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-====== 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 [[system ROM]] and [[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.
-===== Magic Register =====
-The **Magic Register** ([[output port]] $C) byte determines how the data is modified. Each bit flags a specific data transformation. The purpose of each Magic Register bit is shown below:
-^ Bit ^ Function ^ Description ^
-| 0 | LSB of shift amount | |
-| 1 | MSB of shift amount | |
-| 2 | Rotate | Not supported on the Bally Arcade |
-| 3 | Expand | |
-| 4 | OR | |
-| 5 | XOR | |
-| 6 | Flop | |
-| 7 | Not used | Must remain 0 |
-The order in which magic functions are performed is as follows:
-  - Expansion
-  - Rotation or Shift
-  - Flop
-  - OR or XOR
-As many as four functions can be used at any one time, and any function can be bypassed. Rotate and shift as well as OR and XOR cannot be done at the same time.
-==== Expander ====
-The expander expands the 8-bit data bus into 8 pixels (i.e., 16 bits). It expands a 0 on the data bus into a 2-bit pixel and 1 into another 2-bit pixel. Thus, two-color patterns can be stored in ROM in half the normal memory space.
-During each memory write instruction using the expander, either the upper half or the lower half of the data bus is expanded. The half used is determined by the expand flip-flop. The flip-flop is reset by an output to the magic register and is toggled after each [[Magic Register|Magic Memory]] write. The upper half of the data bus is expanded when the flip-flop is 0, and the lower half when the flip-flop is 1.
-The expand register ([[output port]] $19) determines the pixel values into which the data bus will be expanded. A 0 on the data bus will be expanded into the pixel defined by bits 0 and 1 of the expand register. A 1 on the data bus will be expanded into the pixel defined by bits 2 and 3 of the expand register.
-The pixels generated by bit 0 or 4 of the data bus will be the least significant pixel of the expanded byte. The most significant pixel will come from bit 3 or 7.
-^ XPAND Register Bit ^ Controls ^
-| 0 | Bits 0–1 define the color for 0 bits in the expanded pixel pattern |
-| 1 | 00 = color 0, 01 = color 1, 10 = color 2, 11 = color 3 |
-| 2 | Bits 0–1 define the color for 1 bits in the expanded pixel pattern |
-| 3 | 00 = color 0, 01 = color 1, 10 = color 2, 11 = color 3 |
-| 4 | Not used [?] |
-| 5 | Not used [?] |
-| 6 | Not used [?] |
-| 7 | Not used [?] |
-Example:
-<code z80>
-[example]
-</code>
-==== Shift, Flop, and Rotate ====
-The shifter, flopper, and rotator operate on pixels rather than bits. Each byte contains four pixels, each of which has one of four values. The four pixels are referred to as P0, P1, P2, and P3. P0 is composed of the first two bits of the byte.
-=== Shifter ===
-The shifter shifts data 0, 1, 2, or 3 pixels to the right. Bits 0 and 1 of the magic register determine the shift amount. The pixels shifted out of one byte are shifted into the next byte. 0's are shifted into the first byte of a sequence.
-The shifter assumes the first byte of a sequence is the first magic memory write after an output to the magic register. Each sequence must be initialized by an output to the magic register, and data cannot be sent to the magic register in the middle of a sequence.
-=== Flopper ===
-The output of the flopper is a mirror image of its input. Pixel 0 and 3 exchange values, as do pixels 1 and 2.
-=== Rotator ===
-**NOTE:** The rotator is not available in consumer mode, and thus does not function on the Astrocade. To learn more, please consult the Nutting Manual pp. 98–99.
-==== OR and XOR ====
-These functions operate on a byte as 8 bits rather than four pixels. When the OR function writes data to RAM, the input to the OR circuit is ORed with the contents of the RAM location being accessed. The result is then written in RAM.
-The XOR function operates in the same way, except that the data is XORed instead of ORed.
-==== Intercept ====
-[NM:94-100]
-**Reference:** [[http://www.ballyalley.com/basic/Magic_Register_Tutorial_(Bilbrey)/Magic%20Register%20Tutorial%20(December%2010%201981)(Brett%20Bilbrey).pdf|Magic Register Tutorial]] (1981) by Brett Bilbrey [PDF]