Commodore Amiga A1000, Hardware Reference Manual

Page: 63 / 380

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 ($00D0). The normal high-resolution DDFSTOP is
($00D4)
The following example sets data-fetch start to $0038 and data-fetch stop to $00D0 for a
basic playfield.
LEA CUSTOM,a0 ; Point to base hardware address
MOVE.W #$0038,DDFSTRT(a0) ; Write to DDFSTRT
MOVE.W #$00D0,DDFSTOP(a0) ; Write to DDFSTOP
You also need to tell the system exactly which bytes in memory belong on each horizontal
line of the display. To do this, you specify the modulo value. Modulo refers to the number
of bytes in memory between the last word on one horizontal line and the beginning of the
first word on the next line. Thus, the modulo enables the system to convert bit-plane data
stored in linear form (each data byte at a sequentially increasing memory address) into
rectangular form (one "line" of sequential data followed by another line). For the basic
playfield, where the playfield in memory is the same size as the display window, the
modulo is zero because the memory area contains exactly the same number of bytes as
you want to display on the screen. Figures 3-10 and 3-11 show the basic bit-plane layout
in memory and how to make sure the correct data is retrieved.
The bit-plane address pointers (BPLxPTH and BPLxPTL) are used by the system to fetch
the data to the screen. These pointers are dynamic; once the data fetch begins, the
pointers are continuously incremented to point to the next word to be fetched (data is
fetched two bytes at a time). When the end-of-line condition is reached (defined by the
data-fetch register, DDFSTOP) the modulo is added to the bit-plane pointers, adjusting
the pointer to the first word to be fetched for the next horizontal line.
Data for Line 1:
Location: START START+2 START+4 .....START+38
Leftmost Next Word Next Word Last Display
Display Word Word
^
Screen data fetch stops (DDFSTOP) for |
each horizontal line after the last word <----------------------|
on the line has been fetched.
Figure 3-10: Data Fetched for the First Line When Modulo = 0
- 54 Playfield Hardware -