[cc65] Re: signals

> Message-ID: <429C4ADE.6080801@brahms.demon.co.uk>
 
dominic beesley <dominic@brahms.demon.co.uk> wrote:
> What if the user presses break, what if I _dont_ set up an
> Escape handler and the user presses Escape. What if a program is
> in a long loop (doing file stuff) and the user presses Escape
> and nothing happens? they'll press Break and make just as much
> of a mess.
 
There are many instances where pressing Escape "does nothing",
because the computer is in the middle of something and hasn't got
to the end and checked the escape flag yet.
 
  *SAVE name 0+1000000
 
isn't going to respond to the Escape flag being set until it has
finished.  PLOT 96 (draw a circle) isn't going to check the Escape
flag until it has stopped drawing. A really impatient user could
get bored at any time and press Break. If the user presses Break
they deserve whatever they get. 
 
> Also on many you've got to be careful about what you expect to
> be safe; structures may be half updated, variables not correctly
> set (although this is probably worse in 6502 as even a simple
> increment on a 16bit no. will take a number of instructions).
 
If you abort on the background Escape event in the middle of:
 
  strcpy(src,dst);
 
then the destination memory will be left in an inconsistant state.
 
 
 
> It looks as if there are two camps on this, those that dont want any form of
> out-of-band signals sue to escape and those that do. I think I may put
> all this
> stuff in the BBC library but have it compile-time switchable some how
> (either
> when compiling the end program or when compiling the library).
 
That's the solution I was working towards with Small-C. Let the
programmer chose whether to use os_checkescape() themselves, or
allow the Escape event to abort the program instead of setting the
Escape flag.
 
I'm now thinkinking of adding another os_* escape function, that
does something like:
 
os_something()
{
if !os_checkescape() return(void);
os_byte(126, 0);
os_error(17, "Escape");
}
 
so mainline code can just call os_something() and have the error
generation done for it if an escape state is set.
 
Now I've got my BBC hard drive interface working, I really should
copy the C compiler over to is and do some more work on it.
 
-- 
J.G.Harston - jgh@arcade.demon.co.uk - mdfs.net/User/JGH
BBC IDE Hard Drive Interface - http://mdfs.net/Info/Comp/BBC/IDE
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