Display pixel data format

The CPC, CPC+ and KC Compact have 3 official bitmapped graphics modes:

• Mode 0, low resolution, 2 pixels per byte, 16 colours
• Mode 1, medium resolution, 4 pixels per byte, 4 colours
• Mode 2, high resolution, 8 pixels per byte, 2 colours

The CPC and CPC+ have one unofficial bitmapped graphics mode:

• Mode 3, low resolution, 2 pixels per byte, 4 colours

The display mode is defined using the Gate-Array. If the mode is changed, the result will only be seen at the next HSYNC.

The standard resolutions are:

• 160x200pixels; 20x25 characters (low resolution)
• 320x200pixels; 40x25 characters (medium resolution)
• 640x200pixels; 80x25 characters (high resolution)

But, the width and height of the display area can be redefined using the 6845 CRTC, so it is possible to define other resolutions.

Pixel properties:

• a pixel will always be 1 scanline tall, regardless of mode setting
• the width of a pixel is different depending on the mode setting.
• the pixel defines a look-up into a re-defineable palette. The size of the palette depends on the mode setting.

Mode 0

Properties of this mode:

• 2 pixels per display byte. Each pixel is defined from a re-defineable palette of 16 colours (pens 0-15).
• lowest resolution. (a pixel in this mode is 2 times the width of a mode 1 pixel, or 4 times the width of a mode 2 pixel).
• pixels are rectangular
• Each pixel is defined by 4 bits.

Order of pixel display:

pixel 0

pixel 1

Bit mappings for Pixel 0:

	Bit in display byte
bit 3	1
bit 2	5
bit 1	3
bit 0	7

Bit mappings for Pixel 1:

	Bit in display byte
bit 3	0
bit 2	4
bit 1	2
bit 0	6

e.g. if pixel 0 is 13 and pixel 1 is 6, this is encoded as:

pixel 0: 1101 becomes 0-1-0-0-

pixel 1: 0110 becomes -0-1-1-0

and together becomes: 00110100

Mode 1

Properties of this mode:

• 4 pixels per display byte. Each pixel is defined from a re-defineable palette of 4 colours (pens 0-3).
• medium resolution. (a pixel in this mode is half the width of a mode 0 pixel, or 2 times the width of a mode 2 pixel).
• pixels are square
• Each pixel is defined by 2 bits

Order of pixel display:

pixel 0

pixel 1

pixel 2

pixel 3

Bit mappings for Pixel 0:

	Bit in display byte
bit 1	3
bit 0	7

Bit mappings for Pixel 1:

	Bit in display byte
bit 1	2
bit 0	6

Bit mappings for Pixel 2:

	Bit in display byte
bit 1	1
bit 0	5

Bit mappings for Pixel 3:

	Bit in display byte
bit 1	0
bit 0	4

Mode 2

Properties of this mode:

• 8 pixels per display byte. Each pixel is defined from a re-defineable palette of 2 colours (pen 0 and 1).
• high resolution. (a pixel in this mode is quarter the width of a mode 0 pixel AND half the width of a mode 1 pixel).
• pixels are rectangular
• Each pixel is defined by 1 bit

Order of pixel display:

pixel 0

pixel 1

pixel 2

pixel 3

pixel 4

pixel 5

pixel 6

pixel 7

Bit mappings for Pixel 0:

	Bit in display byte
bit 0	7

Bit mappings for Pixel 1:

	Bit in display byte
bit 0	6

Bit mappings for Pixel 2:

	Bit in display byte
bit 0	5

Bit mappings for Pixel 3:

	Bit in display byte
bit 0	4

Bit mappings for Pixel 4:

	Bit in display byte
bit 0	3

Bit mappings for Pixel 5:

	Bit in display byte
bit 0	2

Bit mappings for Pixel 6:

	Bit in display byte
bit 0	1

Bit mappings for Pixel 7:

	Bit in display byte
bit 0	0

Mode 3

This mode is not official and exists on the CPC and CPC+ only and does not exist on the KC Compact.

Properties of this mode:

• 4 pixels per display byte. Each pixel is defined from a re-defineable palette of 4 colours (pens 0-3).
• lowest resolution. (a pixel in this mode is 2 times the width of a mode 1 pixel, or 4 times the width of a mode 2 pixel).
• pixels are rectangular

Order of pixel display:

pixel 0

pixel 1

Bit mappings for Pixel 0:

	Bit in display byte
bit 1	3
bit 0	7

Bit mappings for Pixel 1:

	Bit in display byte
bit 1	2
bit 0	6