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Calculating screen address

Calculating screen address

If we are drawing a sprite, we use our pixel coordinates to define the upper-left coordinate of the sprite.

Since the CPC and KC Compact do not have sprite hardware, we will need to plot the sprites using a software function.

If we want to plot graphics to the display, e.g. to draw a sprite, then we will need to be able to calculate a memory address from our pixel coordinates.

This document will describe how this is done.

The CPC display hardware generates a memory address by using the CRTC MA (MA0-MA13) and RA (RA0-RA5) outputs in the following way:

Notes

• CRTC register 9 defines the number of scanlines per CRTC character.
• CRTC register 1 defines the width of the visible area in CRTC characters. The CPC display hardware fetches two bytes per CRTC character. Therefore the length of a CRTC scanline in bytes is (R1*2).
• CRTC register 6 defines the height of the visible area in CRTC character lines. Therefore the total height of the visible area in CRTC scanlines is (R9+1)*R6

The screen has the following form:

When R9 is 7:

CRTC character row = (y_coordinate/(R9+1))

CRTC character scan line = y_coordinate - (CRTC character row*(R9+1));

Byte offset to CRTC row start = CRTC character row * (R1*2)

Byte offset to CRTC character scan line = CRTC character scan line * &0800

Byte offset to start of line = Byte offset to CRTC row start + Byte offset to CRTC character scan line

For a 16k screen:

screen start byte offset = (((R12 & 0x03)*256) + (R13 & 255))*2

For a 32k screen:

screen start byte offset = (((R12 & 15)*256) + (R13 & 255))*2

On a hardware scrolling screen, there is a problem: