Picture Encoding
This article describes how pictures are encoded in graphics memory. This applies to Graphics RAM and Graphics ROM.
Each card stores a single 8x8 picture in a sequence of 8 bytes. The first byte of the sequence holds the topmost row of pixels. The last byte holds the bottom row of pixels. Bit 7 in each byte holds the leftmost column of pixels, and bit 0 in each byte holds the rightmost column. Pictorally:
Bit #
7 6 5 4 3 2 1 0
+---------------+
0 | |
B 1 | |
y 2 | |
t 3 | |
e 4 | |
5 | |
# 6 | |
7 | |
+---------------+
Suppose, for instance, we want to define a picture for the letter R. That picture might look like so: ('#' indicates pixels that are "on", and '.' indicates pixels that are "off")
Bit #
7 6 5 4 3 2 1 0
+---------------+
0 |# # # # # # . .|
B 1 |. # # . . # # .|
y 2 |. # # . . # # .|
t 3 |. # # # # # . .|
e 4 |. # # . . # # .|
5 |. # # . . # # .|
# 6 |. # # . . # # .|
7 |# # # . . # # #|
+---------------+
All of the "on" pixels are represented with 1 bits, and all of the "off" pixels are represented with 0 bits. The binary pattern for this character would be:
DECLE %11111100
DECLE %01100110
DECLE %01100110
DECLE %01111100
DECLE %01100110
DECLE %01100110
DECLE %01100110
DECLE %11100111
You could copy this exact bit pattern into one of the card slots in GRAM, and voila! You have a stylized R.
