Blitter Mask shift during copy

[LeCaravage]

Folks,
Another technical question
I don't understand what exactly the blitter is doing with mask when shifting bob to be displayed at pixel perfect position in a screen -hope you understand my bad english-.

w=320 ; size in pixel of screen

move.w #$89f0,$40(a6)
clr.w $42(a6)
move.w #$ffff,$44(a6)
move.w #$0000,$46(a6)
move.w #-2,$64(a6)
move.w #w/8-4,$66(a6)
move.l d0,$50(a6)
move.l d1,$54(a6)
move.w #2+64*31,$58(a6)

So, the bob is 16 pixels wide. I shift the bob with 8 pixel but the right mask is null, so why the going over edges 8 pixels are displayed anyhow ? They should be also null ?
Does it means the blitter shift also the 2 masks during copy ?
So it means doing bob clipping is more tricky than expected ?

[mcgeezer]

First off u have a typo, i think you mean to clear bltcon1 (42) but u are clearing 44 - first word mask.

Willlook more later

[LeCaravage]

@mcgeezer : Corrected, thanks.

[LeCaravage]

I should learn to read carefully the HRM before posting questions.
Quoting the HRM :

These masks are "anded" with the source data, before any shifts are applied.

Beside, the example I gave is not appropriate for as it's a straight copy.

[mcgeezer]

Originally Posted by LeCaravage

I should learn to read carefully the HRM before posting questions.
Quoting the HRM :


Beside, the example I gave is not appropriate for as it's a straight copy.

Indeed, there’s a good explanation in the hrm
Of cookie cutter mode (fca) which tells u exactly how to do it.

Will post an example later on for u.

[LeCaravage]

Originally Posted by mcgeezer

Will post an example later on for u.

No need, thank you, as I've already used the cookie copy mode.
In fact, I posted this question because I'd like to code a bob clipping routine which doesn't need to expand the bitplan modulos (copy only bob's pixel that are within screen). Anyway, thanks for taking time for helping me.

Looking forward to see your game finished

[Photon]

Moved to the Asm/Hardware forum