VDC Registers.

00 - 0F : 4x Sprite control

	  Byte 0: Y
	  Byte 1: X
	  Byte 2: (this byte is write only)
		    Bit 0: shift sprite 1 pixel to the right
		    Bit 1: shift even rows 1 pixel to the right
		    Bit 2: double pixel size
		    Bit 3..5: color (543 = BGR)
	  Byte 3: unused

10 - 3F : 12x Character

	  Byte 0: Y
	  Byte 1: X
	  Byte 2: (char * 8) - (Y / 2)
	  Byte 3: Bit 0:    9th bit of char
	          Bit 1..3: color (321 = BGR)

40 - 7F : 4x Quad character

	  Bytes like characters
	  (if address bit 1 = 0 then address bits 2 and 3 are "Don't Care")

80 - 9F : 4x Sprite shape

	  Each byte is one row of 8 pixels, bit 0 is the left-most pixel

A0      : Control

	  Bit 0: Enable horizontal interrupt (?)
	  Bit 1: 1 = Strobe X,Y beam location into $A4,$A5. 0 = $A4,$A5 latched (*)
	  Bit 2: Enable sound interrupt (?)
	  Bit 3: Grid control (0=Off/1=On)
	  Bit 4: Enable external input overlap interrupt (G7400)
	  Bit 5: Foreground (0=Off/1=On) (characters and sprites)
	  Bit 6: Dot Grid (0=Off/1=On)
	  Bit 7: Grid fill Mode (0=Off/1=On)

(*) : OR with STB input (pin 3), falling edge latches values until A5 read.

A1      : Status (reading this register clears bit 2 and 6 (and 3 and 7) and the interrupt output)

	  Bit 0: set 20 usec before start of Hblank, cleared 5 usec before end of Hblank
	  Bit 1: Position Strobe Status. 1 = Follow Beam, 0 = Latched
	  Bit 2: Sound register empty (?)
	  Bit 3: This bit is set at the start of VBLANK
	  Bit 4..5: unused
	  Bit 6: External input overlap (G7400)
	  Bit 7: Character overlap

A2      : Collision (reading this register clears it)

	  Bit 0: Sprite 0
	  Bit 1: Sprite 1
	  Bit 2: Sprite 2
	  Bit 3: Sprite 3
	  Bit 4: Vertical grid
	  Bit 5: Horizontal grid and dots grid
	  Bit 6: External video input (G7400)
	  Bit 7: Characters

A3	: Color (this register is write only)

	  Bit 0..2: Grid color (210 = RGB)
	  Bit 3..5: Background color (543 = RGB)
	  Bit 6: Grid luminance (0=dim/1=bright)
	  Bit 7: unused

A4	: Y position (to read this register, A5 must be read first)
A5	: X position

A7 - A9 : Sound shift register (these registers are write only)

AA	: Sound control

	  Bit 0..3: Sound volume
	  Bit 4 : Enable noise generation
	  Bit 5 : Shift frequency, 0 = 983Hz, 1 = 3933Hz
	  Bit 6 : unused
	  Bit 7 : Sound enable. 0 = sound off, 1 = sound on

B0 - BA : equal to A0 - AA

C0 - C8 : Horizontal grid lines (8)

	  Each byte is one column
	  Bit 0..7 is horizontal line 0..7

D0 - D8 : 9th line of horizontal grid lines

	  Each byte is one segment
	  Bit 0 is horizontal line 8 (other bits unused)

E0 - E9 : Vertical grid lines (10)

	  Each byte is one vertical line
	  Each bit is one vertical segment

	hor: 9x 9 segments
	vert: 10x 8 segments