View source for 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|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&nbsp;!

=== 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:

{| class="FCK__ShowTableBorders"
|-
| ''Title'' 
| ''Year'' 
| ''Vertical'' 
| ''Horizontal'' 
| ''R3'' 
| ''R5'' 
| ''Confirmed''
|-
| [[3D Stunt Rider]] 
| 1985
| No 
| Yes 
| No
| No
| Yes
|-
| [[Action Force]] 
| 1988 
| No 
| Yes 
| No 
| No 
| Yes
|-
| [[The Amazing Shrinking Man|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
|-
| [[The Return of the Jedi|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
|-
| [[Roland on the Ropes]] 
| 1984 
| Yes 
| Yes 
| No 
| No
|-
| [[The Sentinel|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
|-
| [[T.L.L. - Tornado Low Level|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:

{| class="FCK__ShowTableBorders"
|-
| ''Title'' 
| ''Year'' 
| ''Confirmed''
|-
| [[Action Force|Action Force]] 
| 1988 
| Yes
|-
| [[Axys: The Last Battle|Axys]] 
| 1991 
| Yes
|-
| [[Dynamic Duo|Dynamic Duo]] 
| 
| 
|-

| [[Energy Warrior|Energy Warrior]] 
| 1987
| Yes
|-
| [[Enlightenment: Druid 2|Enlightenment: Druid 2]] 
| 
| 
|-
| [[Grell and Falla|Grell and Falla]] 
| 
| 
|-
| [[Mission Genocide|Mission Genocide]] 
| 1987 
| Yes
|-
| [[Octoplex|Octoplex]] 
| 1989 
| Yes
|-
| [[Prehistorik 2|Prehistorik 2]] 
| 1992 
| Yes
|-
| [[Skate Ball|Skate Ball]] 
| 1989 
| Yes
|-
| [[Skatewars|Skatewars]] 
| 1989 
| 
|-
| [[Skateboard Kidz|Skateboard Kidz]]
| 1988
| Yes
|-
| [[Snowstrike|Snowstrike]] 
| 
| 
|-

| [[Sol Negro(opera)|Sol Negro(opera)]] 
| 
| 
|-
| [[Storm Warrior|Storm Warrior]] 
| 1989
| Yes
|-
| [[Super Cauldron|Super Cauldron]] 
| 1992 
| Yes
|-
| [[Turrican|Turrican]] 
| 1990 
| Yes
|-

| [[Twin World (Ubi Soft)|Twin World (Ubi Soft)]] 
| 1990 
| Yes
|-
| [[Ultima Ratio|Ultima Ratio]] 
| 1987 
| Yes
|-
| [[Usagi Yojimbo|Usagi Yojimbo]] 
| 1988
| Yes
|-
| [[Warhawk|Warhawk]] 
| 1987 
| Yes
|
|-
| [[Wonder Boy|Wonder Boy]] 
| 1987 
| Yes
|-
| [[Xyphoes Fantasy|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.

*Xevious