Differences

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--- hardware:magic_system [2016/06/11 12:28]
ex_writer [Intercept Register (Collision Detection)]
+++ hardware:magic_system [2016/06/11 12:35] (current)
ex_writer [Example: Space Invaders]
@@ Line 103: / Line 103: @@
 | 7 | Intercept in pixel 0 in last OR or XOR write |
-> "The Intercept register contains several flag bits which save the result of previous OR or XOR Magic Memory writes. If any of bits [3:0] are set, this indicates that the previous write to the corresponding pixel (bit[3] = pixel[3], etc.,) wrote a non-zero pixel with a non-zero value. Bits [3:0] track the last Magic memory write, while bits [7:4] track //any// write to the corresponding pixel (bit[7]=pixel[3],etc.,) since the last time the Intercept register was read. Thus, once any of bits [7:4] are set to logical 1, they will remain that way until the register is read, at which time these bits will be cleared to logical 0.
+From [[http://www.ballyalley.com/faqs/BPA%20Video%20Hardware%20FAQ/Bally_Professional_Arcade_Video_Hardware%20(2001)(Tony%20Miller).pdf|A Description of the Bally Professional Arcade Video Hardware]], p. 24:
+> "The Intercept register contains several flag bits which save the result of previous OR or XOR Magic Memory writes. If any of bits [3:0] are set, this indicates that the previous write to the corresponding pixel (bit[3] = pixel[3], etc.,) wrote a non-zero pixel with a non-zero value. Bits [3:0] track the last Magic memory write, while bits [7:4] track //any// write to the corresponding pixel (bit[7]=pixel[3],etc.,) since the last time the Intercept register was read. Thus, once any of bits [7:4] are set to logical 1, they will remain that way until the register is read, at which time these bits will be cleared to logical 0."
 === Example: Space Invaders ===
@@ Line 114: / Line 116: @@
 </code>
-Directly after the pattern write, intercept status is loaded in A, then A is ORed to see if any bits are set (indicating an intercept). If the OR result is 0, no collision occurs, and the collision check subroutine exits.
+Directly after the pattern write, intercept status is loaded in A, then A is ORed to see if any bits are set (indicating an intercept). If the OR result is 0, no collision occurs, and the collision check subroutine exits. (//Football// uses an identical check.)
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