Commodore Amiga A1000, Hardware Reference Manual

Page: 51 / 380

You select the number of bit-planes by writing the number into the register BPLCON0 (for
Bit Plane Control Register 0) The relevant bits are bits 14, 13, and 12, named BPU2,
BPU1, and BPU0 (for "Bit Planes Used"). Table 3-5 shows the values to write to these bits
and how the system assigns bit-plane numbers.
Table 3-5: Setting the Number of Bit-Planes
Number of Name(s) of
Value Bit-Planes Bit-Planes
000 None *
001 1 PLANE 1
010 2 PLANES 1 and 2
011 3 PLANES 1 - 3
100 4 PLANES 1 - 4
101 5 PLANES 1 - 5
110 6 PLANES 1 - 6 **
111 7 Value not used.
* Shows only a background color; no playfield is visible.
** Sixth bit-plane is used only in dual-playfield mode and in hold-and-
modify mode (described in the section called "Advanced Topics").
NOTE
The bits in the BPLCON0 register cannot be set independently. To set any one bit, you
must reload them all.
The following example shows how to tell the system to use two low-resolution bit-planes.
MOVE.W #$2200,CUSTOM ; Write to it
Because register BPLCON0 is used for setting other characteristics of the display and the
bits are not independently, the example above also sets other parameters (all of these
parameters are described later in the chapter).
o Hold-and-modify mode is turned off.
o Single-playfield mode is set.
o Composite video color is enabled. (Not applicable in all models.)
- 42 Playfield Hardware -