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System Specification for C65Fred BowenMarch 1, 1991

In BitPlane mode, the C4567R6 does not use the Video Bank select bits, like the old VIC chip did. Instead, You can specify which 8k block (in 320 mode), or which 16k block (in 640 mode) of memory you want a bitplane to come out of. Specify where you want the bitplanes to be fetched from, using Bitplane Address registers 0 through 7. Note, however, that the least significant bits of these registers are ignored in 640 pixel mode, and that register 4 through 7 are never used in 640 pixel mode. Even numbered bitplanes can only be fetched from memory bank 0 (addresses 0-FFFF hex), and odd numbered bitplanes can only be fetched from memory bank 1 (addresses 10000-1FFFF hex). So, the bitplane pointers define which section within the confined bank that bitplane data will be fetched from.

In the Bitplane address registers, there are two bit-fields; One field of bits is for the even vertical scan, and the other field of bits is for the odd scan. The odd scan bits are not used unless both INT and V400 bits are set in control register "B".

Attributes can be enabled in bitplane mode by setting the ATTR bit in control register "B". If this is done, the most significant nybble of bytes fetched for bitplane 3 will contain the attribute specification for each 8 by 8 pixel cell, exactly as is done in character modes. One exception is that the "hilite" attribute will be disabled. The attributes are only applied to bitplane 2, which is also the foreground/background plane for sprite collisions and priority purposes.

To properly utilize this feature, bitplane 2 must be enabled to provide attributed bitplane data, and bitplane 3 must be disabled, since it will be providing attribute data. Data fetches for the attribute data will occur, because bitplanes 2 and 3 are both fetched in the same memory cycle. You may also enable any other bitplanes as needed. Bitplane 2, and any other bitplane may be complemented, but complementing bitplane 3 will only cause its bit weight to contribute a "1", and will not invert the attribute data.


Addresses 1F800-1FFFF hex are the Color and Attribute RAM used in the old video modes. You can use this area for bitplane if you do not plan on switching between old and new video modes and expect the data for both modes to be there.

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Read the small print. Last updated March 24, 2003.