T -TIMING CYCLE- T+7
+ * + *
---------------------------------
| | 4 | 6 | 2 | | 3 | 5 | 1 |
---------------------------------
Figure 6-11: Time Slots Used by a Six Bit Plane Display
If you specify four high-resolution bit-planes (640 pixels wide), bit-plane DMA needs all of
the available memory time slots during the display time just to fetch the 40 data words
for each line of the four bit-planes (40 4 = 160 time slots). This effectively locks out the
68000 (as well as the blitter or Copper) from any memory access during the display,
except during horizontal and vertical blanking.
T -TIMING CYCLE- T+7
---------------------------------
| 4 | 2 | 3 | 1 | 4 | 2 | 3 | 1 |
---------------------------------
Figure 6-12: Time Slots Used by a High Resolution Display
Each horizontal line in a normal, full-sized display contains 320 pixels in low-resolution
mode or 640 pixels in high-resolution mode. Thus, either 20 or 40 words will be fetched
during the horizontal line display time. If you want to scroll a playfield, one extra data
word per line must be fetched from the memory.
Display size is adjustable (see Chapter 3, "Playfield Hardware"), and bit-plane DMA takes
precedence over sprite DMA. As shown in Figure 6-9, larger displays may block out one or
more of the highest-numbered sprites, especially with scrolling.
- 192 Blitter Hardware -
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