Technical information

CHAPTER 2
Hardware Design
26 I/O System
Video System ICs 2
Two ICs provide the video support for the PC:
■ 82C450 VGA controller
■ MU9C9760 SynDAC
82C450 VGA Controller 2
The 82C450 is an integrated VGA video controller that is backward compatible with
EGA, CGA, and MDA video modes. With the card’s 512 KB of video DRAM (four
256K-by-4 DRAM ICs), the SynDAC supports all standard VGA modes as well as 800 by
600 pixels at 4 bits per pixel (noninterlaced), 640 by 480 pixels at 8 bits per pixel, and
132-column text mode.
The video controller is connected to the system by way of the ISA bus (the XD bus on the
DOS Compatibility Card).
MU9C9760 SynDAC 2
The video system on the DOS Compatibility Card uses a device called the SynDAC: an
IC (MU9C9760) that combines a lookup table, a triple video DAC, and a dual clock
synthesizer. The SynDAC IC drives the video output line directly and is compatible with
the Brooktree BT475 CLUT/DAC IC. The SynDAC IC provides 256 colors from a palette
of 256K colors. The SynDAC IC also provides an internal pixel clock with eight
programmable frequencies.
I/O System 2
The I/O system on the DOS Compatibility Card consists of the Pretzel Logic IC and the
8242 keyboard and mouse controller.
Pretzel Logic I/O Controller IC 2
The main component of the I/O system is the Pretzel Logic IC. It acts as a bus converter
between the PC processor bus and the Macintosh processor bus. The Pretzel Logic IC
integrates many of the I/O functions required to support the PC and also helps support
the communication between the PC and the Macintosh host computer. The Pretzel Logic
IC has the following features:
■ Two DMA channels (one for shared memory and one for disk I/O)
■ Two serial ports (16C450 compatible)
■ One Centronics parallel printer port
■ Keyboard and mouse controller (8047 compatible)