Commodore Amiga A2000 - Using the Copper in Interlaced Mode; Figure 2-1 Interlaced Bit-Plane in RAM

DC.W COPJMP2,$0 ; Force a jump to COP2LC
; Whatever cleanup copper code that might be needed here...
; Since there are 262 lines in NTSC, and we stopped at 255, there is a
; bit of time available
DC.W $FFFF,$FFFE ; End of Copper list
USING THE COPPER IN INTERLACED MODE
An interlaced bit-plane display has twice the normal number of vertical lines on the
screen.
Whereas a normal NTSC display has 262 lines, an interlaced NTSC display has 524 lines.
PAL has 312 lines normally and 625 in interlaced mode. In interlaced mode, the video
beam scans the screen twice from top to bottom, displaying, in the case of NTSC, 262
lines at a time. During the first scan, the odd-numbered lines are displayed. During the
second scan, the even-numbered lines are displayed and interlaced with the odd-
numbered ones. The scanning circuitry thus treats an interlaced display as two display
fields, one containing the even-numbered lines and one containing the odd-numbered
lines. Figure 2-1 shows how an interlaced display is stored in memory.
Odd Field Even field
(time t) (time t+16.6ms) Data in memory
_____________
| |
| 1 |
|_____________|
| |
_____________ _____________ | 2 |
| | | | |_____________|
| 1 | | 2 | | |
|_____________| |_____________| | 3 |
| | | | |_____________|
| 3 | | 4 | | |
|_____________| |_____________| | 4 |
| | | | |_____________|
| 5 | | 6 | | |
|_____________| |_____________| | 5 |
|_____________|
| |
| 6 |
|_____________|
Figure 2-1: (Interlaced Bit-Plane in RAM)
The system retrieves data for bit-plane displays by using pointers to the starting address
of the data in memory. As you can see, the starting address for the even-numbered fields
is one line greater than the starting address for the odd-numbered fields. Therefore, the
bit-plane pointer must contain a different value for alternate fields of the interlaced
display.
Simply, the organization of the data in memory matches the apparent organization on the
screen (i.e., odd and even lines are interlaced together). This is accomplished by having a
separate Copper instruction list for each field to manage displaying the data.
- 30 Coprocessor Hardware -