Splitting the Atom

page 51

CHAPTER 8
THE MEMORY MAPPED VDU

I. Text mode / Graphics mode 0

In this mode the VDU display is mapped from 512 memory locations (0.5K) in the format 32 across and 16 down.

In text mode this allows 32 columns by 16 rows of text characters to be printed. In graphics mode, it allows 64 columns by 48 rows of pixels to be individually access. Each memory location, or graphics cell, is divided into 6 pixels corresponding to the 6 lower bits of the number in that location.

When the bit is set (=1) the pixel is white (or grey). When the bit is clear (=0) the pixel is black. If the last bit (bit 7) of the number is set, the graphics are change from white to grey.

5 4 3 2 1 0 Organisation of graphics cell	0 = full tone 1 = half tone 7 6 5 4 3 2 1 0   0 = text 1 = graphics
e.g.
	0 1 1 1 1 1 1 0 = #7E

Graphics modes

In the true graphics modes 1 to 4 the screen is bit-mapped. The VDU memory maps show the screen divided into cells, each a byte (8-bits) wide. Each bit may be either set or clear.