jump on reset - for the Z80 CPU card

for the Z80 CPU card The Z80 program commences to run at reset on address 0000. It is, however, desirable for many applica- tions to access a RAM range (for in- stance, a RAM card) at this address. The present circuit ensures that the bus driver remains at high impedance until the CPU reaches, for instance, the required start address of the monitor program through execution of the hard-wire NOP instruction. As this instruction has an execution time of 1us (with a clock of 4 MHz), the bus driver cannot remain in that state for more than 0.06 s. The CPU is taken from its socket and IC5 removed from the Z80 card. A 40-way wire-wrap socket together with the remainder of the additional circuit is then mounted onto a small wiring board. Pin 26 of the socket is bent because it carries the BUSACK signal of the CPU. The data and ad- dress terminals on the bus, as well as RESET, MREQ, +5 V, and earth are taken directly from the wire-wrap socket. The completed additional cir- cuit is plugged...