The Slogger and Elektuur Turbo boards were born out of a hack initially devised at Acorn. By shadowing the lowest 8 KB of RAM with a static RAM chip outside of reach of the ULA, the CPU could always access it at 2 MHz. The tradeoff was that the screen could not be located in that 8 KB. In practice the operating system ROMs always put the screen into the top 20 KB and as a result this probably only broke compatibility with around 2% of software. Speeding up the low portion of memory is particularly useful on 6502 derived machines because that processor has a faster addressing mode for the first 256 bytes and so it is common for software to put any variables involved in time critical sections of program into that region.