There is an improvement that I would like to make to the reset circuitry. When the computer is switched on, it is necessary to press the reset button to ensure that the microprocessor is in a known state.
As mentioned previously, I envisaged the combination of a resistor and capacitor, coupled with the 6502’s internal oscillator as a temporary clock circuit. As a permanent solution, I wish to use an oscillator.
An oscillator contains everything required to create a stable and reliable signal. This is not the same as a crystal, which requires extra components to be externally connected to it.
Address decoder logic
I have used more than one type of logic gate to build this computer. It is possible to change this, so that it uses just one logic chip. This is achievable by using just NOR or NAND gates! NOR and NAND gates are special, because you can create any other gate from it! For this reason, they are known as universal gates.
This will mean that less space is taken up and a reduction in the current that is drawn.
The LCD can actually be used in two different modes, 4-bit and 8-bit. Currently, it is in 8-bit, which requires the use of 8 data pins. If the LCD is operated in 4-bit mode, it requires 4 data pins. This means 4 less wires but more importantly, I can free up 4 of the port pins. I could then keep PORT B for the LCD and not need any PORT A pins for it!