Right then, after digging out my calculator and going back over everything, I have it again 🙂

I did miss out a step, here is the worked example

32 bit Address Bus – 64 bit Data Bus

• 2^{Address Bus} * Data Bus
• 2³² * 64 bits is equivalent to
• 2³² * 8 Bytes
• 8 * 2³² is equivalent to
• 8 * 2² * 2³⁰
• 8 * 4 * 2³⁰
• 32 * 2³⁰ Bytes

2³⁰ is a GigaByte so the answer is 32GB

Remember

• 2¹⁰ = KiloBytes
• 2²⁰ = MegaBytes
• 2³⁰ = GigaBytes
• 2⁴⁰ = TerraBytes
• 2⁵⁰ = PetaBytes

Here is a graphic from wikipedia, notice the IEC binary prefix, this is to try and remove the confusion when manufacturers refer to a hard drive as having 30GB capacity they often mean 30×10⁹ rather than 30×2³⁰ or 30,000,000,000 Bytes rather than 32,212,254,720 Bytes a difference of 2,212,254,720 Bytes (Aprox. 2GB)