Programming methods used in games

This section describes specific programming methods which are used in games and the games that use them.

Hardware Double Buffer

Most good CPC games use the hardware double buffer technique in order to display sprites and/or to scroll the play area smoothly.

In particular, two memory areas (instead of one) are reserved for the screen ram.

In each frame, one of the two screens is displayed while the other is not visible but is drawn to. After the draw is done, the screens are exchanged using the hardware, so that the previously invisible one is now visible and the previously visible one is invisible.

The exchange is done using the hardware specifically by changing the screen-offset, Reg 12 and 13 of the CRTC).

This process is repeated.

The big advantage of this technique is that we can use a whole frame (or more) machine-time for our code (with no problems that arise when we alter screen ram at the same time the electron beam displays it on the monitor such as tearing or flickering).

However, page flipping eats up a lot of memory area (which is crucial especially for 64Kb machines). This explains why a lot of games can have small game-areas !

Hardware Scrolling

Hardware scrolling is performed by changing the CRTC registers 12 and 13 (start address).

The scrolling is normally 1 CRTC char (4 mode 0 pixels, 8 mode 1 pixels or 16 mode 2 pixels) in the horizontal and 1 char line (scroll amount dependant on R9, but normally 8 scan lines) vertically.

Horizontal scrolling can be made smoother by using R3 (Hsync Width) and makes the scrolling effectively half a CRTC char (2 mode 0 pixels, 4 mode 1 pixels, 8 mode 2 pixels). (Changing between values 5 and 6 for the horizontal sync causes a half CRTC char movement. This is an effect on the monitor).

This effect works well on Amstrad monitors but produces a black and white image on a MP-2 modulator because of the adjusted HSYNC timing.

Vertical scrolling can be made smoother by using R5 (Vertical Adjust) in combination with Rupture/Splitting to make perfect 1 scanline vertical scrolling.

This works on all monitors.

This following is an incomplete list of games using hardware scrolling:

Title	Year	Vertical	Horizontal	R3	R5	Confirmed
3D Stunt Rider	1985	No	Yes	No	No	Yes
Action Force	1988	No	Yes	No	No	Yes
Amazing Shrinking Man (The)	1986	Yes	Yes	No	No	Yes
Anarchy	1988	No	Yes	No	No
Axys: The Last Battle	1991	Yes	No	No	Yes	Yes
BMX Kidz	1988	No	Yes	No	No	Yes
Bob Morane Science Fiction	1987	Yes	Yes	No	No
Boulder Dash	1984	Yes	Yes	No	No	Yes
Builderland	1991	No	Yes	No	No
Canadair	1987	No	Yes	No	No	Yes
Cyborgs	1991	Yes	Yes	Yes	Yes	Yes
Doctor Who and the Mines of Terror	1986	Yes	Yes	No	No	Yes
Energy Warrior	1987	No	Yes	No	No	Yes
FlySpy	1986	Yes	Yes	No	No
Fusion 2	1988	No	Yes	No	No
Gothic	1988	Yes	Yes	No	Yes
Ghosts 'n' Goblins	1986	No	Yes	No	No	Yes
Ghouls 'n' Ghosts	1989	No	Yes	No	No	Yes
Into The Eagle's Nest	1987	Yes	Yes	No	No	Yes
Jinks	1988	No	Yes	Yes	No	Yes
Killer Cobra	1987	No	Yes	Yes	No	Yes
Led Storm	1988	Yes	No	No	Yes	Yes
Legend of Kage	1986	Yes	Yes	Yes	Yes
Leviathan	1987	Diagonal	scroll!	No	No	Yes
Le 5eme Axe	1985	Yes	Yes	No	No
Mission Genocide	1987	Yes	No	No	Yes	Yes
Monty Python's Flying Circus	1990	Yes	Yes	No	No	Yes
Octoplex	1989	Yes	Yes	No	Yes(?)	Yes
Out of This World	1987	No	Yes	Yes	No	Yes
Paraplane	1992	Yes	Yes	No	No
Plate-Forme	1988	No	Yes	No	No	Yes
Prehistorik 2	1992	Yes	Yes	Yes	Yes	Yes
Prohibition	1987	Yes	Yes	No	No	Yes
Realm!	1987	Yes	Yes	No	No	Yes
Return of the Jedi (The)	1989	Diagonal	scroll!	No	No	Yes
Rick Dangerous 2	1990	No	Yes	No	No
Rig Attack	1985	No	Yes	No	No	Yes
Rockford		Yes	Yes	No	No	Yes
Roland on the Ropes	1984	Yes	Yes	No	No
Sentinel (The)	1987	Yes	Yes	No	No	Yes
Skate Ball	1989	No	Yes	No	No	Yes
Skateboard Kidz	1988	Yes	No	No	Yes	Yes
Skate Rock	1987	No	Yes	No	No	Yes
Skate Wars	1990	No	Yes	No	Yes	Yes
Sly Spy Secret Agent	1990	Yes	Yes	No	No	Yes
Star Avenger	1984	No	Yes	No	No
Street Machine	1986	Yes	Yes	No	No	Yes
Striker in the Crypts of Trogan	1992	Yes	Yes	No	No
Super Cauldron	1992	Yes	Yes	Yes	Yes	Yes
Titan	1988	Yes	Yes	No	No
Tornado Low Level	1985	Yes	Yes	No	No	Yes
Ultima Ratio	1987	Yes	No	No	Yes	Yes
Vector Ball	1988	Diagonal	scroll!	No	No	Yes
Warhawk	1987	Yes	No	No	Yes	Yes
Xeno	1986	No	Yes	No	No	Yes

Rupture (or splitscreen)

A CRTC programming technique used to split the screen into more than one block vertically.

The sizes of each block are defined using CRTC register R4. Vsync is turned off in all but one block (by setting CRTC R7 to a value larger than R4). The start address of each block can be changed using CRTC registers R12 and R13.

This allows each block to reference different ram or to hardware scroll one block while another is static.

This method requires careful timing for the CRTC register updates, it also needs testing on all CRTC because there are differences of when each will accept and use the values programmed. However, the result can be made to work on all with more simple ruptures. Care must also be taken to ensure the timings are setup for a 50Hz screen.

It is worth noting that some monitors are more tolerant to longer or shorter frames so the result may appear correctly on them and not on others. In addition some games may have been programmed and tested on only 1 CRTC type so may not work on others.

This is a an incomplete list of games that use this technique:

Title	Year	Confirmed
Action Force	1988	Yes
Axys	1991	Yes
Dynamic Duo
DJ Puff's Volcanic Eruption	1992	Yes
Energy Warrior	1987	Yes
Enlightenment: Druid 2
Grell and Falla	1992
Mission Genocide	1987	Yes
Octoplex	1989	Yes
Prehistorik 2	1992	Yes
Rockford	1988	Yes
Skate Ball	1989	Yes
Skatewars	1989
Skateboard Kidz	1988	Yes
Snowstrike
Sol Negro(opera)
Storm Warrior	1989	Yes
Super Cauldron	1992	Yes
Turrican	1990	Yes
Twin World (Ubi Soft)	1990	Yes
Ultima Ratio	1987	Yes
Usagi Yojimbo	1988	Yes
Warhawk	1987	Yes
Wonder Boy	1987	Yes
Xyphoes Fantasy

Firmware

The following games are known to use firmware functions. This probably explains why they are poor. If the programmer had used the hardware directly they would have lots more cycles free which they could have used to make the game better.

This list is not exhaustive.

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