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 = 128K paging enable (inverse of port 0x7ffd, bit 5) 
         Unlike the paging lock in port 0x7ffd, 
         this may be enabled or disabled at any time.
         Use "1" to disable the locked paging.
 bit 6 = "1" to disable RAM contention. (0 after a reset) 
 bit 5 = Stereo mode (0 = ABC, 1 = ACB)(0 after a PoR or Hard-reset)
 bit 4 = Enable internal speaker (1 = enabled)(1 after a PoR or Hard-reset)
 bit 3 = Enable Specdrum/Covox (1 = enabled)(0 after a PoR or Hard-reset)
 bit 2 = Make $FF Timex port readable (covers "floating bus") (0 after a PoR or Hard-reset)
 bit 1 = Enable TurboSound (1 = enabled)(0 after a PoR or Hard-reset)
 bit 0 = (core 3.0) Selects issue 2 keyboard (port $FE reads as early ZX boards)

https://www.specnext.com/tbblue-io-port-system/

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.

NEW in core 3.0:

 bit 4 default 0
 bit 0 set selects issue 2 keyboard