Peripheral 3 Register

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Number $08
Readable Yes
Writable Yes
Short Description ABC/ACB Stereo, Internal Speaker, SpecDrum, Timex Video Modes, Turbo Sound Next, RAM contention and [un]lock 128k paging.

(R/W) 0x08 (08) => Peripheral 3 setting:

 bit 7 = Unlock(1)/lock(0) port $7FFD paging (read 1 indicates port $7FFD is not locked)
 bit 6 = 1 to disable RAM and I/O port contention (soft reset = 0)
 bit 5 = AY stereo mode (0 = ABC, 1 = ACB) (hard reset = 0)
 bit 4 = Enable internal speaker (hard reset = 1)
 bit 3 = Enable 8-bit DACs (A,B,C,D) (hard reset = 0)
 bit 2 = Enable port $FF Timex video mode *read* (hides floating bus on 0xff) (hard reset = 0)
 bit 1 = Enable Turbosound (currently selected AY is frozen when disabled) (hard reset = 0)
 bit 0 = Implement Issue 2 keyboard (port $FE reads as early ZX boards) (hard reset = 0)

The Timex modes are operational even when bit 2 is set to zero, you can still write to port $FF with desired mode changes. Bit 2 does only enable readability of port $FF.

When loading 48k SNA files through NextZXOS, be aware it does lock down many features of Next to better simulate ZX48 machine, including lock of Memory Paging Control ($7FFD / 32765) paging feature. If you are using 48k SNA file for convenience, but you are actually using full ZX Next features, make sure you enable/unlock all relevant bits, before using the particular feature.