Machine Type Register
From SpecNext official Wiki
| 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).