Commodore Amiga A1000, Hardware Reference Manual

Page: 89 / 380

SPECIFYING AMOUNT OF DELAY
The amount of delay in horizontal scrolling is controlled by bits 7-0 in BPLCON1. You set
the delay separately for each playfield; bits 3-0 for playfield 1 (bit-planes 1, 3, and 5) and
bits 7-4 for playfield 2 (bit-planes 2, 4, and 6).
NOTE
Always set all six bits, even if you have only one playfield. Set 3-0 and 7-4 to the same
value if you are using only one playfield.
The following example sets the horizontal scroll delay to 7 for both playfields.
MOVE.W #$77,CUSTOM
SCROLLED PLAYFIELD SUMMARY
The steps for defining a scrolled playfield are the same as those for defining the basic
playfield, except for the following steps:
o Defining the data fetch. Fetch one extra word per horizontal line and start it 16 pixels
before the normal (unscrolled) data-fetch start.
o Defining the modulo. The modulo is two counts less than when there is no scrolling.
These steps are added:
o For vertical scrolling, reset the bit-plane pointers for the amount of the scrolling
increment. Reset BPLxPTH and BPLxPTL during the vertical blanking interval.
o For horizontal scrolling, specify the delay. Set bits 7-0 in BPLCON1 for 0 to 15 bits of
delay.
- 80 Playfield Hardware -