add Virtual Boy docs

papers/doc/7-systems/virtual-boy.md

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+# Virtual Boy
+
+a "portable" video game console made by Nintendo in the '90's.
+
+it supposedly was the beginning of virtual reality... nah, instead it failed to sell well because you use it for 15 minutes and then you get a headache.
+
+its sound generation chip is called Virtual Sound Unit (VSU), a wavetable chip that is a lot like PC Engine, but unlike that, the waves are twice as tall, it doesn't go too low in terms of frequency (~D-2), and the last channel (yep, it has 6 channels) is a noise one.
+
+additionally, channel 5 offers a modulation/sweep unit. the former is similar to FDS' but has much reduced speed control.
+
+# effects
+
+- `10xx`: set waveform.
+- `11xx`: set noise length (0 to 7).
+  - only in the noise channel.
+- `12xy`: setup envelope.
+  - `x` determines whether envelope is enabled or not.
+    - 0: disabled
+    - 1: enabled
+    - 3: enabled and loop
+    - yeah, the value 2 isn't useful.
+  - `y` sets the speed and direction.
+    - 0-7: down
+    - 8-F: up
+- `13xy`: setup sweep.
+  - `x` sets the speed.
+    - 0 and 8 are "speed 0" - sweep is ineffective.
+  - `y` sets the shift (0 to 7).
+    - 8 and higher will mute the channel.
+  - only in channel 5.
+- `14xy`: setup modulation.
+  - `x` determines whether it's enabled or not.
+    - 0: disabled
+    - 1: enabled
+    - 3: enabled and loop
+    - 2 isn't useful here either.
+  - `y` sets the speed.
+    - 0 and 8 are "speed 0" - modulation is ineffective.
+    - no, you can't really do Yamaha FM using this.
+  - only in channel 5.
+- `15xx`: set modulation wave.
+  - `xx` points to a wavetable. it should have a height of 255.
+  - this is an alternative to setting the modulation wave through the instrument.