The FDX 80C card has much the same architecture as the IBM MDA design, like the MDA, the 80C includes a 2K character memory and a 2K attribute memory.

The MDA did not have any pixel addressable graphics modes, it had only a single text mode and all screen characters were generated from the 256 characters available in the character generator PROM.

Although not featuring colour, the MDA used a number of text attributes, e.g., underline, bright, reverse video and blink to provide some differentiation between the characters.

The 80C board supports additional attributes for RGB colour and incorporates an additional character generator ROM for graphics characters.

As the diagram shows, the 6845 has 18 internal, 8 bit, registers that are programmed by the CPU and used to control all of the video timing attributes etc. Depending on the individual register, it may be read-only (ro), write-only (wo) or read-write (rw), and is accessed via the system data bus.

The functions of these internal registers is fully described in the MC6845 datasheet and won't be discussed in detail on this page but the table summarises the registers and their function.

The FDX 80 column card, having 80 x 24 character positions, requires 1920 storage locations to buffer the data used for screen refresh. Each of the 1920 character locations has one 16-bit data word associated with it, provided by two Toshiba TMM2016AP-15 (2K x 8-bit) SRAM chips.

The most significant byte of the character word stores the address of the shape in the character generator PROM and the least significant word, called the attribute byte, controls the way in which the character is displayed and which PROM is selected. The effect of the bits in the attribute byte depends on whether a colour or monochrome monitor is being used.

All of the characters that can be generated by the 80 column card are defined in two character generator (CG) PROMs, one for ALPHA characters and one for for bit-mapped GRAPHICS characters, each PROM containing 256 characters or shapes.

The effect of the bits in the Attribute byte is different depending on whether a colour or monochrome display is being used.

The bits in the Attribute byte can be combined, for example, to produce an alpha-character in white text on a blue background with no blink, the attribute byte would be 00100111

As described earlier, a character set for raster scanning could be formed using an 8 x 8 matrix, with all letters in upper case, the example text shown below is very readable.

However, to display lower case characters using the same matrix, letters with tails, such as "g", "j", "q", etc. would need to be displayed with an offset from the baseline - for example :

As illustrated above, this would result in ugly screen output which would be tiring to read. To display these lower case letters correctly when using a mixed upper & lower case graphics set, the descenders need to be displayed below the normal character baseline. In order for the 80 Column card to be able to display true descenders, characters are formed using a 7 x 10 matrix.

For example, the standard ALPHA character set is as shown.

The GRAPHICS character generator PROM contains the 256 permutations of 8 pixels per character. Bits D0..D7 represent the pixels in the the graphics character block.