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The shl and shr instructions are often documented to take both a Vx and a
Vy argument. It’s a bit unclear what the behaviour here is meant to be.
The original CHIP-8 implementation shifted Vy by 1 and stored the result
in Vx. Some GitHub projects shift the value in Vx by the value in Vy.
Ahoy takes the approach of SCHIP where Vx is shifted by 1 and Vy is
ignored. The Ahoy assembler does not recognize a second Vy argument to
either instruction and the Y section of the instruction is always set to 0.
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