Can't get higher half bare bones working

[Patrick]

Hi. I'm following the "Higher Half bare bones" tutorial, and it's not working. I'm trying to translate the NASM assembly in the tutorial to gas, which could be my problem. I know the basics of how assembly works, but I am mostly just learning as I go.

Here's the code:
http://pastebin.com/wL1AMbHs

When I run it in VirtualBox it hangs at line 64 but there is no error message. If I run it on my actual hardware it makes it to line 65 and just reboots.

Thanks.

[thepowersgang]

I would suggest running in bochs to begin with, it's far easier to debug.

That said, have you checked the values of the symbols used, specifically the stack?

[gerryg400]

Code: Select all

    # jump to higher half
    lea (higher_half), %ecx
    jmp *%ecx
 
higher_half:

What is the value of higher_half ?

You haven't shown your linker script. You need to link your code so that the load address is in low memory but the link (virtual) address in in higher memory. Did you do that ? Disassemble your binary and see what value if loaded into ecx.

[Solar]

I'm trying to translate the NASM assembly in the tutorial to gas, which could be my problem.

Hint: Use NASM to assemble the tutorial code, then use objdump -d to disassemble the binary. Comparing the disassembly with your GAS source could give you pointers to where your translation didn't quite work out.

[Patrick]

Code: Select all

    # jump to higher half
    lea (higher_half), %ecx
    jmp *%ecx
 
higher_half:

What is the value of higher_half ?

You haven't shown your linker script. You need to link your code so that the load address is in low memory but the link (virtual) address in in higher memory. Did you do that ? Disassemble your binary and see what value if loaded into ecx.

My linker script is basically the same as in the tutorial. That line assembles to

Code: Select all

lea 0xc0100031,%ecx

.

I'm trying to translate the NASM assembly in the tutorial to gas, which could be my problem.

Hint: Use NASM to assemble the tutorial code, then use objdump -d to disassemble the binary. Comparing the disassembly with your GAS source could give you pointers to where your translation didn't quite work out.

Thanks, I'll try that tomorrow.