Re: Cell commands

On Fri, 25 Jan 2013 21:12:42 +0100, Alex Thüring wrote:

> if I specify conditional cell command such as "if (g0 == 1) jump
> title 1;", I get error ERR:  Cell command can only compile to one VM
> instruction. But some other software (e.g. DVDReMake) supports cell
> conditional commands. And I also didn't find this restriction at
>      http://dvd.sourceforge.net/dvdinfo/pgc.html
> Is it possibly a bug in dvdauthor?

No, this is a limitation of the DVD-Video spec itself. If you look at
the detailed instruction layouts at
<http://dvdnav.mplayerhq.hu/dvdinfo/vmi.html>, you will see that a
conditional JumpTT instruction (the green line below the
NOP/Exit/Jump/Call lines) only allows both operands being compared to be
registers, there is no option for comparing a register to a literal
value. Comparing a register to a literal is only allowed for goto/link
instructions (top two green blocks) and conditional-set instructions
(bottom green block in that top table). So dvdauthor compiles your
statement above to multiple instructions (using a goto), which will not
fit into a cell command.

The only way around this is to set up your own manual redirect—perhaps
instead of trying to jump directly to title 1, you jump to another cell
which contains an unconditional “jump title 1” command. This might be a
dummy cell that shows a fraction of a section of a black screen, or
something.

You’re not the first to be baffled by the peculiar characteristics of
the DVD-Video format. That’s why I collected every bit of info I could
find into this wikibook:
<http://en.wikibooks.org/wiki/Inside_DVD-Video>.

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