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The VGA3 ,as the name suggests, is an alphanumeric board which operates at VGA frequencies, making it usable on a far larger number of monitors that the CVDU, which it replaces. The core of the board is the HD6445CP4 (PLCC) CRTC, which is capable of operating at character frequencies up to 4.5mhz. The VGA3 uses 9-bit wide characters with a dot frequency of 28.322mhz (VGA standard), or a character rate of about 3.15mhz. A standard VGA screen 80 columns by 25 lines refreshes at a rate of 70hz. An alternate format, 80 columns by 30 lines refreshes at 60hz.
The board uses a single 32k x 8 memory chip to hold both the display memory, attributes, and font data. Character, attribute, and raster data from the font are read from memory during each character clock. The display memory may be accessed in two modes: via address registers (similar to the CVDU), or direct memory-mapped (PC-style). The method used will depend upon the number of address bits provided by the CPU in use. SBC-188 and MC68000 systems will probably use direct memory mapping, whereas, the Z80 and Z180 systems will use the address registered approach.
The color attribute display is the same as the CVDU; i.e., the color palette is fixed. However, attributes are stored with characters (char at even addresses, attributes at char address+1), as on the IBM PC display adapters. One bit of the background color may alternately be used as a 9th character code bit, allowing for 512 byte character sets. Normal operation is with 256 (8-bit) characters.
As on the CVDU, a PS/2 keyboard may be attached to an 8242 keyboard controller chip. The VIA chip, as shown in the photo above, has proved to be very reliable. The keyboard generates interrrupts on the ECB bus on B_/INT, or B_/IR1 when an interrupt controller is present. IRQ1 is the channel used on the IBM PC's.