x86 Assembly - Clear Console Text Screen?

Webmonkey

Hi There,

I just touching up on an assignment there for college.
It is a 386 console program that has a menu system.
Everything works great but the output could be improved by clearing the screen eachtime the menu loads back again.
Is there any equivilent to the 'cls' command in dos in x86.
Come across a lot of the internet but none seem to want to work.
It will be running on XP. Thought you could access the video memory and clear it but i'm finding it hard to get any information on the net.

Hopefully someone with bit of experience could help.

By way i'm using Masm 6.1 for assembling and linking.

Cheers.

Martyr

to keep it simple, use system() from msvcrt.dll

[PHP]invoke system,CStr("cls")[/PHP]

you would prototype it in file called msvcrt.inc

[PHP]system proto c :dword[/PHP]

to get the library created, use polib by pelle
i know link.exe can do this also, and masm32 by hutch has inc2l.exe but polib is better (just in future)

pelles executables and help

there are also excellent include files by Japheth

i wouldn't use masm32 anymore, because its very badly maintained, but its good for beginner.
a combination of japheths win32inc files and vc++6 processor pack from microsofts website make writing assembly programs for windows alot easier than some might think.

Webmonkey

Interesting. Thank you very much

I'll look into that. Well you know with college they want basic code with no help from other librarys! Anyways i'll look into that site and start messing around with it myself.

Cheers.

DaSilva

As far as I can remeber, you can use the set cursor position function to clear the screen, by setting it too the first column, row and page.

xor   dx,dx        ;dh,dl contain row column, forget which is which
xor   bh,bh        ;0 = page 1
mov   ah,2h
int   10h

Webmonkey

xor   dx,dx        ;dh,dl contain row column, forget which is which
xor   bh,bh        ;0 = page 1
mov   ah,2h
int   10h

Noticed and tried this before but seems to crash the program. Perhaps that is used outside Windows protected environment I amn't sure.

Wish i could use something as simple as that but doesn't want to work.

Any ideas why it might be crashing.

Webmonkey

Just looking at the Kernel32 API procedures there in a reference PDF. Some nice handy stuff in there but seem to be unable to run some of them.

I can do a sleep no bother:

Sleep PROTO NEAR32 stdcall, dwMilliseconds:DWORD

and i can run it by:

Invoke Sleep,3000[code] to pause program for 3 seconds.

Now this is fine as the argument for Sleep is a DWORD.

[quote]Sleep: procedure
(
dwMilliseconds: dword
);
stdcall;
returns( "eax" );
external( "__imp__Sleep@4" );[/quote]

But I am having a go at others now, for example the setConsoleTitle.

[quote]SetConsoleTitle: procedure
(
lpConsoleTitle: string
Win32 API Reference
Page 431
);
stdcall;
returns( "eax" );
external( "__imp__SetConsoleTitleA@4" );[/quote]

Now this expects a String but how do I define this in my Assembly program. I tried doing
[code]SetConsoleTitle PROTO NEAR32 stdcall, lpConsoleTitle:PTR BYTE

and calling it using

Invoke SetConsoleTitle, ADDR titleStr

where titleStr is a defined db in the data section as titleStr db "Title",0

Assembles fine but wont like. It is complaining of:

Error LNK2001: unresolved external symbol _SetConsoleTitle@4

Any ideas what the problem might be? Bit new to this assembly stuff.
I know its not exactly what my first post was kinda drifted to this now.

Cheers.

