The Commodore 64 scene is divided into two sections, based on incompatibility differences in the machines. These are based upon differences in the video standards of the different countries. Most European countries use the PAL standard, while the countries in the Americas primarily use the NTSC standard. Because of these differences, software designed on European 64's often fails to work correctly on an American 64 and vice versa.
To cope with these problems, a discipline known as fixing has arisen. In this article, I will attempt to cover the basic differences in the 64 and what problems arise.
The first difference is in the screen-refreshing rate. PAL screens are redrawn 50 times a second, while NTSC screens are redrawn 60 timers per second. This usually does not prevent software from working properly, but anything which is synched to the screen refresh will occur at different rates when run on the different systems. The place this is most obvious is in the music players. Most European tunes are fine, as they sound ok when played a bit faster. Some NTSC tunes may seem too slow when played on a European 64. The difference is noticeable but not usually a problem, as PAL routines will run about 20% faster on an NTSC machine and NTSC routines will run about 16% slower on a PAL machine. This does not apply on all processing, only to movements or other routines that make one cycle on every screen refresh.
Two other differences account for most of the problems related to the PAL/NTSC division. PAL systems have 312 raster lines and 63 cycles on each line. This means that PAL systems have 19, 656 cycles per frame, while NTSC systems have only 17, 095.
PAL systems thus have about 15% more processing time for each video frame. Any PAL routine which uses up most of the available processing time will not complete in a single frame on an NTSC machine and will probably have a distorted display. Note that this only applies to routines that work with the graphics chip and are synced to the refresh rate. The problem becomes even worse when the processing must be done outside of the visible display area. Since the normal display area is 200 raster lines high, there are 112 left over on a PAL machine, but only 63 on an NTSC 64.
Another problem arises due to the differing number of raster lines. If a program wishes to perform a task at some raster line greater than 263, it will work fine on a PAL 64, but will wait in an endless loop on an NTSC machine. If interrupts are used instead of a busy wait, the interrupts will never occur.
Also, with the differing number of cycles per raster line, raster routines – routines that need to perform one or more operations each raster line and thus need to take up exactly 63 or 64 cycles in each interaction, in the simplest case – will become out of sync, looking distorted or not displaying at all. Thus an FLI routine for a PAL machine will often work like an FLD routine instead when run on a NTSC machine.
One final difference exists, which causes a displayer distortion, but which is less troublesome than the others. The sprite coordinates above 404 are displayed at the left of a PAL screen, while coordinates above 412 are displayed at the left of an NTSC screen.
Often these coordinates are used with a hyperscreen routine to display continuous graphics even in the left and right borders. But a PAL routine will have an eight pixel wide gap between two of the sprites when run on an NTSC 64 and an NTSC routine will have an eight pixel overlap when run on a PAL computer.
http://www.cei.net/~rreed/ (Editor note: This URL is broken since long time.)