Bits 3, 4 and 5 control whether the noise generator is output on channels A, B and C respectively. Any given channel can output a pure tone, pure noise, or a mixture of the two. Note that all six of these bits are inverted; output is enabled when the corresponding bit is a zero. Bits 6 and 7 control whether the two I/O ports, IOA and IOB, are set for input (0) or output (1).

R10, R11 and R12 control the volume for the three channels. The four lower bits produce a value from 0 (off) to 15 (loud). Bit 4 controls the "amplitude mode", or