programmable_bitmap_sequencer
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programmable_bitmap_sequencer [2020/09/28 03:00] – silverdr | programmable_bitmap_sequencer [2022/07/14 15:20] – [Principles of operation] silverdr | ||
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The first condition is normally met, so no point worrying about it. \\ | The first condition is normally met, so no point worrying about it. \\ | ||
- | As soon as '' | + | As soon as '' |
Once PBS decides to act, it performs following operations in every cycle: | Once PBS decides to act, it performs following operations in every cycle: | ||
- | - It uses an internal copy of S_BASE register to determine where to fetch the graphics data from. '' | + | - It uses an internal copy of '' |
- Once the byte is fetched, VASYL can do some simple bit operations on it (see below), then writes it to the VIC-II data bus. | - Once the byte is fetched, VASYL can do some simple bit operations on it (see below), then writes it to the VIC-II data bus. | ||
- | - Finally, the internal copy of S_BASE is incremented by S_STEP, yielding the address of a byte to use in the next cycle. With a typically organized graphics data, this value is equal to 1. | + | - Finally, the internal copy of '' |
- | One cycle after the last active cycle in a rasterline, PBS does one more thing - it increases its internal copy of S_BASE by the value of register S_PADDING, yielding the memory address of the first byte of the next line. If our graphics data is laid out sequentially in memory (i.e. the last byte in a line is directly followed by the first byte of the next line), S_PADDING is going to be 0. But if you wanted to open a virtual screen larger than the display window, you would use S_PADDING to "skip over" the currently invisible (off-screen) part of the virtual display - as it happens to the rightmost part of C= logo in the illustration above. | + | One cycle after the last active cycle in a rasterline, PBS does one more thing - it increases its internal copy of '' |
==== Examples ==== | ==== Examples ==== |
programmable_bitmap_sequencer.txt · Last modified: 2022/07/14 15:23 by silverdr