--- "H. Peter Anvin" <[email protected]> wrote:
> >> - The VGA recalc has the same bug as the assembly version where a VGA
> >> write protected register is written (Overflow register) without setting
> >> the enable bit (see VGA docs).
>
> I dug into this, and it turns out you're incorrect. Both the assembly
> code and the C code are, in fact, 100% correct:
>
> The only instance of writing the vertical overflow register is this code
> in vga_set_480_scanlines():
>
> out_idx(0x0c, crtc, 0x11); /* Vertical sync end, unlock CR0-7 */
> out_idx(0x0b, crtc, 0x06); /* Vertical total */
> out_idx(0x3e, crtc, 0x07); /* Vertical overflow */
> out_idx(0xea, crtc, 0x10); /* Vertical sync start */
> out_idx(end, crtc, 0x12); /* Vertical display end */
> out_idx(0xe7, crtc, 0x15); /* Vertical blank start */
> out_idx(0x04, crtc, 0x16); /* Vertical blank end */
>
> Register 0x11 has the Protect (not enable!) bit in it, it is bit 7. As
> you can see, it is cleared (meaning writable) at the beginning of this
> sequence, and the fact that it's being done is even documented.
The only time I ever needed this "end line recalculation" was when the heigh in
graphic lines was not a multiple of the character heigh - i.e. 640x350 with 8x16
or 8x8 chars - some VGA adapters do not hide the bottom graphic lines.
The function vga_set_480_scanlines() is not called, and the protect bit is never
cleared - the video BIOS leaving those low index register protected.
The function vga_recalc_vertical() (or its assembler equivalent) is probably
perfectly called but because the protect bit is never cleared, the few graphic
line are displayed during the whole Linux text session...
I have myself never seen any other problems when the graphic heigh is a multiple
of the character heigh - tested on ~40 video boards.
Etienne.
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