Nintendo GAME BOY Programming Manual page 35

Note: When bit 0 of register Key 1 is set to 1, the standby function cannot be
used. When using the standby function, always confirm that bit 0 of
register Key 1 is set to 0. When switching the CPU speed, all interrupt-
enable flags should be reset and a STOP instruction executed with bits 4
and 5 of the P1 port register set to 1, as with the standby function (STOP
mode). When the CPU speed is switched, a return from STOP mode is
automatic, so it is not necessary to generate a STOP mode cancellation.
However, until the CPU speed has been changed and the system clock
returns, bits 4 and 5 of the P1 port register should be made to hold the
value 1.
Approximately 16 ms is required to switch from normal to double-speed mode, and
approximately 32 ms is needed to switch from double-speed to normal mode. In double-speed
mode, the DIV register (FF04h) and the TIMA register (FF05h) both operate at double
speed. Battery life is shorter in double-speed mode than in normal mode. The use of double-
speed mode requires the corresponding mask ROM and MBC.
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Chapter 1: System