Machine Type Register

Number	TBRegisterNumber::$03
Readable	TBRegisterReadable::Yes
Writable	TBRegisterWritable::Yes
Short Description	ShortDesc::Identifies timing and machine type.

A write to this register disables the bootrom in config mode

A write with bit 7 set will be accepted in any mode to change only display timing (bits 6-4).

Bit	Function
7	(W) 1 to allow changes to bits 6:4
7	(R) Next write to {{#ask: TBRegisterNumber::$44 }} ($44) will affect colour byte: 0 = RRRGGGBB, 1 = p000000B
6-4	Display timing (affects I/O port decoding (since core 3.1.1)): %000: internal usage %001: ZX 48k %010: ZX 128k/+2 (Grey) %011: ZX +2A-B/+3e/Next Native %100: Pentagon.
3	1 to toggle user lock on display timing (hard reset = 0) (since core 3.1.0)
2-0	Machine type (determines roms loaded and multiface type (since core 3.1.1)): %000: Config mode %001: ZX 48k %010: ZX 128k/+2 (Grey) %011: ZX +2A-B/+3e/Next Native %100: Pentagon. Writeable only in config mode.

Core 3.1.1 change: port-decoding now depends on the selected display-mode, not machine-type. So while machine is still in Next type, by changing to 128 display the port decoding does change too (for example $7FFD port is decoded differently on ZX128 vs ZX128+3 machines).