zeugma/plotmasks at master · hexwab/zeugma · GitHub

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width 9      8      7      6      5      4      3      2      1
     L  R   L  R   L  R   L  R   L  R   L  R   L  R   L  R   L  R
0   00 7f  00 ff  01 ff  03 ff  07 ff  0f ff  1f ff  3f ff  7f ff
1   80 3f  80 7f  80 ff  81 ff  83 ff  87 ff  8f ff  9f ff  bf ff
2   c0 1f  c0 3f  c0 7f  c0 ff  c1 ff  c3 ff  c7 ff  cf ff  df ff
3   e0 0f  e0 1f  e0 3f  e0 7f  e0 ff  e1 ff  e3 ff  e7 ff  ef ff
4   f0 07  f0 0f  f0 1f  f0 3f  f0 7f  f0 ff  f1 ff  f3 ff  f7 ff
5   f8 03  f8 07  f8 0f  f8 1f  f8 3f  f8 7f  f8 ff  f9 ff  fb ff
6   fc 01  fc 03  fc 07  fc 0f  fc 1f  fc 3f  fc 7f  fc ff  fd ff
7   fe 00  fe 01  fe 03  fe 07  fe 0f  fe 1f  fe 3f  fe 7f  fe ff
^
--- x offset

R==ff -> onebyte


font format:

0: nchars
1: char height
for nchars:
   LSB of pointer
for nchars:
   MSB of pointer (bits 0-3)
   width-1 (bits 4-7)

pointers are char height (width=1..8) or char height*2 (width=9..) bytes


maybe second byte of font should be +256 not +height?


7654321076543210
abcdefgh
       abcdefgh

asl a \2
sta tmp \3
adc #0 \2

R9 scan lines per char
set to 0..7
R6 vertical displayed reg
normally 32

mode 3,6,7:
10(R9)*31(R4) + 2(R5) = 312 lines
other modes:
8(R9) * 39(R4) + 0(R5) = 312 lines

sync pos = 34/39
-> 5 lines before start of frame

1 frame = 312 lines = 39936 2MHz cycles
= 19968 ($4e00) 1MHz cycles

1 scanline = 64 = $40 1MHz cycles


total displayed lines = 32
line 0: height 2
lines 1..30: height 8
line 31: height 6

line 1: set lines/char to n
line n-1: set lines/char to 8


MA0 RA2 RA1 RA0

four possibilities (for max char width of 16)
onebyte: one byte char (width<=8), fits into one byte of screen
twobyte: ony byte char (width<=8), split over two bytes of screen
twobytetwo: two byte char (width>8), fits into two bytes of screen
threebyte: two byte char (width>8), split over three bytes of screen