Understanding RAM64

[Technik]

I figured out RAM64 after a few minutes, but I'm not sure I totally "get" what's going on.

I understand that some of the address pins are going to the mux/dmux, while the others are going to the RAM chips. And I understand that one RAM chip is handling 0-7, while another one might handle 56-63. I'm just having a hard time understanding the big picture/overall "hierarchy" of the higher vs lower address pins, I suppose.

Could anybody help clarify this?

[cadet1620]

The way you did the RAM64 is the most common: using the higher address bits to control which sub-block is activated and passing the lower address bit to the sub-block.

This same hierarchy is used in general for most addressing schemes. When you build your Hack computer you will make the Memory component that takes a 15-bit address and routes data appropriately to/from the RAM, Screen and Keyboard based on the memory address. The highest order bits in the address select between the three components, and the lower order bits are used by the component parts as needed. (The RAM and Screen are not the same size, and the Keyboard only uses one address.)

--Mark

[ybakos]

Technik, the thing that helped me most was drawing the smaller RAM arrays on paper. The larger arrays follow the same pattern of hierarchy and relationship to address bits, so once you grok the design of the first few RAM chips, I think you'll see it.

A drawing I use to illustrate this for students looks like this:



Do you see how the bits of the overall address serve as components that specify how to "navigate" to a particular block in the diagram?

[Technik]

Yeah, it's starting to make a bit more sense. Thanks guys! It helped more after completing RAM512 too.

ybakos, your picture reminds me of the Fibonacci spiral in some ways: