OSDev.org

Posted: **Fri Feb 01, 2002 8:48 am**

hello,
i have a program loaded to 080000h in realmode
it switches then to pmode. That works fine.
at least i call the _main function, which is written in c (djgpp)

it looks like this:

char * message = "H A L L O ";
int main()
{
....
movedata(datasel, message, screensel, 0, 10);
}

THE PROBLEM is: movedata works. But the pointer may hold the wrong adress. Cause 5 stupid signs are written to the screen.

When i take
char * message = "H A L L O ";

into the main() function it works!!!!
I know that local variables are on the stack. BUT:

a for(i=0....
causes the pc to hang! (i is declared local!!)
where can the problem be?

Posted: **Sun Feb 03, 2002 9:07 pm**

Could you maybe give some more info on this?

K.J.

Posted: **Mon Feb 04, 2002 10:16 am**

sure....what kind of info?

Posted: **Mon Feb 04, 2002 3:08 pm**

Well, the source to your movedata function might help and all the values that you feed into the movedata function in your code:

movedata(datasel, message, screensel, 0, 10);
what's datasel equal, what's screensel equal, what's the 0 and 10 for?

Also, the commands you use to link your kernel might also help.

K.J.

Posted: **Mon Feb 04, 2002 8:38 pm**

Now that I think about it you're probably using the movedata from string.h(right?). If this is so try:

#include <string.h> // movedata()

#define LINEAR_SEL 0x08
#define SYS_DATA_SEL 0x18

int main(void)
{
const char Msg[] = "h e l l o ";

movedata(SYS_DATA_SEL, (unsigned)Msg,
LINEAR_SEL, 0xB8000,
sizeof(Msg));
return 0;
};

Please note that I haven't tested the above code but found floating around on the 'net.

Still, please post the commands used to link your kernel, and the rest of your for(i=0.... loop.

K.J.

Posted: **Tue Feb 05, 2002 12:47 am**

1st: movedata works (copied from an older version of this project)

2nd: i dont link against the libc:
ld -o loader.oos -e start --oformat binary -Ttext 0x0 $(OFILES)

where ofiles are the objectcodefiles produced by gcc -c ...

the problem is:
this works:

Code: Select all

int main()
{
        movedata(DATA_SEL, (unsigned)"H A L L O ", SCREEN_DATA_SEL, 0, 10);
        while(1);
}

this dont:

Code: Select all


char *message = "H A L L O ";
int main()
{
movedata(DATA_SEL, message, SCREEN_DATA_SEL, 0, 10);
        while(1);
}

movedata looks like this (altough it cant be the problem):

Code: Select all

/*
ssel...sourcesegment selector
s.......sourceoffset
dsel...destinationsegment selector
d.......destination offset
len.....how many bytes to copy

void movedata(unsigned ssel, unsigned s, unsigned dsel, unsigned d, unsigned len)
{
        asm(
        "pushw %%ds\n"
        "pushw %%ax\n"
        "popw  %%ds\n"
        "pushw %%es\n"
        "pushw %%bx\n"
        "popw  %%es\n"
        "moveloop: \n"
        "lodsb     \n"
        "stosb     \n"
        "loop moveloop\n"
        "popw   %%es\n"
        "popw   %%ds"
        ::"a" (ssel), "b" (dsel), "D" (d), "S" (s), "c" (len));
}

gdt looks like this (descriptor is a makro, that copies all the values into the gdt
1st one: nil
2nd one: kernel codeseg (yet not used)
3rd: kernel data (not used)
then stack, codeseg (for this program), dataseg(for this program (the loader)) and then the screensegment

descriptor 0,0,0,0,0,0,0
;codeseg base 0 limit FFFFF, gran
descriptor 8, 0FFFFh, 0, 0, 9Ah, 0CFh, 0
;dataseg base 0 limit FFFFF, gran
descriptor 16, 0FFFFh, 0, 0, 092h, 0CFh, 0
;stackseg, base 70800h
descriptor 24, 0F800h, 0800h, 07h, 092h, 040h, 0
;codeseg base 0x80000 limit FFFFF, gran
descriptor 32, 0FFFFh, 0, 08h, 09Ah, 0CFh, 0
;dataseg base 0x80000 limit FFFFF, gran
descriptor 40, 0FFFFh, 0, 08h, 092h, 0CFh, 0
;videoseg base 0xB8000 limit 4000
descriptor 48, 0FA0h, 08000h, 0Bh, 092h, 040h, 0

you see the problem is: why doesnt it work when message is declared globally?

