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:

Memory address Signal	Signal Source	Signal name
A15	6845	MA13
A14	6845	MA12
A13	6845	RA2
A12	6845	RA1
A11	6845	RA0
A10	6845	MA9
A9	6845	MA8
A8	6845	MA7
A7	6845	MA6
A6	6845	MA5
A5	6845	MA4
A4	6845	MA3
A3	6845	MA2
A2	6845	MA1
A1	6845	MA0
A0	Gate-Array	CCLK

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:

line 0	0 + &000
line 1	0 + &800
line 2	0 + &1000
line 3	0 + &1800
line 4	0 + &2000
line 5	0 + &2800
line 6	0 + &3000
line 7	0 + &3800
line 8	((R1*1)*2) + &000
line 9	(R1*2) + &800
line 10	(R1*2) + &1000
line 11	(R1*2) + &1800
line 12	(R1*2) + &2000
line 13	(R1*2) + &2800
line 14	(R1*2) + &3000
line 15	(R1*2) + &3800
line 16	(R1*2) + &000
line 17	(R1*2) + &800
line 18	(R1*2) + &1000
line 19	(R1*2) + &1800
line 20	(R1*2) + &2000
line 21	(R1*2) + &2800
line 22	(R1*2) + &3000
line 23	(R1*2) + &3800

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:

C7FF->C000

CFFF->C800

D7FF->D000

DFFF->D800

E7FF->E000

EFFF->E800

F7FF->F000

FFFF->F800