Nintendo GAME BOY Programming Manual page 19

Sound Functions
Each system is equipped with 4 types of sound synthesis circuitry.
! Sound 1: Quadrangular waveform, sweep and envelope functions
! Sound 2: Quadrangular waveform, envelope functions
! Sound 3: Arbitrary waveform, generated
! Sound 4: White noise, generated
! 2 output channels (output can be allocated to a channel)
! Synthesized output with external sound input <CGB only>
Miscellaneous
! An internal monitor program is built into DMG/CGB CPUs. When power is
turned on or the Game Boy is reset, the internal monitor program first
initializes components such as the ports, then passes control to the user
program.
! Instruction cycles
<DMG> 0.954 µs (source oscillation: 4.1943 MHz)
<CGB> 0.954 µs/0.477 µs, switchable (source oscillation: 8.3886 MHz)
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Chapter 1: System