The SG84 is a fast, high resolution colour graphics board. It contains an advanced display controller and powerful graphics-optimised processor in a single chip, the Hitachi 63484 ACRTC.
The graphics processor uses hardware and microcode to perform very high speed pixel operations, including lines, arcs, ellipses, filling, and block transfers.
The display controller provides an upper, lower and middle base screen, one window, and zooming, directly from hardware. The cursor can be a block-cursor, up to two cross-hairs, as chosen by software.
The resolution is programmable (up to 640x256 non-interlaced, 640x500 interlaced) in either 16 or 256 logical colours, from a palette of a over a quarter of a million (262144 to be exact) physical colours. This allows a range of special effects such as instant colour changing, or false colour images. Colours can also be made to flash.
The SG84 supports two frame-access. modes. In single-access mode the user can choose between flicker-free display-priority mode where drawing is stopped during display periods, or the faster drawing-priority mode where drawing may occur during display periods. In dual-access mode, display and drawing accesses are interleaved, allowing fast drawing at any time without disturbing the display. This is only available to 16-colour displays.
The 256k of on-board memory provides either one 256-colour screen, or two 16-colour screens. Any memory not used for display can be used to hold fonts, icons, windows etc.
The SG84 provides analogue and TTL level video signals (RGB or monochrome) with selectable sync polarity.
This manual provides a description of the SG84 board, an ACRTC command summary, and some example programs. For a more detailed description of the ACRTC registers and commands, see the document titled "HD63484 ACRTC Advanced CRT Controller Users Manual" available from Hitachi or distributors of the ACRTC.