An interlaced bitplane 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 Bitplane in RAM The system retrieves data for bitplane 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 bitplane 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. To get the Copper to execute the correct list, you set an interrupt to the 680x0 just after the first line of the display. When the interrupt is executed, you change the contents of the COP1LC location register to point to the second list. Then, during the vertical blanking interval, COP1LC will be automatically reset to point to the original list. For more information about interlaced displays , see Chapter 3, "Playfield Hardware."
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