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The VIC-II (Video Interface Chip II), specifically known as the MOS Technology 6567/8562/8564 (NTSC versions), 6569/8565/8566 (PAL), is the microchip tasked with generating Y/C/composite video graphics and DRAM refresh signals in the Commodore 64 and C128 home computers.

Succeeding MOS's original VIC (used in the VIC-20), the VIC-II was one of the two chips mainly responsible for the C64's success as the best-selling computer model of all time (the other chip being the 6581 SID).

Development history

The VIC-II chip was designed by primarily Al Charpentier and Charles Winterble at MOS Technology as a successor to the MOS Technology VIC (6560). The team at MOS Technology had previously failed to produce two graphics chips named MOS Technology 6562 for the Commodore TOI computer, and MOS Technology 6564 for the Color PET, much due to memory speed constraints.

In order to construct the VIC-II, Charpentier and Winterble made a market survey of current home computers and video games, listing up the current features, and what features they wanted to have in the VIC-II. The idea of adding sprites came from the Texas Instruments TI-99/4A computer and its TMS9918 graphics coprocessor. About 3/4 of the chip surface is used for the sprite functionality.

The chip was partly laid out using electronic design automation tools from Applicon (nowadays a part of UGS Corp.), partly layed out manually on vellum paper. The design was partly debugged by fabricating chips containing small subsets of the design, which could then be tested separately. This was easy since MOS Technology had both its design workshops and manufacturing plant at the same location.

The work on the VIC-II was done in the latter half of 1981 while Robert Yannes was simultaneously working on the SID chip. Both chips, like the Commodore 64, were finished in time for the Consumer Electronics Show in the first weekend of January 1982.

VIC-II features

16 KB address space for screen, character and sprite memory
320 × 200 pixels video resolution (160 × 200 in multi-color mode)
40 × 25 characters text resolution
three character display modes and two bitmap modes
16 colors
concurrent handling of 8 sprites, each of 24 × 21 pixels (12 × 21 multicolor)
raster interrupt (see details, below)
smooth scrolling
independent dynamic RAM refresh
bus mastering for a 6502-style system bus; CPU and VIC-II accessing the bus during alternating half-clock cycles (the VIC-II will halt the CPU when it needs extra cycles)

Technical details

Programming

The VIC-II was programmed by manipulating its 47 control registers (up from 16 in the VIC), memory mapped to the range $D000–$D02E in the C64 address space. Of all these registers, 34 dealt exclusively with sprite control (sprites being called MOBs, from Movable Object Blocks, in the VIC-II documentation). Like its predecessor, the VIC-II handled light pen input, and, with help from the C64s standard character ROM, provided the original PETSCII character set from 1977 on a similarly dimensioned display as the 40-column PET series, allowing some degree of PET BASIC program emulation on the C64.

By reloading the VIC-II's control registers via machine code hooked into the raster interrupt routine (the scanline interrupt), one could program the chip to generate significantly more than 8 concurrent sprites (a process known as sprite multiplexing), and generally give every program-defined slice of the screen different scrolling, resolution and color properties. The hardware limitation of 8 sprites per scanline could be increased further by letting the sprites flicker rapidly on and off. Mastery of the raster interrupt was essential in order to unleash the VIC-II's capabilities. Many demos and some later games would establish a fixed "lock-step" between the CPU and the VIC-II so that the VIC registers could be manipulated at exactly the right moment.

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