Reducing Power Consumption

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The following programming techniques can be used to reduce the power consumption of the GameBoy hardware and extend the life of the batteries.

Using the HALT Instruction

The HALT instruction should be used whenever possible to reduce power consumption & extend the life of the batteries. This command stops the system clock, reducing the power consumption of both the CPU and ROM.

The CPU will remain stopped until an interrupt occurs at which point the interrupt is serviced and then the instruction immediately following the HALT is executed.

Depending on how much CPU time is required by a game, the HALT instruction can extend battery life anywhere from 5 to 50% or possibly more.

When waiting for a vblank event, this would be a BAD example:

 @@wait:
  ld   a,(0FF44h)      ;LY
  cp   a,144
  jr   nz,@@wait

A better example would be a procedure as shown below. In this case the vblank interrupt must be enabled, and your vblank interrupt procedure must set vblank_flag to a non-zero value.

  ld   hl,vblank_flag  ;hl=pointer to vblank_flag
  xor  a               ;a=0
 @@wait:               ;wait...
  halt                 ;suspend CPU - wait for ANY interrupt
  cp   a,(hl)          ;vblank flag still zero?
  jr   z,@@wait        ;wait more if zero
  ld   (hl),a          ;set vblank_flag back to zero

The vblank_flag is used to determine whether the HALT period has been terminated by a vblank interrupt, or by another interrupt. In case your program has all other interrupts disabled, then it would be okay to replace the above procedure by a single HALT instruction.

Another possibility is, if your game uses no other interrupt than VBlank (or uses no interrupt), to only enable VBlank interrupts and simply use a halt instruction, which will only resume main code execution when a VBlank occurs.

Remember when using HALT to wait between VBlanks, your interrupt routines MUST enable interrupts (ie with ei during the execution, or better, using the RETI instruction)

Using the STOP Instruction

The STOP instruction is intended to switch the gameboy into VERY low power standby mode. For example, a program may use this feature when it hasn't sensed keyboard input for a longer period (for example, when somebody forgot to turn off the gameboy).

Before invoking STOP, it might be required to disable Sound and Video manually (as well as IR-link port in CGB). Much like HALT, the STOP state is terminated by interrupt events. STOP is commonly terminated with a joypad interrupt.

During STOP mode, the display will turn white, so avoid using it in your game's main loop.

Disabling the Sound Controller

If your program doesn't use sound at all (or during some periods) then write 00h to register FF26 to save 16% or more on GB power consumption. Sound can be turned back on by writing 80h to the same register, all sound registers must be then re-initialized. When the gameboy is turned on, sound is enabled by default, and must be turned off manually when not used.

Not using CGB Double Speed Mode

Because CGB Double Speed mode consumes more power, it's recommended to use normal speed when possible. There's limited ability to switch between both speeds, for example, a game might use normal speed in the title screen, and double speed in the game, or vice versa. However, during speed switch, the display collapses for a short moment, so it's not a good idea to alter speeds within active game or title screen periods.

Using the Skills

Most of the above power saving methods will produce best results when using efficient and tight assembler code which requires as little CPU power as possible. Using a high level language will require more CPU power and these techniques will not have as big as an effect.

To optimize you code, it might be a good idea to look at this page, although it applies to the original Z80 CPU, so one must adapt the optimizations to the GBZ80.