Description: Apple IIGS: Video Generation Chip (VGC)
Header: Apple IIGS: Video Generation Chip (VGC)


 This article last reviewed: 23 October 1986

 The Video Generation Chip (VGC) supports video output from the Mega II for
 both Apple II graphics and Super Hi-res graphics, provides an interface to
 the Real Time Clock chip, supports interrupt handling, and assists disk drive
 interfacing.

 The VGC accepts color information from the Mega II, modifies it according to
 the current Control Panel selections, and puts out appropriate display
 information for the NTSC composite video jack and the Video RBG port. The VGC
 accesses text, background, and border information maintained in the Text and
 Background Color Register and Border Color Register. 4-bit value determines
 each of the three color areas, so that there are 16 possible colors for each.
 These colors correspond to the 16 Apple II Lo-res colors.

 Apple II Graphics and Text

 If Apple II text mode is used, the VGC removes color information from the
 NTSC composite output signal, so that color fringing does not occur on a
 color composite monitor. If a mixed text/graphics mode is chosen, color
 fringing is unavoidable, since most composite monitors do not have the
 ability to respond quickly enough to a change in the chroma information. In
 this case, the bottom four lines of text will show a color fringing anomaly.

 The VGC polls the Monochrome/Color Register to determine which type of video
 signal should be output. If monochrome has been chosen, the VGC will output
 appropriate dot patterns to represent the chosen colors, so that a monochrome
 composite monitor will display gray-scale images. An AppleColor RGB Monitor
 displaying double Hi-res graphics will also display gray-scale images if
 monochrome is selected.

 Super Hi-res Graphics

 After the selection of one of the new Apple IIGS Graphics modes, the VGC is
 responsible for implementing the color mode. It uses memory in the auxiliary
 64K bank of Apple II RAM to implement Super Hi-res graphics. In this display
 buffer, locations $2000-$9CFF are used for pixel information, $9D00-$9DFF are
 pointers that determine the charactistics of each line, and $9E00-$9FFF hold
 color pallette information.

 For 640 or 320 graphics modes, each pixel may be represented respectively by
 either 2 or 4 bits, wherein the value is a number of a color in the
 appropriate color palette. Each of the 200 pointers (one for each line)
 stores the display mode used, the color palette associated with that line,
 and a flag of enabled or disabled for scan line interrupts. Each of the 16
 color palettes contains information on 16 colors. Each of the colors takes
 two bytes: 4 bits each for the value of red, green, and blue, which allows
 the three primary colors can be combined in 4096 different ways.

 Real Time Clock Interface

 The Video Generation Chip also works as an interface between the 65816
 microprocessor and the Real Time Clock Chip (RTC). A Real Time Clock register
 in the VGC is used as a command register for the RTC. The RTC then maintains
 calendar and clock information within parameter RAM.

 VGC Interrupts

 Two types of internal interrupts are handled by the VGC: the One-Second
 interrupt generated by the Real Time Clock Chip and the Scan-Line interrupt
 generated by scan line information in Super Hi-res mode. The status and
 enable states of these interrupts are found in the VGC Interrupt Register and
 the VGC Interrupt Clear Register. The VGC also handles one external interrupt
 line.

 VGC Disk Register

 The VGC Disk Register, used as a control register for the disk drive
 interface, functions in choosing the head to use and the type of drive
 selected.



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