B69000 Asiliant Technologies, B69000 Datasheet - Page 87

• Manufacturer Part Number: B69000
• Manufacturer: Asiliant Technologies

Specifications of B69000

• Lead Free Status / Rohs Status: Supplier Unconfirmed

Chapter 8
General Control and Status Registers
These are direct-access registers -- they are NOT read from or written to using any form of sub-indexing
scheme.
Various bits in these registers have bits that provide control over the real-time status of the horizontal sync
signal, the horizontal retrace interval, the vertical sync signal, and the vertical retrace interval.
The horizontal retrace interval is the time when the drawing of each horizontal line has active video data,
when the active video data is not being displayed. It is the time that includes the horizontal front and back
porches, and the horizontal sync pulse. The horizontal retrace interval is always longer than the horizontal
sync pulse.
The vertical retrace interval is the period during the drawing of each screen, when the horizontal lines with
active video data are not drawn. This period includes the vertical front and back porches, and the vertical
sync pulse. The vertical retrace interval is always longer than the vertical sync pulse.
The ‘Display Enable’ status bit (bit 0) in Input Status Register 1 indicates that either a horizontal retrace
interval or a vertical retrace interval is in progress (the name ‘Display Enable’ is misleading for this status
bit because the bit does not enable nor disable the graphics system as it’s name suggests). In the IBM
EGA graphics system (and the ones that preceded it, including MDA and CGA) it was important to check
the status of this bit to ensure that one or the other of the retrace intervals was taking place before accessing
the graphics memory. In these earlier systems reading from or writing to graphics memory outside the
retrace intervals meant that the CRT controller would be cut off from accessing the graphics memory in
order to draw pixels to the display, resulting in either “snow” or a flickering display.
&+,36
Name
ST00
ST01
MSR
FCR
69000 Databook
Input Status Register 0
Input Status Register 1
Feature Control Register
Miscellaneous Output Register
General Control and Status Registers
Subject to Change Without Notice
Function
3BA/3DA
Read
3CA
3CC
3C2
Revision 1.3 8/31/98
3BA/3DA
Write
3C2
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