Date   : Sun, 10 May 2015 21:16:07 +0100
From   : jgh@... (J.G.Harston)
Subject: The mythical chunky graphics mode

Try:
10 MODE4:VDU 28,0,31,39,0
20 VDU23,0,12,&28,0,0,0,0,0,0
30 FOR B%=1 TO 10
40 FOR A%=32 TO 126:VDU A%:NEXT A%
50 NEXT B%

This defines a text window to stop hardware scrolling reprogramming the 
CTRC registers, and prints lots of characters to demonstrate what is 
happening.

MODE 4 puts the display into bitmap mode, but VDU 23,0,12,&28,etc sets 
MA13 which tells the video hardware to address memory as though in 
teletext mode, so scans through &7C00-&7FFF to fetch the bitmap, with 
the same byte read for every scan line.

Oddly, there's a bit in the video ULA that selects the teletext RGB 
source, and the bit from the CRTC that selects the teletext memory 
addressing. You would have thought there would have been a single 
hardware flag that both selected the MODE 7 memory map and told the 
video ULA to read RGB from the teletext IC. Though it does allow you to 
program the system to display teletext data from anywhere in the bitmap 
memory.

-- 
J.G.Harston - jgh@...      - mdfs.net/jgh