn't get traction.

Though English still has some traces of it, as 11 and 12 are called eleven and twelve instead of oneteen and twoteen.

26 Jan 2023, 09:21

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20303
Location: In your JS exploiting you and your system

revolution 26 Jan 2023, 09:41

al_Fazline wrote:

I''d rather have everybody switch to hex everywhere, why not?

Agreed. It is the genetic modification to grow six more fingers that gets tricky.

al_Fazline wrote:

Though then, preferably there would be completely new digits for A-F so it can''t be confused with written words.

Use Arabic: ۰۱۲۳۴۵.

26 Jan 2023, 09:41

al_Fazline

Joined: 24 Oct 2018
Posts: 54

al_Fazline 26 Jan 2023, 09:53

I think they are not quite distinct. Also try to think something you can display on 7-segment indicator.

Quote:

It is the genetic modification to grow six more fingers that gets tricky.

This is completely unnecessary. Does anybody count with fingers still?

26 Jan 2023, 09:53

DimonSoft

Joined: 03 Mar 2010
Posts: 1228
Location: Belarus

DimonSoft 26 Jan 2023, 20:08

al_Fazline wrote:

I'd rather have everybody switch to hex everywhere, why not? Though then, preferably there would be completely new digits for A-F so it can't be confused with written words.

For backwards compatibility it’s not a matter of choosing one silver bullet for every purpose but to choose the most convenient for particular usecase. There’re $7C00 bytes (not 31744) from the beginning of the real-mode address space till the place a bootloader starts from, and PE EXE by default has image base of $400000 (not 4'194'304), but there’re 365 or 366 days in a year (not $16D or $16E). Recognizability causes better mental performance for those who read the code, including the code’s author.

26 Jan 2023, 20:08

geekbasic@gmx.com

Joined: 25 Oct 2022
Posts: 71
Location: Arizona

geekbasic@gmx.com 26 Jan 2023, 21:51

I got your example working, thank you.

People will write code in various ways. We should be thankful that FASM has such a flexible syntax.

There seems to be conflicting documentation regarding int 2fh ax=4810h

My source is "The Programmers PC Sourcebook" by Thom Hogan from Microsoft Press dated 1991.

What is my book referring to if not the Command Line?

Description:
Filesize:	637.56 KB
Viewed:	7862 Time(s)

_________________
http://www.lucidapogee.com

26 Jan 2023, 21:51

sinsi

Joined: 10 Aug 2007
Posts: 789
Location: Adelaide

sinsi 26 Jan 2023, 23:15

http://www.ctyme.com/intr/rb-4804.htm

26 Jan 2023, 23:15

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