[OpenBIOS] PATCH: Correct b?branch for negative offset branches

[Blue Swirl]

On 4/6/09, Mark Cave-Ayland <mark.cave-ayland at siriusit.co.uk> wrote:
> Hi everyone,
>
>  Please find enclosed a patch against current SVN that allows b?branch to
> work for negative offset branches. With this patch applied, the following
> output can be seen under openbios-unix on x86 (32-bit):
>
>
>  For the non-branch case (flags is set to -1):
>
>  0 > here  ok
>  1 > 10 c, ff c, ff c, ff c, ff c,  ok
>  1 > b1 c,  ok
>  1 > 10 c, aa c, aa c, aa c, aa c,  ok
>  1 > 9d c,  ok
>  1 > 02 c, 16 c,  ok
>  1 > 14 c, f7 c,  ok
>  1 > 02 c, 16 c,  ok
>  1 > 00 c,  ok
>  1 > dup 1 byte-load
>  byte-load: exception caught!
>   ok
>
>
>  For the branch case (flags is set to 0):
>
>  0 > here  ok
>  1 > 10 c, 00 c, 00 c, 00 c, 00 c,  ok
>  1 > b1 c,  ok
>  1 > 10 c, aa c, aa c, aa c, aa c,  ok
>  1 > 9d c,  ok
>  1 > 02 c, 16 c,  ok
>  1 > 14 c, f7 c,  ok
>  1 > 02 c, 16 c,  ok
>  1 > 00 c,  ok
>  1 > dup 1 byte-load -55555556
>  byte-load: exception caught!
>   ok
>
>
>  Note that in terms of docbranch, the branch action being taken is actually
> the opposite to what you may expect due to the use of setup-tmp-comp (hence
> the extra invert is required). So as an example, if flags is set to 0 then
> docbranch does not change the PC, and so we execute the contents of the
> temporary buffer which contains everything from b(<mark) to b(>resolve). On
> the other hand, if flags is non-zero then docbranch will branch back to the
> b(>resolve), and so the temporary buffer contents are ignored.
>
>  AFAICT the behaviour is correct from the above examples, but again I'd
> suggest that someone with more OpenBIOS experience should verify to make
> sure that this is the right approach.

Thanks, applied.