Push button to act as toggle switch?
The switch needs to be debounced, which adds components to the circuit. And that's affected by your switch, whether it's SPST or SPDT.

You don't state the supply voltages or what kind of output your need from the switch. But as you mention a PROM, I'll assume you're looking for logic levels.

If you have a SPDT switch, it's somewhat easier...







simulate this circuit &ndash; Schematic created using CircuitLab

U1a debounces the switch and U1b provides your toggle function. R3/C1 deliver a reset pulse to U1b so its output is logic low after power-up. D1 discharges C1 into the Vdd rail when the power's switched off. (U1b's CLR pin has a similar input diode but not as strong as D1.)

If you have to use a SPDT switch, you need a different debounce circuit...







simulate this circuit

Here, monostable U2 is triggered by your switch being pressed and by all of its contact bounces. The re-triggered monostable doesn't expire for 11 ms, well after your switch contacts have stopped bouncing on closure. So the 555 outputs a single pulse high of slightly varying pulse width. This clocks U1a and causes it to invert, as before.

If you use the SPST circuit, you can use 556 dual timers and get two switch circuits for the two ICs needed.