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hardware:magic_system [2016/06/11 12:15] ex_writer [Magic System] |
hardware:magic_system [2016/06/11 12:35] (current) ex_writer [Example: Space Invaders] |
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| ====== Magic System ====== | ====== Magic System ====== | ||
| - | The Magic System is enabled when data is written to a memory location between 0 to 16K ($0–$3FFF), aka Magic Memory. Since this memory range is occupied by [[software:system ROM]] and [[software:cartridges|cartridge ROM]], respectively, data cannot be written to this area. Instead, a modified form of the data is actually written to memory location X + 16K, but with the Magic function applied. The Magic function generator uses a pair of registers, MAGIC ($C) and XPAND ($19), to perform the requested transformations. | + | The Magic System is enabled when data is written to a memory location between 0 to 16K ($0–$3FFF), aka Magic Memory. Since this memory range is occupied by [[software:system ROM]] and [[software:cartridges|cartridge ROM]], respectively, data cannot be written to this area. Instead, a modified form of the data is actually written to memory location X + 16K. The Magic function generator uses a pair of registers, MAGIC ($C) and XPAND ($19), to perform the requested transformations. |
| ===== Magic Register ===== | ===== Magic Register ===== | ||
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| === Intercept Register (Collision Detection) === | === Intercept Register (Collision Detection) === | ||
| - | Software reads the intercept register ([[i o ports|input port]] $8) to determine if an intercept occurred on an OR or XOR write. An intercept is defined as the writing of a non-zero pixel location that previously contained a non-zero pixel. A non-zero pixel is a pixel with a value of 01, 10, or 11. A 1 in the intercept register means an intercept has occurred. Bits 0–3 give the intercept information for all OR or XOR writes since the last input from the intercept register. An input from the intercept register resets these bits. A bit is set to 1 if an intercept occurs in the appropriate position and will not be reset until after the next intercept register input. | + | Software reads the intercept register ([[i o ports|input port]] $8) to determine if an intercept occurred on an OR or XOR write. An intercept is defined as the writing of a non-zero pixel (i.e., 01B, 10B, or 11B) location that previously contained a non-zero pixel. |
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| + | A bit corresponding to the pixel position in the intercept register will be written to logical 1. Bits 0–3 give the intercept information for all OR or XOR writes since the last input from the intercept register. An input from the intercept register resets these bits. A bit is set to 1 if an intercept occurs in the appropriate position and will not be reset until after the next intercept register input. | ||
| ^ Bit ^ Meaning ^ | ^ Bit ^ Meaning ^ | ||
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| | 7 | Intercept in pixel 0 in last OR or XOR write | | | 7 | Intercept in pixel 0 in last OR or XOR write | | ||
| + | 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: | ||
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| + | > "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 === | === Example: Space Invaders === | ||
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| </code> | </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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