Commodore Amiga A1000, Hardware Reference Manual

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playfield or scroll the playfield through the window. You can also define display windows
larger than the basic playfield. These larger playfields and different-sized display windows
are described in The section below called "Bit-Planes and Display Windows of All Sizes."
You determine the size of the display window by specifying the vertical and horizontal
positions at which the window starts and stops and writing these positions to the display
window registers. The resolution of vertical start and stop is one scan line. The resolution
of horizontal start and stop is one low-resolution pixel. Each position on the screen defines
the horizontal and vertical position of some pixel, and this position is specified by the x
and y coordinates of the pixel. This document shows the x and y coordinates in this form:
(x,y). Although the coordinates begin at (0,0) in the upper left-hand corner of the screen,
the first horizontal position normally used is $81 and the first vertical position is $2C. The
horizontal and vertical starting positions are the same both for NTSC and for PAL.
The hardware allows you to specify a starting position before ($81,$2C), but part of the
display may not be visible. The difference between the absolute starting position of (0,0)
and the normal starling position of ($81,$2C) is the result of the way many video display
monitors are designed. To overcome the distortion that can occur at the extreme edges of
the screen, the scanning beam sweeps over a larger area than the front face of the screen
can display. A starting position of ($81,$2C) centers a normal size display, leaving a
border of eight low-resolution pixel around The display window. Figure 3-9 shows the
relationship between the normal display window, the visible screen area, and the area
actually covered by the scanning beam.
(0,0)
/ ($81,$2C)
/______/____________________________
| ___/__________________________ |
| | /_________________________ |\ |
| | | /\ | | \|
| | |<--|-------320----------->| | \
| | | | | | |\
| | | |200 | | | \Visible screen
| | | | | | | boundaries
| | | | | | |
| | |___\/_____________________| | |
| |__\________________________/__| |
|______\_____________________ /______|
\ /
\_____Display _____/
window starting &
stopping positions
Figure 3-9: Positioning the On-screen Display
- 51 Playfield Hardware -