Commodore Amiga A500 Technical Reference Manual page 206

This register contains the width and height of the blitter operation
(in line mode width must = 2, height = line length). Writing to this
register starts the Blitter, and should be done last after all pointers
and control registers have been initialized.
BIT# 15,14, 13, 12, 11, IO, 09,08, 07,06.05, 04. 03.02, 01, 00
h9 h8 h7 h6 h5 h4 h3 h2 hi hO, w5 w4 w3 w2 wl wO
h = Height = Vertical lines (10 bits= 1024 lines max)
w = Width = Horiz. pixels (6 bits = 64 words = 1024 pixels max)
LINE DRAW: BLTSIZE controls the line length and starts the line
draw when written to. The h field controls the line
length (10 bits gives lines up to 1024 dots long). The
w field must be set to 02 for all line drawing.
BPL x PTH
BPL x PTL
This pair of registers contains the 18 bit pointer to 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 to point to the beginning of
Bit Plane data every vertical blank time.
BPL 1 MOD
BPL2MOD
These registers contain the Modulos for the odd and even bit planes.
A Modulo is a number that is automatically added to the address at
the end of each line, in order that the address then points to 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
This register controls the operation of the Bit Planes and various as-
pects of the display.
203
Bit plane x pointer (high 3 bits)
Bit plane x pointer (low 15 bits)
Bit plane modulo (odd planes)
Bitplane modulo (even planes)
Bit plan control register
(miscellaneous control bits)