Misc/abrash-black-book
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James Gregory 2013-12-30 12:25:34 +11:00
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23-04.html
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@ -49,7 +49,7 @@
<P>Smooth horizontal panning is provided by the Horizontal Pel Panning register, AC register 13H, working in conjunction with the start address. Up to 7 pixels worth of single pixel panning of the displayed image to the left is performed by increasing the Horizontal Pel Panning register from 0 to 7. This exhausts the range of motion possible via the Horizontal Pel Panning register; the next pixel&#146;s worth of smooth panning is accomplished by incrementing the start address by one and resetting the Horizontal Pel Panning register to 0. Smooth horizontal panning should be viewed as a series of fine adjustments in the 8-pixel range between coarse byte-sized adjustments.</P>
<P>A horizontal panning oddity: Alone among VGA modes, text mode (in most cases) has 9 dots per character clock. Smooth panning in this mode requires cycling the Horizontal Pel Panning register through the values 8, 0, 1, 2, 3, 4, 5, 6, and 7. 8 is the &#147;no panning&#148; setting.</P>
<P>There is one annoying quirk about programming the AC. When the AC Index register is set, only the lower five bits are used as the internal index. The next most significant bit, bit 5, controls the source of the video data sent to the monitor by the VGA. When bit 5 is set to 1, the output of the palette RAM, derived from display memory, controls the displayed pixels; this is normal operation. When bit 5 is 0, video data does not come from the palette RAM, and the screen becomes a solid color. The only time bit 5 of the AC Index register should be 0 is during the setting of a palette RAM register, since the CPU is only able to write to palette RAM when bit 5 is 0. (Some VGAs do not enforce this, but you should always set bit 5 to 0 before writing to the palette RAM just to be safe.) Immediately after setting palette RAM, however, 20h (or any other value with bit 5 set to 1) should be written to the AC Index register to restore normal video, and at all other times bit 5 should be set to 1.</P>
<TABLE WIDTH="100%"><TD WIDTH="5%" VALIGN="TOP"><IMG SRC="images/23-02i.jpg"><TD WIDTH="95%" VALIGN="TOP"><SMALL><I>By the way, palette RAM can be set via the BIOS video interrupt (interrupt 10H), function 10H. Whenever an VGA function can be performed reasonably well through a BIOS function, as it can in the case of setting palette RAM, it should be, both because there is no point in reinventing the wheel and because the BIOS may well mask incompatibilities between the IBM VGA and VGA clones.</I></SMALL>
<TABLE WIDTH="100%"><TD WIDTH="5%" VALIGN="TOP"><IMG SRC="images/i.jpg"><TD WIDTH="95%" VALIGN="TOP"><SMALL><I>By the way, palette RAM can be set via the BIOS video interrupt (interrupt 10H), function 10H. Whenever an VGA function can be performed reasonably well through a BIOS function, as it can in the case of setting palette RAM, it should be, both because there is no point in reinventing the wheel and because the BIOS may well mask incompatibilities between the IBM VGA and VGA clones.</I></SMALL>
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