Figure 3-10 Data Fetched For The First Line When Modulo - Commodore Amiga Hardware Reference Manual

You also need to tell the system exactly which bytes in memory belong on each horizon-
tal 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 begin-
ning 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
Leftmost
Display Word
START+2
Next Word
START+4
Next 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
START+38
Last Display
T
After the first line is fetched, the bit-plane pointers BPLxPTH and BPLxPTL contain
the value START+40. The modulo (in this case, 0) is added to the current value of the
pointer, so when the pointer begins the data fetch for the next line, it fetches the data
you want on that line. The data for the next line begins at memory location
START+40.
50 Playfield Hardware