Fast Clock

To permit the new system to run certain types of the old C64 software, the C4567R6 provides a normal (slow) CPU clock with a long term (63us) average of 1.02 Mhz (exactly the C64 clock rate). This is accomplished by setting up a pattern of 1.79Mhz (560ns) cycles to give a total of 65 cycles be horizontal scanning line (also, like C64). In addition, logic is provided on the C4567R6 to determine when the microprocessor chip is executing an enhanced opcode, and, if so, subtracts a clock cycle from it.

By setting the FAST bit in Control Register "B", you can instruct the C4567R6 to clock the CPU at 3.58 Mhz, and permit the microprocessor to execute its enhanced instructions at full speed. This can increase CPU speed up to 400%.

BitPlane mode

In addition to the usual video modes provided by the old VIC chip, the C4567R6 provides a bitplane mode, which allows up to eight bitplanes to be used in the 320, or up to four bitplanes to be used in the 640 horizontal pixel modes.

Enabling BitPlane mode is done by setting the BPM bit in Control Register "B". Doing this will override all of the other video modes. To specify which bitplanes (0-7) to use, set the corresponding bit for each bitplane you want, in the Bitplane Enable register. Bitplane mode may be used with sprites. Bitplane 2 is the foreground/background plane used for sprite/background collision detection and priority.

The bitplanes, whether enabled, or not, provide the eight color value bits used to define what color will be displayed for any pixel on the screen. Bitplane 0 provides the least significant bit of the color value, and bitplane 7 provides the most significant bit. Bitplanes that are not enabled will contribute a "0" to their bit position in the color select code, unless the complement bit for that bitplane, in the complement register, is set.

Any bitplane's data can be inverted, whether or not the bitplane is enabled by setting its respective bit in the Bitplane Complement register. Inversion on unenabled bitplanes will cause them to contribute a "1" instead of their usual "0".