bug on BTX

[John Baldwin]

On Thursday 19 October 2006 22:06, Bruce M. Simpson wrote:
> Hi all,
> 
> FYI, this change adds  32 bytes to btx leaving 139 bytes free, according 
> to btxld(8). As you probably all know, and just as a reminder, size in 
> boot2 is at a premium -- it can't go over 8192 bytes as this is the 
> boot-sector limit in the BSD disk-label.

My other changes trim btx by 48 bytes.  I actually plan on MFC'ing this stuff 
into 6.2.  We actually have 100+ free bytes in boot2 right now anyways.

> Per my IRC discussions with jhb@ : given that BTX reflects the hardware 
> interrupts, we shouldn't need to reprogram the AT-PICs; indeed GRUB does 
> not. This was one of his concerns. We can probably get away with a 
> simple trampoline a la GRUB providing the INTs invoked by BTX or LOADER 
> don't attempt to rewrite or redirect interrupt vectors once we're up and 
> running.

BTX currently uses different IDT offsets for the PICs, it puts them right
next to each other instead of using 0x70-0x78 (IIRC) for the slave PIC.
That can be adjusted though to make BTX use the same IDT offsets as the
BIOS in which case we wouldn't have to touch the PIC at all.  That would
require a larger IDT though, and I'm not sure if the IDT is statically
allocated in BTX (if so, it might result in a space problem), however
reusing the BIOS IDT offsets would be preferable to re-programming the
PICs on every mode switch.

-- 
John Baldwin