Commodore Plus 4 - Page 234

Bit Maps
ZZZ Programming Graphics
Bit Pair Color Source
00 $FF15
01 $FF16
10 $FF17
11 color memory
The bits of color memory are interpreted differently. The high (flashing) bit is
ignored. The next three bits determine luminance as usual. But the foreground
color is determined only by the lower 3 bits of the low nybble. The high bit from
the low nybble determines whether this character is interpreted as high resolution
or multicolor. Thus, high-resolution characters and multicolor characters may
be mixed, but only the first eight colors are available for both types.
There is no hardware reverse, but that bit (bit 7 of $FF07) still determines the
size of the character set. If it is clear, screen codes from 128 to 255 result in the
same characters as 0 through 127. When it is set, the full 256 codes correspond to
separate character definitions.
In bit-map mode, every dot (or pair of dots) on the screen is individually
controlled by a bit (or pair of bits) in memory. To enter bit-map mode, bit 5 of
$FF06 must be set to 1. Normally, your bit map will come from RAM, so bit 2 of
$FF12 must be cleared to make the graphics chip look at RAM. The location of
the bit map itself is also set with $FF12. Bits 5 through 3 correspond to bits 15
through 13 of the address of the bit map. It must be on an 8K boundary.
Luminance for your bit map is located in the area of memory pointed to by
$FF14. Bits 7 through 3 of $FF14 correspond to bits 15 through 11 in the address
of the luminance memory (which occupies the position of color memory in
character mode). It must be on a 2K boundary. The color memory for your bit
map (which occupies the position of screen memory in character mode) is always
located IK above the luminance memory.
The screen organization for a bit map is oriented toward character cells. The
first byte represents the upper left corner of the screen. That and the next 7 bytes
fill the upper left character cell. The eighth byte begins the character cell imme
diately to the right of the first one. Page 223 has a diagram of the screen layout.
High-Resolution Bit Maps In a high-resolution bit map, every dot on the screen
is individually controlled by a bit. The screen is organized as shown. Within each
of the 8000 bytes that make up the bit map, the high bit determines the status of
the leftmost dot on the screen, and the low bit determines the status of the
rightmost dot on the screen. The resolution is 320 by 200 (a total of 64000
individual dots).