Commodore Amiga A500 Technical Reference Manual page 209

This register contains the width and height of the blitter operation
(in line mode width must
=
2, height
=
line length). Writing t o this
register starts the Blitter, and should be done last, after all pointers
and control registers have been initialized.
h
=
Height
=
Vertical lines
( l 0
bits
=
1 0 2 4 lines max)
W =
Width
=
Horiz. pixels (6 bits
=
64
words
=
1 0 2 4 pixels max)
LINE DRAW: BLTSIZE controls the line length and starts the line
draw when written to. The h field controls the line
length
( l
0
bits gives lines up t o 1 0 2 4 dots long). The
W
field must be set t o 02 for all line drawing.
BPL
X
PTH
Bit plane
X
pointer (high 3 bits)
BPL
X
PTL
Bit plane
X
pointer (low 15 bits)
This pair of registers contains the 1 8 bit pointer t o the address of Bit
plane
X (X =
1,2.3,4,5,6)
DMA
data. This pointer must be reinitialized
by the processor or Copper t o point t o the beginning of Bit Plane
data every vertical blank time.
BPLl MOD
Bit plane modulo (odd planes)
BPL2MOD
Bit plane modulo (even planes)
These registers contain the Modulos for the odd and even bit planes.
A Modulo is a number that is automatically added t o the address a t
the end of each line, in order that the address then points t o the start
of the next line. Since they have separate modulos, the odd and even
bit planes may have sizes that are different from each other, as well
as different from the Display Window size.
BPLCONO
Bit plan control register
(miscellaneous control bits)
This register controls the operation of the Bit Planes and various as-
pects of the display.