Posted: **Tue Feb 05, 2002 12:56 am**

have you tried:

char message[] = "HALLO";

??

- Nick

Posted: **Tue Feb 05, 2002 1:03 pm**

Yeah, I think that you ought to try what Nick suggests, just change it to:

char message[] = "H A L L O ";

And if that doesn't work then I don't have a clue why it won't work when declared globaly

. One thing I do suggest though is that if you are wanting to make a printf function then I suggest using something different than movedata.

K.J.

Posted: **Wed Feb 06, 2002 5:39 am**

what else should i use to move datas?
when i change char *messge into message[] ... the pc reboots
also it does not make sense to me:

this c-code:

Code: Select all

char *global1="GLOBAL*";
char global2[]="GLOBAL[]";

int main()
{
        char *local1="LOCAL*";
        char local2[]="LOCAL[]";
}

assembles to:

Code: Select all

      .file      "test.c"
.globl _global1
      .section .text
LC0:
      .ascii "GLOBAL*\0"
      .section .data
      .p2align 2
_global1:
      .long      LC0
.globl _global2
_global2:
      .ascii "GLOBAL[]\0"
      .section .text
LC1:
      .ascii "LOCAL*\0"
LC2:
      .ascii "LOCAL[]\0"
      .p2align 4
.globl _main
_main:
      pushl      %ebp
      movl      %esp, %ebp
      subl      $24, %esp
      andl      $-16, %esp
      movl      $LC1, -4(%ebp)
      movl      LC2, %eax
      movl      LC2+4, %edx
      movl      %eax, -16(%ebp)
      movl      %edx, -12(%ebp)
      movl      %ebp, %esp
      popl      %ebp
      ret
      .ident      "GCC: (GNU) 3.0.3"

it seems that the only difference is that message[] will keep the real name (_message) in asm code.

So i took a look with the hexeditor and found that
H A L L O is contained in the file. I manually changed the sourceoffset in movedata to the offset read in the hexeditor.
And it worked. It also cant be a problem with the dataseg cause local strings are also in it.

Posted: **Wed Feb 06, 2002 6:17 am**

The only reason why I suggested it was that GCC normally sets constant strings as read only. Presumably to save memory because you can lump similar strings together. I don't know a lot about compilers or linkers so I don't know which stage it actually does this. Anyway.. if you set it as an array instead of a pointer, it won't set it as readonly.

I noticed you're using GCC v3.0.3.. have you tried the v2.9xx range? v3 is pretty recent and some people have had problems with it still.

- Nick

Posted: **Wed Feb 06, 2002 2:02 pm**

What else should you use to move data? Do you mean "what else can I do for a printf type function?" if so then this is what I use, with a little bit of editing you should be able to easily make a printf type function:

main()
{
char *vidmem = (char *) 0xb8000;
char message[]="Hello";

vidmem[0]=message[0];
vidmem[1]=0x7; // for white on black text
vidmem[2]=message[1];
vidmem[3]=0x7;
vidmem[4]=message[2];
vidmem[5]=0x7;
vidmem[6]=message[3];
vidmem[7]=0x7;
vidmem[8]=message[4];
vidmem[9]=0x7;
};

K.J.

Posted: **Thu Feb 07, 2002 1:51 am**

and wheres the difference?...if i move a block of data or every sign for itself?....also movedata is much faster and you can also use uit for other datatransfers.

BUT that is not the PROBLEM.
I tried gcc 2.952 & 2.953 ...it still doesnt work.

Posted: **Thu Feb 07, 2002 3:08 pm**

Does your bootloader set up your DS(data segment) and cs(code segment) properly(or at all)?

K.J.

Posted: **Fri Feb 08, 2002 12:23 am**

the bootsector loads loader.oos (FAT FS) to 08000h:0
loader.oos then starts with

push cs
pop ds

after switching to pmode:
mov ax, datasel
mov ds, ax
mov es, ax
mov fs, ax
mov ax, screensel
mov gs, ax
mov ax, stacksel
mov ss, ax
mov esp, stacksize
then calls main()

Posted: **Fri Feb 08, 2002 12:28 am**

i can send you the code if want to have a look at it.

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