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| ROM 0 | ROM 1 | Either ROM may be switched in. | Bank 2 | Any one of these pages may be switched in. ROM 0 is the 128k editor and menu system ROM 1 contains 48K BASIC.
Bit 5: If set, memory paging will be disabled and further output to this port will be ignored until the computer is reset.Ġxffff +-+-+-+-+-+-+-+-+

Note that this does not affect the memory between 0x4000 and 0x7fff, which is always bank 5.
Bit 4: ROM select. The normal screen is in bank 5, whilst the shadow screen is in bank 7. The byte output will be interpreted as follows:
Bits 0-2: RAM page (0-7) to map into memory at 0xc000.
Bit 3: Select normal (0) or shadow (1) screen to be displayed. Reading from 0x7ffd produces no special results: floating bus values will be returned as would be returned from any other port not attached to any hardware. If normal interrupt code is to run, then the system variable at 0x5b5c (23388) must be kept updated with the last value sent to port 0x7ffd. When memory is being paged, interrupts should be disabled and the stack should be in an area which is not going to change. However, 0x7ffd should be used if at all possible to avoid conflicts with other hardware. As normal on Sinclair hardware, the port address is in fact only partially decoded and the hardware will respond to any port address with bits 1 and 15 reset. The additional memory features of the 128Ke are controlled to by writes to port 0x7ffd. The ULA bug which causes snow when I is set to point to contended memory still occurs, and also appears to crash the machine shortly after I is set to point to contended memory. Note that this means that there are 70908 T states per frame, and the '50 Hz' interrupt occurs at 50.01 Hz, as compared with 50.08 Hz on the 48K machine. As with the 48K machine, on some machines all timings (including contended memory timings) are one T state later. To modify the border at the position of the first byte of the screen, the OUT must finish after 14365, 14366, 14367 or 14368 T states have passed since interrupt. There are 63 scanlines before the television picture, as opposed to 64.There are 311 scanlines per frame, as opposed to 312.There are 228 T-states per scanline, as opposed to 224.The main processor runs at 3.54690 MHz, as opposed to 3.50000 MHz.

The 128Ke is similar to the 48K machine, but with extra memory accessed by paging it into the top 16K of RAM. This reference is based on the Spectrum 128 reference from the FAQ. It can be built in hardware by modifying a Spectrum +2A. The 128Ke is an 'ideal' Spectrum 128 that eliminates these bugs and provides maximum compatibility with existing 48K and 128K software. Each of the official Spectrum 128 models (MkI, MkII, +2, +2A, +3) had hardware bugs and compatibility issues.