Machine Type Register

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Number $03
Readable Yes
Writable Yes
Short Description 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 Enhanced ULA Palette Extension ($44) will affect colour byte: 0 = RRRGGGBB, 1 = p000000B
6-4 Display timing. %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:

%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, may affect port decoding and enabling of some hardware.

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).