Wrong addresses being pushed on the stack

[Davidm44]

Title kind of sums it up.
Details:

I have a basic bootloader that loads the kernel, the kernel ( not really a kernel.. ) is just supposed to output a few letter to the screen. The problem is it doesn't push the right addresses onto the stack for the function to use.

The kernel is basically a linked object file compiled in C and an object file assembled in NASM.

The weird thing is, it will only fail if pointers are involved ( i.e. character arrays ), when I call the function to print using a single character, rather than an array, it works fine.

Anyone know whats going on? o_O

Note: I'm new to all this OS development stuff, don't go too hard on me..

[Chandra]

Title kind of sums it up.
Details:

I have a basic bootloader that loads the kernel, the kernel ( not really a kernel.. ) is just supposed to output a few letter to the screen. The problem is it doesn't push the right addresses onto the stack for the function to use.

The kernel is basically a linked object file compiled in C and an object file assembled in NASM.

The weird thing is, it will only fail if pointers are involved ( i.e. character arrays ), when I call the function to print using a single character, rather than an array, it works fine.

Anyone know whats going on? o_O

Note: I'm new to all this OS development stuff, don't go too hard on me..

No enough information provided to answer your question.

Still, assuming you are in Real Mode, I guess there's a problem with setup of segment registers.

[Davidm44]

Title kind of sums it up.
Details:

I have a basic bootloader that loads the kernel, the kernel ( not really a kernel.. ) is just supposed to output a few letter to the screen. The problem is it doesn't push the right addresses onto the stack for the function to use.

The kernel is basically a linked object file compiled in C and an object file assembled in NASM.

The weird thing is, it will only fail if pointers are involved ( i.e. character arrays ), when I call the function to print using a single character, rather than an array, it works fine.

Anyone know whats going on? o_O

Note: I'm new to all this OS development stuff, don't go too hard on me..

No enough information provided to answer your question.

Still, assuming you are in Real Mode, I guess there's a problem with setup of segment registers.

Sorry about that.

I'm in protected mode at the moment. What other information would I need to provide for a definite answer? Or atleast a push in the right direction. I'm debugging but I'm not sure what to look for..

[DavidCooper]

You could try pasting a relevant chunk of your source code into a post (then highlight it and click on the "Code" button higher up the screen). Make sure it's well commented so that people don't have to work hard to follow it.

[Chandra]

You could try pasting a relevant chunk of your source code into a post

and the linker script as well.

[Davidm44]

Sorry for the late reply, I wasn't on my usual computer, I didn't have my files with me.

Okay here you go, it's pretty much the basic of the basics at the moment until I continue with my readings on OS development.

Here's the portion of the bootloader that is jumping to the kernel.

Code: Select all

[bits 32]



enter_32:



	mov ax,10h

	mov ds,ax

	mov ss,ax

	mov esp,090000h



	call 0x08:0x1000

Here is the assembly portion of the kernel ( jumps to an externel function in my C object file )

Code: Select all

[bits 32]

global main
extern kmain

section .text
main:

	call kmain

	hlt

And here's the C function I'm jumping to.

Code: Select all

unsigned char *D = "TA";

extern int kmain()
{
	unsigned char* vidMem = (unsigned char*)0xB8000;

	vidMem[0] = *D;
	vidMem[1] = 0x0E;

	return 0;
}

There isn't any function to print yet because I'm having troubles with the global variables at the moment. But pointers don't seem to be working, regardless.

Here's how I'm putting it all together. I'm using gcc and NASM.

Code: Select all

nasm -f elf bootmain.asm

gcc -ffreestanding -m32 -c -o main.o main.c

ld -Ttext 0x1000 -melf_i386 --oformat elf32-i386 -o kernel.o bootmain.o main.o

objcopy -R .note -R .comment -S -O binary kernel.o kernel.bin

Thanks.

[Tosi]

I'm assuming you have a GDT set up with a 32-bit privileged code selector of 0x08 and a data one of 0x10, as the code seems to imply.

Code: Select all

 mov esp,090000h

How do you know it's safe to put your stack at 0x00090000? There might be important data structures there you will want later. It's better to reserve memory in your kernel's BSS section for the stack. And yes, it should work with a flat binary file.

Code: Select all

gcc -ffreestanding -m32 -c -o main.o main.c

Since it appears you are not using a cross compiler, you may be linking in things that you don't want, among other problems. It is recommended to use one for OS development.

Code: Select all

ld -Ttext 0x1000 -melf_i386 --oformat elf32-i386 -o kernel.o bootmain.o main.o

You are not setting the addresses of your .data, .bss, and .rodata sections at least. Either specify that on the command line, or use a linker script.

[Combuster]

call 0x08:0x1000

So you are calling what's supposed to be the start of your text section. Do you have any idea what's there? (and how did you verify that?) Why is ES left undefined?

Do you have any experience with the bochs debugger? You can use that to see if your code is actually loaded where you expect it to be, in the way you expect it to be.

[Solar]

Have you checked that your string actually ends up in the binary?

I have only skimmed the thread, but this looks like an old classic (.rodata not getting linked into the binary).

[Davidm44]

call 0x08:0x1000

So you are calling what's supposed to be the start of your text section. Do you have any idea what's there? (and how did you verify that?) Why is ES left undefined?

Do you have any experience with the bochs debugger? You can use that to see if your code is actually loaded where you expect it to be, in the way you expect it to be.

The call to kmain is in my text section, see above. I have used bochs debugger, I found where my string is, its underneath the main function ( I would assume thats where it should be, since thats my only function at the moment ).

I gave the ld command -Tdata 0x1000 but I get the same results.

When I use the linker script provided from osdev's wiki, exact same results. I'm completely stuck.

[Tosi]

Are you sure the bootloader loads your kernel correctly at 0x1000 (which is a bad place for protected mode anyway)?

[Davidm44]

Are you sure the bootloader loads your kernel correctly at 0x1000 (which is a bad place for protected mode anyway)?

Yes I'm sure, before attempting to make a kernel in C I tried doing some basic printing using assembly, and it worked fine. I would assume it's because I can declare the data section and text section in the code itself, but I'm not sure...

[b.zaar]

The call to kmain is in my text section, see above. I have used bochs debugger, I found where my string is, its underneath the main function ( I would assume thats where it should be, since thats my only function at the moment ).

I gave the ld command -Tdata 0x1000 but I get the same results.

When I use the linker script provided from osdev's wiki, exact same results. I'm completely stuck.

When you say the string is underneath your main function do you mean before main in memory? I haven't used Bochs debugger so I don't know how the output looks.
If your string is before your main then you'd be jumping into your data section not your code.
Use -Map with LD to see the order of your sections in the compiled kernel.

[Tosi]

The ELF format uses the .rodata section for strings and other read-only data, make sure you include that section in your final executable.

[Davidm44]

The ELF format uses the .rodata section for strings and other read-only data, make sure you include that section in your final executable.

Add it while linking or add it in the code itself..?

Also, Idk if I mentioned it, but I can access my variables if it isn't a character array, like if I do:

Code: Select all

char singleChar = 'A';

If I use a character array, stops working..