Capt'n Midnight

since the console screen is so small you could just fill it with spaces, bypassing the BIOS
not neat or elegant though
http://burks.brighton.ac.uk/burks/language/asm/asmtut/asm8.htm

Listing 15: DIRECTWR.ASM
; write a string direct to video memory
[php].model small
.stack
.code

mov ax,@data
mov ds,ax

mov ax,0B800h ; segment of video buffer
mov es,ax ; put this into es
mov ah,3 ; attribute - cyan
mov cx,17 ; length of string to print
mov si,OFFSET Text ; DX:SI points to string
xor di,di

Wr_Char:

lodsb ; put next character into al
mov es:[di],al ; output character to video memory
inc di ; move along to next column
mov es:[di],ah ; output attribute to video memory
inc di

loop Wr_Char ; loop until done

mov ax,4C00h ; return to DOS
int 21h[/php]

Another not neat solution - page down 26 lines and restore cursor posititon

you could also change the video mode / page
http://www.cs.uregina.ca/Links/class-info/250/f06/lab9/index.html

int 10 , Ah 6 ?
it's in the Peter Norton book ( hidden upstairs somewhere )

Webmonkey

Excellent stuff Paul. I'll have a look into all that now soon.
Thanks for that.

Martyr

Noticed and tried this before but seems to crash the program. Perhaps that is used outside Windows protected environment I amn't sure.

Wish i could use something as simple as that but doesn't want to work.

Any ideas why it might be crashing.

if you are running 386 console program using flat model, (running in protected mode), then you cannot access the video hardware using 16-bit code.(perhaps under win9x, maybe)
Under MS-DOS the writing to video memory buffer code would work fine, or MS-DOS emulator, like NTVDM

from what i can see, Webmonkey.you want 32-bit console mode program, so you have to use 32-bit Windows API, not 16-bit MS-DOS code.

here is a list of console functions

Here is code examples on MSDN to clear screen using console functions

here is example.

[PHP]; ml /coff /Cp /c clear.asm
; link /subsystem:console clear.obj kernel32.lib
;
; using win32 include headers by Japheth - http://www.japheth.de/
;
; the kernel32.lib can be found in platform SDK or lib
; directory of visual studio..etc
; ..or you could generate it with polib by pelle
;
; polib /out:kernel32.lib %SYSTEMROOT%\\system32\\kernel32.dll
;
.386
.model flat,stdcall

WIN32_LEAN_AND_MEAN equ 1

include <windows.inc>

; Cls prototype
Cls PROTO :HANDLE

.data
hStdOut DWORD ?

.code

start:
invoke GetStdHandle,STD_OUTPUT_HANDLE
mov [hStdOut],eax

invoke Cls,[hStdOut]
invoke ExitProcess,0

Cls PROC USES esi ebx edi hConsole:HANDLE

LOCAL coordScreen :COORD
LOCAL cCharsWritten :DWORD
LOCAL csbi :CONSOLE_SCREEN_BUFFER_INFO
LOCAL dwConSize :DWORD

mov [coordScreen.X], 0
mov [coordScreen.Y], 0

invoke GetConsoleScreenBufferInfo,[hConsole],addr csbi

.if eax ; if eax is non-zero

mov eax,dword ptr[csbi.dwSize.X]
mov ecx,dword ptr[csbi.dwSize.Y]
mul ecx ; calculate size of console
mov [dwConSize],eax

; fill the entire screen with spaces

invoke FillConsoleOutputCharacter,
[hConsole],VK_SPACE,[dwConSize],
coordScreen,addr cCharsWritten

.endif
ret
Cls ENDP

end start[/PHP]

and using system() from msvcrt.dll

[PHP]; ml /coff /Cp /c cls2.asm
; link /subsystem:console cls2.obj msvcrt.lib
;
; polib /out:msvcrt.lib %SYSTEMROOT%\\system32\\msvcrt.dll

.386
.model flat,stdcall

system proto c :dword
exit proto c :dword

.data

szCls db 'cls',0

.code

start:
invoke system,addr szCls
invoke exit,0

end start[/PHP]

..if you wanted to use text on same line, you could use a macro

[php]
CStr macro pszText:REQ ; macro by Japheth
local szText
.const
szText db pszText,0
.code
exitm <offset szText>
endm
[/php]

then on one line
[php]
invoke system,CStr(<'cls'>)
[/php]

Webmonkey

You're right I using Flat Model. Thanks for that, at least i know now why it was crashing.

That code looks good cheers. Even though i've handed up the assignment now - It looked ok at the end even without the screen clear.

Thanks for that code though, definetely use it in future!