There are two ways in which the transition from 2MHz to 1MHz takes place depending on which phase lE was on when the request was received from IC41, see figure 2. Figure 2 2MHz to 1MHz transition 5.3 Reset circuitry The system has two reset circuits, one is a general reset from a 555 timer (IC43)., the other is an RC network which just resets the system VIA (IC20) on power-up. This allows the software to detect the difference between a power-on reset and a BREAK key reset. The keyboard BREAK key connects via S10 (a PCB made link) to the 555 timer. The 555 generates reset pulse RS which is inverted to give the CPU notRS signal. 5.4 Address decoding and memory Figure 3 shows the memory map. 13