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29 lines
1.2 KiB
Plaintext
29 lines
1.2 KiB
Plaintext
T7.bas a test program for cycle-counting.
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The basic idea: we can find out how the real Atari does things by
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changing a register after a certain number of NOP's on a certain
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scanline. By aligning a player with the colour change, the horizontal
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position can be determined and checked on the emulator.
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Second idea: We can use RANDOM as a cycle-timer. Set the 9-bit
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random number generator (repeating 511 different values in a sequence),
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then generate all 511 values and store them in an array, with
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the value as the index and the index as the value.
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Then given two different readings of RANDOM at different times,
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look up the index of both in the table, and the difference , mod 511
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is the number of cycles elapsed between the readings.
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Only problem is that we can only read 8 bits of RANDOM, so there's
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two possible values for each reading, giving 4 possible offsets.
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But only one of these will remain constant if the procedure is
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repeated.
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The 4 values printed by the program at the bottom are the computed
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possible offsets, the one which remains constant is the acutal
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offset.
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N=# of NOP cycles
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P=HPOSP0
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C=register to change (0=player 0, 9=COLBK, 10=DMACTL, 11-13 HPOS,SIZE,GRAF)
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V=value to change to
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PG=player graphics
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M=antic IR mode
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