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Display pixel data format

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

 

Article créé le : Lundi 16 Novembre 2009 à 13 h 20
Dernière mise à jour le : Mardi 28 Février 2012 à 21 h 32
 
 

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