how to reset custom VBI to normal operation?

[Marius]

Hi.

 

I'm writing a program in which I have my own VBI routine.

 

At the beginning of the program I have thise code:

 

LDY #<VBI
LDX #>VBI
LDA #$06
JSR $E45C

<the main routines of the program; not needed for this post to type that all over>

VBI  some code
      JMP $E45F

 

This works perfect.

 

But... the program itselfs has a normal EXIT (at some point there is a RTS, and the program exits to DOS)

But then the VBI keeps running.

 

How do I re-initialise the VBI so it is just the way it WAS before I had my program?

 

Thanks

Marius

[Bryan]

I would imagine you'd need to save the original and call E45C again on exit.

[Rybags]

Do the same as before, but with E4/5F in X/Y.

 

Or if you're real fussy,

 

LDY $E460

LDX $E461

 

But the save/restore method might be more desirable given that something before you might have already been using the vector, e.g. a Time display routine.

 

You could even get real tricky and create a VBI chain, e.g. save the old vector and test the high byte, if it's less than $C0 then it means something is already using it.

[Shawn Jefferson]

Here's how I did it (actually may have nicked this from someone else ) It doesn't take into account anything else that may have hooked the VBI before my program.

 

SETVBV = $E45C ;vector to set VBLANK parameters

XITVBV = $E462 ;vector to process deferred VBLANK

 

_reset_vbi: lda #7 ; deferred vblank

ldx #>XITVBV ; hi address of vbi routine

ldy #<XITVBV ; lo address of vbi routine

jsr SETVBV ; jump to os set vblank routine

rts

Edited January 5, 2011 by Shawn Jefferson

[Heaven/TQA]

yes, simply point to the exit routine XITVBV (depending on the VBL $e462 or $e45f) and with the OS routine SETVBV. As it sets the vectors in a safe mode.

[Marius]

Is this also a good way? (EDIT: oh now I read that is just what Bryan advised...)

 

The first thing in the program is:

storing $222 and $223 in a 'safe place' (labels stored222 and stored223) ... and right before the end of the program (the RTS) I restore the info this way:

 

LDY stored222
LDX stored223
LDA #$06
JSR e45c

 

This worked too. I guess this is the 'safest' way... so that everyting was just the way it was before lauching the program.

 

Am I right or this a wrong method?

 

Thanks

Marius

 

p.s. Mapping the Atari writes that Stage1 VBI address is in VVBLKI ($222 and $223).

[drac030]

Save and restore is the best method.