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Input Status Register One Format
B~
7
6
5
4
3
2
1
0
Disptay Enable
Light Pen Strobe
Light Pen Switch
'--------i"'~
Vertical Retrace
Diagnostic 1
Diagnostic 0
'--...Ii..-_ _ _ _ _ _ _
-I.~
Not Used
Bit 0
Bit 1
Bit 2
Bit 3
Bits 4 and 5
Display Enable-Logical 0 indicates the CRT
raster is in a horizontal or vertical retrace
interval. This bit is the real time status of the
display enable signal. Some programs use this
status bit to restrict screen updates to inactive
display intervals. The Enhanced Graphics
Adapter does not require the CPU to update the
screen buffer during inactive display intervals to
avoid glitches in the display image.
Light Pen Strobe-A logical 0 indicates that the
light pen trigger has not been set; a logical 1
indicates that the light pen trigger has been set.
Light Pen Switch-A logical 0 indicates that the
light pen switch is closed; a logical 1 indicates
that the light pen switch is open.
Vertical Retrace-A logical 0 indicates that video
information is being displayed on the CRT
screen; a logical 1 indicates the CRT is in a
vertical retrace interval. This bit can be
programmed to interrupt the processor on
interrupt level 2 at the start of the vertical
retrace. This is done through bits 4 and 5 of the
Vertical Retrace End Register of the CRTC.
Diagnostic Usage-These bits are selectively
connected to two of the six color outputs of the
16 IBM. Enhanced GraphiCS' Adapter
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