APPENDIX C

DISPLAY CONTROLLER LIMITATIONS

KNOWN LIMITATIONS OF THE MINIMODULE/PRIVATE EYE
DISPLAY CONTROLLER BOARD

The MiniModule/Private Eye display controller board incorporates an ASIC to implement many of its functions. The present version of this ASIC has several known design deficiencies which affect its functionality. (This ASIC has the designation “VCGA” and part number “060205-001” stamped on it.) Many of these problems are corrected by other circuitry on the MM/Private Eye board, and so are of no concern to its users. However, some limitations affecting CGA software compatibility remain and are described below.

(For a complete list of the ASIC “bugs” see the Private Eye Display Controller ASIC Specification in Appendix B. Note that Items (1) through (4) in that document have been corrected on the MM/Private Eye board.)

Remaining Limitations with MM/Private Eye Board:

a)	Cursor slow blink mode does not work.

b)	Some flashing and reverse video characters may not be displayed properly:
Characters with the “Flashing” attribute are not displayed when the Flash Enable bit in the MCR is disabled.
Characters with the “Red Flashing” attribute (OxF7h) flash when the Flash Enable bit in the MCR is disabled.
Flashing underlined characters are not displayed properly, regardless of the state of the MCR Flash Enable bit.

c)	In shadow mode only, characters may be dropped or incorrect characters may appear in text mode. This intermittent problem depends on the speed of the Primary CGA board. It does not occur with two MMlPrivate Eye boards.

d)	Extended Vertical Mode (720 x 280 graphics, vertical bitmap) is unusable. Instead, use Extended Horizontal Mode (720 x 280 graphics, horizontal bitmap).

e)	In Extended Horizontal Mode, the contents of the RTSI Command Register are not reliably transmitted to the display. Write the register in a loop, so that it eventually gets transmitted. (Empirically determine loop count for reliable transmission.)

f)	NON-PCMODE is not implemented. Only the standard CGA memory and I/O register addresses are available. Therefore, the MM/Private Eye board cannot co-exist with an EGA or VGA display controller.

g)	“Flight Simulator” does not display properly in graphics mode if initialized for CGA display, color monitor. Instead, initialize for CGA display, monochrome monitor.

h)	Some standard programs may not be usable due to mapping of color attribute bytes to monochrome displayed characters. For example, the “TOPS 2.0” menu screen is not usable because the highlighted menu items (green background, black foreground on normal CGA) are not displayed differently from the unhighlighted menu characters. Similarly, “Columns” (public domain version of Tetris) is unusable because it uses character background colors.