joels@tektronix.UUCP (Joel Swank) (04/29/84)
Some things I found browsing around the C-64 ROMS: There is a flag at $2A6 called the NTSC/PAL flag. 0=NTSC 1=PAL. (NTSC is the TV standard for the U.S., PAL is the standard used in Europe.) This flag is set by the Kernal initialization routine at $FF5B. This routine uses the VIC-II registers at $D012 and $D019 to determine which way to set this flag. Two other routines in the Kernal use this flag. One is the RS232 OPEN routine. It uses one of 2 RS232 timing tables depending on this flag. The other routine is the jiffy clock routine. It uses one of two timer values depending on this flag. These routines were not in early versions of the C-64 ROMS. Inside the C-64, inside the metal box containing the VIC-II chip, is an empty jumper block marked 'PAL'. The schematic shows that there are two possible jumper locations. One is marked PAL and the other NTSC. The position of this jumper determines the frequency of the DOT CLOCK input to the VIC-II chip. The DOT CLOCK is what the VIC-II uses to generate the processor phase 2 clock signal as well as the timing for the video raster scan. My guess is that all this is to allow a C-64 to easily be modified to work on PAL monitors. Whether it will work on a PAL TV depends on the compatibility of the RF modulator. There is a fuse block in the modulator circuit, but its purpose is not marked. I don't know what frequencys PAL TVs accept. Joel Swank Tektronix, Beaverton OR