The Z80 also supports a non-maskable interrupt. The /NMI signal is on Pin 17 of the original 40 pin DIP package. Since a non-maskable interrupt will regain control of the CPU even after executing the instruction sequence DI / HALT, that pair does not represent a true HCF. It will only result in a HCF condition if either the /NMI pin is connected directly to the +5V rail, making the generation of that signal impossible, or if the interrupt routine that services /NMI ends with a return, placing it back in the HALT state.