o If the
6-5
bit combination
is
01,
the color of the pixel immediately
to
the left of
this pixel
is
duplicated and then modified. The bit-combinations from planes
4 - 1 are used to replace the four "blue" bits
in
the corresponding color register.
o
If
the
6-5
bit combination
is
10,
the color of the pixel immediately
to
the left of
this pixel
is
duplicated and then modified.
The
bit-combinations from planes
4 - 1 are used to replace the four "red" bits.
o
If
the
6-5
bit combination
is
11,
the color of the pixel immediately
to
the left of
this pixel
is
duplicated and then modified.
The
bit-combinations from planes
4 - 1 are used to replace the four "green" bits.
Using hold-and-modify mode,
it
is
possible to get by with defining only one color regis-
ter, which
is
COLORO,
the color of the background. You
treat
the entire screen as a
modification of
that
original color, according to the scheme above.
Bit
11
of register BPLCONO selects hold-and-modify mode.
The
following bits
in
BPLCONO must
be
set for hold-and-modify mode
to
be active:
oBit
HOMOD, bit
11,
is
1.
oBit
DBLPF,
bit
10,
is ° (single-play
field
mode specified).
o Bit HIRES, bit
15,
is
° (low-resolution mode specified).
oBits
BPU2, BPU1, and
BPUO
- bits
14, 13,
and
12,
are
101
or 110 (five or six
bit-planes active).
The following example code generates a six-bit-plane display with hold-and-modify mode
turned on. All
32
color registers are loaded with black
to
prove
that
the colors are being
generated by hold-and-modify. The equates are the usual and are not repeated here.
; First,
set
up the control registers.
LEA CUSTOM,AO
MOVE.W
#$6AOO,BPLCONO(AO)
MOVE.W #0,BPLCON1(AO)
MOVE.W #0,BPL1MOD(AO)
MOVE.W #0,BPL2MOD(AO)
MOVE.W #$0038,DDFSTRT(AO)
MOVE.W
#$OODO,DDFSTOP(AO)
MOVE.W #$2C81,DIWSTRT(AO)
MOVE.W #$F4C1,DIWSTOP(AO)
80 Playfield Hardware
;Point AO at custom chips
;Six bit-planes, hold-and-modify mode
;Horizontal scroll
= 0
;Modulo for odd bit-planes
= 0
;Ditto for even bit-planes
;Set data-fetch
start
;Set data-fetch stop
;Set display window start
;Set display window stop
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