Programming methods used in games
This section describes specific programming methods which are used in games and the games that use them.
- Prehistorik 2
- Xyphoes Fantasy
The sampled speech is often stored as 4-bit values with 2 values packed into each byte. Each value is sent to one of the AY volume registers at a fixed rate.
- Chase HQ
- Bad Dudes vs Dragon Ninja
Rasters is a colour changing effect.
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 !
For example: Pac-Man Emulator
The following types of scroll are used:
1. The player controls a character on the screen. When the character is moved the screen scrolls but the character remains in the same position on the screen. If they get to the edge of the map, they may walk up to the sides of the screen. If they walk back the other way when they reach a particular point on the screen it starts to scroll again and they remain in the same position.
2. The player controls a character on the screen. When the character is moved towards the edges of the screen, the scroll moves a whole screen in width or height. The character is now on the opposite side. e.g. the character moves left, the screen scrolls by a whole screen width, the character is now on the right. The screen only scrolls when they get close to the sides.
3. The screen always scrolls. The player controls a character who is either static or moves freely around the screen.
Parallax/Multi Layer Scroll
Parallax or Multi Layer Scroll is an effect where there are 2 layers of tiles which scroll, one has priority over the other and often they move at different scroll rates.
For example, the following games have parallax or multi layer scroll:
- Wizard Willy
- Teenage Mutant Hero Turtles
- Astro Marine Corps (AMC)
- Turrican I and II
- Shadow Dancer
The screen is scrolled using the CPU only. This can be done by moving the data on the screen, or re-drawing it in a different position to give the illusion of scrolling.
- Batman the Movie.
Hardware scrolling is performed by changing the CRTC registers R12 and R13 to define the start address of the screen. The display is automatically wrapped at 16K, so continuous incrementation of these values will scroll the screen and wrap around. The code must then update the display for new graphics.
The scrolling is in CRTC character sizes.
- 1 CRTC char horizontally is 2 bytes of RAM at a time which corresponds to 4 mode 0 pixels, 8 mode 1 pixels or 16 mode 2 pixels.
- The height of a CRTC char is defined by CRTC Register 9 (Max Raster), which is normally set to 7. Resulting in 8 scanlines per character.
Horizontal scrolling can be made smoother by using R3 (Hsync Width), in addition to using R12 and R13 and makes the scrolling effectively half a CRTC char (therefore 2 mode 0 pixels, 4 mode 1 pixels or 8 mode 2 pixels). This can be done by changing the HSYNC width values between 5 and 6. The movement is an effect of how the monitor handles the HSYNC. This effect works well on Amstrad monitors, doesn't work on modern TFT screens and 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). This works on all monitors.
More recent games (e.g. Prehistorik 2, Super Cauldron) use the rupture technique when doing vertical scroll to ensure the display is a constant number of scan lines in height, and so the refresh rate is always the same.
Older games adjust R5 and don't use rupture (e.g. LED Storm, Legend of Kage), the time for a frame therefore varies by up to 7 extra scan-lines. This means the VSYNC to the monitor also varies in position by up to 7 extra scan-lines, and the stability of the image is then dependent on how the monitor handles the VSYNC.
If the game uses a status panel, there are 2 ways to handle this.
- Redraw the panel when the screen is scrolled to compensate for it.
- Use rupture technique, the panel is in a different part of RAM and will not be affected by the scroll.
This following is an incomplete list of games using hardware scrolling. From this list it can be seen that hardware scrolling has been used from 1984 :
|3D Stunt Rider||1985||No||Yes||No||No||Yes|
|Amazing Shrinking Man (The)||1986||Yes||Yes||No||No||Yes|
|Axys: The Last Battle||1991||Yes||No||No||Yes||Yes|
|Bob Morane Science Fiction||1987||Yes||Yes||No||No|
|Cessna Over Moscow||1987||No||Yes||No||No||Yes|
|DJ Puff's Volcanic Eruption||1992||Yes||Yes||No||No||No|
|Doctor Who and the Mines of Terror||1986||Yes||Yes||No||No||Yes|
|Glen Hoddle Soccer||1985||No||Yes||No||No||Yes|
|Ghosts 'n' Goblins||1986||No||Yes||Yes||No||Yes|
|Ghouls 'n' Ghosts||1989||No||Yes||No||No||Yes|
|Into The Eagle's Nest||1987||Yes||Yes||No||No||Yes|
|Legend of Kage||1986||Yes||Yes||Yes||Yes||Yes|
|Le 5eme Axe||1985||Yes||Yes||No||No||Yes|
|Monty Python's Flying Circus||1990||Yes||Yes||No||No||Yes|
|Out of This World||1987||No||Yes||Yes||No||Yes|
|Rick Dangerous 2||1990||No||Yes||No||No|
|Roland on the Ropes||1984||Yes||Yes||No||No|
|Return of the Jedi (The)||1989||Diagonal||scroll!||No||No||Yes|
|Thing on a Spring||1986||No||Yes||Yes||No||Yes|
|Sly Spy Secret Agent||1990||Yes||Yes||No||No||Yes|
|Striker in the Crypts of Trogan||1992||Yes||Yes||No||No|
|Tornado Low Level||1985||Yes||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.
Wonderboy sets R4 every frame to the same value - reason unknown. But it doesn't use Rupture, because if this code is removed it works fine.
This is a an incomplete list of games that use this technique, but according to this list the earliest games used this technique was 1987:
|DJ Puff's Volcanic Eruption||1992||Yes|
|Enlightenment: Druid 2||1988|
|F1 Tornado Simulator||1991||No|
|Grell and Falla||1992|
|Q10 Tank Buster||1991||Yes|
|Santa's Christmas Capers||1990||Yes|
|Twin World (Ubi Soft)||1990||Yes|
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.