Telling The System How To Fetch And Display Data - Commodore Amiga Hardware Reference Manual

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 hor-
izontal 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 pix-
els 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. In low-resolution mode the difference is 8.5 clocks; in high-resolution mode the
difference is 4.5 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:
($81/2 - 8.5)
=
($38)
($81/2 - 4.5)
=
($3C)
The relationship between data-fetch start and stop is
DDFSTRT
=
DDFSTOP - (8*(word count - 1) for low resolution
DDFSTRT
=
DDFSTOP - (4*(word count - 2) for high resolution
The normal low-resolution DDFSTOP is ($OODO). The normal high-resolution
DDFSTOP is ($00D4).
The following example sets data-fetch start to $0038 and data-fetch stop to $ooDO for a
basic playfield.
DDFSTRT EQU
DDFSTOP EQU
MOVE.W
MOVE.W
$DFF092
$DFF094
#$0038,DDFSTRT
#$OODO,DDFSTOP
; Write to DDFSTRT
; Write to DDFSTOP
Playfield Hardware 49