Commodore Amiga A1000, Hardware Reference Manual

Page: 62 / 380

The following example sets DIWSTOP for a basic playfield to $F4 for the vertical position
and $C1 for the horizontal position.
LEA CUSTOM,a0 ; Get base address of custom hardware...
MOVE.W #$F4C1,DIWSTOP(a0) ; Display window stop register...
Table 3-9: DIWSTRT AND DIWSTOP Summary.
-Nominal Values- -Possible Values-
NTSC PAL MIN MAX
DIWSTRT:
VSTART $2C $2C $00 $FF
HSTART $81 $81 $00 $FF
DIWSTOP:
VSTOP $F4 $2C (=$12C) $80 $7F (=$17F)
HSTOP $C1 $C1 $00 (=$100) $FF (=$1FF)
TELLING THE SYSTEM HOW TO FETCH AND DISPLAY DATA
After defining the size and position of the display window, you need to give the system the
on screen location for data fetched from memory. To do this, you describe the horizontal
positions where each line starts and stops and write these positions to the data-fetch
registers. The data-fetch registers have a four-pixel resolution (unlike the display window
registers, which have a one-pixel resolution). Each position specified is four pixels from
the last one. Pixel 0 is position 0; pixel 4 is position 1, and so on.
The data-fetch start and display window starting positions interact with each other. It is
recommended that data-fetch start values be restricted to a programming resolution of 16
pixels (8 clocks in low-resolution mode, 4 clocks in high-resolution mode). The hardware
requires some time after the first data fetch before it can actually display the data. As a
result, there is a difference between the value of window start and data-fetch start of 4.5
color clocks.
The normal low-resolution DDFSTRT is ($0038).
The normal high-resolution DDFSTRT is ($003C).
Recall that the hardware resolution of display window start and stop is twice the hardware
resolution of data fetch:
$81
--- -8.5=$38
2
$81
--- -4.5=$3c
2
- Playfield Hardware 53 -