How to access to flat memory ?

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fafastrungen

Joined: 12 Aug 2006
Posts: 27

fafastrungen 16 Feb 2007, 02:16

Hi, I want to write a byte value in the memory position: 15728640.
I know that first of all I must have at least 15728641 MB of ram in the system, this is ok for me, I have it, but how do I access to this position using something like this:

Code:

mov  edi, 15728640
mov  al, 'X'
mov  [edi], al

I want to avoid the use of segmentation becouse all the current processors can access to memory in a flat way.
I don't use DOS at all.

16 Feb 2007, 02:16

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4624
Location: Argentina

LocoDelAssembly 16 Feb 2007, 02:31

In fact you need no more than 16 MB (15728640/1024^2). The code is correct but it requires protected mode (or unreal mode) and some code that sets DS with a descriptor with full access to the 4 GB address space. There are lots of these codes on OS construction subforum. The sources of FASM (SOURCE\DOS) has code that enters to this unreal mode too.

16 Feb 2007, 02:31

Dex4u

Joined: 08 Feb 2005
Posts: 1601
Location: web

Dex4u 16 Feb 2007, 16:27

Something like this would work:

Code:

;************************************; Pmode, by Dex; Assemble with fasm; c:\fasm Pmode.asm Pmode.com;************************************org 0x100use16;****************************; Realmode startup code.;****************************start:        xor   ax,ax        mov   ds,ax        mov   es,ax;****************************; enable  A20 line;****************************        pusha        cli                                    ; Disable all irqs        cld        mov   al,255                           ; Mask all irqs        out   0xa1,al        out   0x21,all.5:    in    al,0x64                          ; Enable A20        test  al,2                             ; Test the buffer full flag        jnz   l.5                              ; Loop until buffer is empty        mov   al,0xD1                          ; Keyboard: write to output port        out   0x64,al                          ; Output command to keyboardl.6:    in    al,0x64        test  al,2        jnz   l.6                              ; Wait 'till buffer is empty again        mov   al,0xDF                          ; keyboard: set A20        out   0x60,al                          ; Send it to the keyboard controller        mov   cx,14hl.7:                                           ; this is approx. a 25uS delay to wait        out   0edh,ax                          ; for the kb controler to execute our        loop  l.7                              ; command.        sti        popa;*****************************; Setting up, to enter pmode.;*****************************        cli        lgdt  [gdtr]        mov   eax, cr0        or    al,0x1        mov   cr0,eax        jmp   0x10: protected;*****************************; Pmode. ;*****************************use32protected:        mov   ax,0x8         mov   ds,ax        mov   es,ax        mov   ss,ax;*****************************; Turn floppy off (if space).;*****************************        mov   dx,3F2h        mov   al,0        out   dx,al;*****************************; put your code here;*****************************        jmp   $;*************************************; GDT.;*************************************gdt:        dw    0x0000, 0x0000, 0x0000, 0x0000sys_data:   dw    0xFFFF, 0x0000, 0x9200, 0x00CFsys_code:   dw    0xFFFF, 0x0000, 0x9800, 0x00CFgdt_end:gdtr:       dw gdt_end - gdt - 1            dd gdt

Note: you should assemble the above code as a com file and run it from dos, but it will not return to Dos.
If you want to return to Dos, you can use some of this demo:
http://www.dex4u.com/tuts/DemoVesa.zip

16 Feb 2007, 16:27

fafastrungen

Joined: 12 Aug 2006
Posts: 27

fafastrungen 16 Feb 2007, 20:03

Thanks for the answers.
I forgot to say that I use an AMD64, so the real mode and segmentation is not necesary.
I don't need compatibility with 32 bits, I just want to do this only for 64 bits.

16 Feb 2007, 20:03

Dex4u

Joined: 08 Feb 2005
Posts: 1601
Location: web

Dex4u 17 Feb 2007, 17:11

Then you should look here:
http://board.flatassembler.net/topic.php?t=6206

17 Feb 2007, 17:11

fafastrungen

Joined: 12 Aug 2006
Posts: 27

fafastrungen 17 Feb 2007, 23:00

Dex4u wrote:

Then you should look here:
http://board.flatassembler.net/topic.php?t=6206

Perfect, this is what I was looking for.

thanks.

17 Feb 2007, 23:00

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