Turbo Control Register

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Number $07
Readable Yes
Writable Yes
Short Description Sets accelerated clock speed, reads actual speed.


Bit 1-0 = turbo: %00 = 3.5MHz, %01 = 7MHz, %10 = 14MHz (%00 after PoR or Hard-reset).

When 14MHz turbo is used, and Layer 2 is enabled (bit 1 Layer 2 Access Port ($123B)), the 14MHz speed is used only when BORDER/VBLANK/HBLANK areas of screen image are generated, during PAPER (centre 256x192 pixel area of screen) the CPU speed is dropped down to 7MHz. With Layer 2 disabled, 14MHz CPU speed is stable.

Reads actual CPU speed.