[lkml]   [2000]   [Nov]   [29]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: why volatile on vgacon.c?
On Wed, Nov 29, 2000 at 05:24:15PM +0100, Santiago Garcia Mantinan wrote:
> Hi!
> I used to be able to run my 12 ethernet ports pentium based bridge without
> vga card, but with tty1, tty2, ... still working, as the kernel used to
> recognice a kind of a cga card on my machine even though there was none. But
> the kernel could write to the memory were the card was supposed to be, and
> so it worked.
> That was on 2.2 series, but since I moved it to 2.4 series I don't have that
> cga card found anymore. I have looked on the kernel code and followed it to
> the __init function in vgacon.c, more concretely this piece of code...
> scr_writew(0xAA55, p);
> scr_writew(0x55AA, p + 1);
> if (scr_readw(p) != 0xAA55 || scr_readw(p + 1) != 0x55AA) {
> Well, the thing is that this code and the code in this function is almost
> the same in 2.4 as in 2.2, however reading returns the written values on 2.2
> and different ones (0xffff) on 2.4
> This is caused by the volatile declaration of *p on 2.4, so the questions
> are:
> was the old (I have found a CGA) behaviour considered a bug and is the
> volatile declaration its fix?

Yes. The compiler was optimizing too much.

> If so, is there any way to have /dev/tty1 on a no graphic card i386 machine?
> (besides unvolatilizating *p wich works for me)

I think you can use serial console instead, if you have serial ports in
the machine.

Vojtech Pavlik
SuSE Labs
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to
Please read the FAQ at

 \ /
  Last update: 2005-03-22 12:47    [W:0.067 / U:1.200 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site