C++ video memory access

[jpal]

In my kernel I am trying to write to the screen.

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*((char*)0xB8000) = 72;
*((char*)0xB8001) = 15;

which compiles to

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movb $72, ($0xB8000)
movb $15, ($0xB8000)

displays an 'H', but

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char* ptr = (char*)0xB8000;
*ptr = 72;
*(ptr + 1) = 15;

which compiles to

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movl $0xB8000, -0x4(%ebp)
mov -0x4(%ebp), %eax
movb $72, (%eax)

mov -0x4(%ebp), %eax
add %eax, 1
movb $15, (%eax)

does not show any characters.

I do use a cross compiler so that is not the problem.
Please help! Thanks.

[YaYo]

maybe because you are using a char* pointer,
i.e. :
char* ptr = (char*)0xB8000;
*ptr = 72; // now ptr equals 0xB8000
*(ptr + 1) = 15; // but now is (0xB8000 + sizeof(char*)) equals 0xB8004
// try casting.

[thepowersgang]

@YaYo - Actually, that's not the problem.

@OP - Are you running on bochs/qemu and putting a cli;hlt loop soon afterwards? Iirc one (or both) of these will stop updating the screen when the CPU is halted in this manner.

The code looks correct to me (pretty obviously compiled with optimisaton off), so it's probably either the code not running, or something else hiding its effect.

[YaYo]

try to change this line

*(ptr + 1) = 15;

to this *((char*)(((unsigned long)ptr) + 1)) = 15;.

Note: Bochs, Qemo, VBox,...etc have nothing to do with this problem.

[thepowersgang]

@YaYo - Please actually check what you're saying, and don't comment unless you have actually read and understood the post.

The generated assembly is correct (It's adding one to the address, not four). Which points to an error outside of the provided code, which is most likely bochs/qemu not updating the screen (this has caught almost everyone once, myself included)

[Combuster]

In my kernel I am trying to write to the screen.

Code: Select all

*((char*)0xB8000) = 72;
*((char*)0xB8001) = 15;

which compiles to

Code: Select all

movb $72, ($0xB8000)
movb $15, ($0xB8000)

That's obviously wrong. I get

0: c6 05 00 80 0b 00 48 movb $0x48,0xb8000
7: c6 05 01 80 0b 00 0f movb $0xf,0xb8001

What else did you not copy-paste properly?

[evoex]

What else did you not copy-paste properly?

I'd daresay this line:

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add %eax, 1

Which ought to be:

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add $1, %eax