Re: newbie question on gnu 'as' => explaination about the why (for really newbie ;-))

[Frederic Marmond <fmarmond@eprocess.fr>]

Hi,

Here are some explainations you may want to know.

When your PC boots, it is in real mode.
This mode (!= protected mode) manage memory in this way:
segment+offset
The segment is a 64ko range , step by 16 bytes.
So, if you want to write to b8000 (video memory start in text mode), you 
may set your segment at 0xb800 (=> 0xb800 * 16 = 0xb8000)
Then, you can adress this segment of 65536 bytes with an offset of 16bits.
(that's why in real mode you can only see the first 1Mo of your memory: 
0xFFFF*16 = 0xFFFF0  ~= 1024*1024  (in fact, a little bit more than 1Mo, 
because of the offset of 0xffff max))
For your problem, in pseudo code:
mov 0x411f to 0xb800:0    (write 0x411f to the corner high-left)
mov 0x411f to 0xb800:(80*2 *10+ 20*2)    (write 0x411f to the 10th line 
(80*2 *10= 80(charPerLine) *2(1char=ascii+attribut) *10(10th line)) and 
20th char)

in assembly, segments are stored in special registers: ds (Data 
Segment), CS(Code Segment), Es,Fs,Gs (Extra Segments), SS(Stack Segment)
Offset may be any of the followings (Ax,Bx,Cx,Dx,Si,Di,Sp). (and, in 
32bits mode: eAx,eBx,eCx....)

So, the pseudo code may be write like that ((in intel syntax, which I 
prefer, and that compile with nasm):
mov    ax,0xb800
mov    es,ax                ; we cannot affect directly value to seg 
register (cannot write mov es,0xb800)
mov    bx,0x0            ; start of the segment
mov     [es:bx],0x411f    ;the compilator will assume automaticaly movw 
(mov word => 16 bits)

That is what Emanuele wrote you (in At&t syntax):

           movw    $0xB800, %ax
           movw    %ax, %es			; affect the start adress to the segment
           movb    $0x41, %es:0  			; store value in the video memory
           movb    $0x1f, %es:1					; these 2 lines may be = to movw $0x411f,%es:0


hope it helps!

Fred

Anticipating a Reply wrote:

>Hello Emanuele ,
>
>   Thank you for the below code 
>you sent . It works perfectly and
>will help me learn more of assembly .
>
>   Can you please help me figure 
>out where I'am wrong in the code below,
>which I have witten for the same 
>functionality .
>
>.code16
>
>.globl _start 
>
>_start: 
>       movw $0xb800,%bx  # Video Memory address   
>       movb $0x41,(%bx)  # ASCII value to be displayed
>
>
>       movw $0xb801,%bx  # Video Memory address    
>       movb $0x1f,(%bx)  # Attribute of the Character 
>loop1: 
>       jmp loop1
>
>
>    Thanks once again .
>
>With Best Regards
>
> --- Emanuele Altieri <ealtieri@hampshire.edu> wrote:
>  
>
>>I'm assuming that you want to keep your code as a
>>boot program:
>>
>>           .code16               # needed for
>>booting
>>           .global _start
>>   _start:
>>           movw    $0xB800, %ax
>>           movw    %ax, %es
>>           movb    $0x41, %es:0  
>>           movb    $0x1f, %es:1
>>   loop1:  jmp     loop1
>>
>>
>>To assembly and link the program:
>>
>>   as -o boot.o boot.s
>>   ld -s -oformat binary -Ttext 0x0 -o boot boot.o
>>
>>The last command creates the "boot" file that is
>>ready to be 
>>installed in the first sector of the floppy disk:
>>
>>   dd seek=0 if=boot of=/dev/fd0
>>
>>
>>Hope this helps
>>
>>  -Emanuele
>>
>>
>>On Tue, 2002-07-30 at 07:59, Anticipating a Reply
>>wrote:
>>    
>>
>>>Hi Everybody,
>>>
>>>        I have got a small piece of Assembly code
>>>      
>>>
>>,
>>    
>>
>>>which writes a character "A" on the screen .
>>>
>>>  The below program is compiled and written 
>>>to the boot sector of a floppy. It compiles
>>>using the command :
>>>
>>>	as86 boot.s -o boot.o
>>>	ld86 -d boot.o -o boot 
>>>
>>>   I want to rewrite it in AT&T syntax and
>>>want to use GNU "as" assembler to compile it .
>>>I tried doing it ,but with no success .
>>>
>>>  Can anybody please help me rewite the
>>>below code in AT&T syntax and tell me how
>>>to compile & link it ?
>>>
>>>   Thanks in Advance .
>>>
>>>With Best Regards
>>>
>>>
>>>THE CODE :
>>>------------
>>>
>>>
>>>entry start 
>>>
>>>start: 
>>>       mov ax,#0xb800 ; Video Memory address is
>>>      
>>>
>>0xb800
>>    
>>
>>>  
>>>       mov es,ax      ; Video Memory is made as
>>>      
>>>
>>Extra
>>    
>>
>>>Segment
>>>       seg es         ; Makes next instruction
>>>      
>>>
>>execute
>>    
>>
>>>wrt Extra Segment 
>>>       mov [0],#0x41  ; ASCII value of the
>>>      
>>>
>>Character
>>    
>>
>>>to be displayed 
>>>                      ;  in 1-row & 1-column
>>>       seg es         ; Makes next instruction
>>>      
>>>
>>execute
>>    
>>
>>>wrt Extra Segment 
>>>       mov [1],#0x1f  ; Attribute of the Character
>>>      
>>>
>>to
>>    
>>
>>>be displayed
>>>                      ;  in 1-row & 1-column
>>>                      ; 0x1f represents White
>>>Character on a Blue Background
>>>loop1: 
>>>       jmp loop1
>>>
>>>
>>>
>>>      
>>>
>________________________________________________________________________
>  
>
>>>Want to sell your car? advertise on Yahoo Autos
>>>      
>>>
>>Classifieds. It's Free!!
>>    
>>
>>>       visit http://in.autos.yahoo.com
>>>-
>>>To unsubscribe from this list: send the line
>>>      
>>>
>>"unsubscribe linux-assembly" in
>>    
>>
>>>the body of a message to majordomo@vger.kernel.org
>>>More majordomo info at 
>>>      
>>>
>>http://vger.kernel.org/majordomo-info.html
>>
>>
>>    
>>
>>># boot.s
>>>      
>>>
>>#
>># To assembly and link as a **boot** program:
>>#
>>#    as boot.s -o boot.o
>>#    ld -s -oformat binary -Ttext 0x0 -o boot boot.o
>>#
>># To copy it to the boot sector of a floppy disk:
>>#
>>#    dd seek=0 if=boot of=/dev/fd0
>>#
>># ---
>># Emanuele (ealtieri@hampshire.edu)
>>	
>>	.code16
>>	.global _start
>>_start:
>>	movw	$0xB800, %ax
>>	movw	%ax, %es
>>	movb	$0x41, %es:0
>>	movb	$0x1f, %es:1
>>loop1:	jmp	loop1
>> 
>>    
>>
>
>________________________________________________________________________
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>       visit http://in.autos.yahoo.com
>-
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>  
>