Reducing the memory address bus by adding banks

Question

If you are trying to control memory from a CPU, then if you use two banks instead of one, you can remove the first address line. If you use four banks, then you can also remove the second as well. Why is this?

Answer 1

I think W5VO's reply is a great description of a modern memory system but does not really describe Bank Switching.

You want bank switching when you have a narrow address bus (like 8- or 16-bit) and you want to add more memory. You can then use separate GPIO lines to switch memory chips just like W5VO wrote. You have to be careful because all your pointers are still 8-bit and the switching is manual so you can easily cause a mess.

This technique was/is popular in small micros, but also in 16-bit DOS (it was born on the 80286) and kind of in modern 32-bit processors to address more than 4GB of memory (but here the trick is done invisibly by the kernel and one program can still only see 4GB).

Answer 2

I'm going to assume that you're talking about parallel memory, and that when you say "use two banks instead of one" you are using two chips that have the same total memory, but each are half as large.

Here's the thing: you can't actually remove that address line from your CPU. It might not go directly to the memory chip, but if you remove it there is no way to distinguish between the two chips. The missing ingredient is the "chip select" function. Most memory chips will have a pin or set of pins that are used to basically tell that particular memory chip that the processor want to talk to only that chip.

Lets consider a simple example: 256 words of memory. This results in an 8-bit address bus.

• One chip with 256 words: then we are always using only one chip, and it has an 8-bit address.
• Two chips with 128 words each: Only one memory can be active at the same time, so we will be using the "chip select" pin. The MSB (Most Significant Bit) of the 8-bit address bus will go to the chip select pin on each chip, and the rest of the 7 bits will go to the 7-bit address bus on both chips.
• Four chips with 64 words each: Only one memory can be active at the same time, so we will be using the "chip select" pin. Some memory chips have multiple chip select pins, but for this example we will assume there is only one. In order to generate the chip select signals for all four chips, we will need a 2-to-4 decoder (also known as a $n\mbox{ }to\mbox{ } 2^{n}$ decoder, row decoder, address decoder - all the same things). This decoder takes the first two MSB from the address bus, and determines which chip select signal should be active. The lower 6 bits of the address go to the 6-bit address bus of all 4 memory chips.

This is a really simple example, but it scales fine. This is basically the procedure for making multiple memory chips work together to increase the address space.

Answer 3

If one has a half-egabyte of byte-addressable memory, there has to be a way of generating a 19-bit address. One nice easy approach is to have a CPU whose address bus has 19 or more address lines (e.g. an 8088). Another approach is to have some of the address lines generated by something other than the CPU's address bus. For example, suppose one has a CPU with a 16-bit address bus and one wants to construct a system with 32K of RAM and ~512KB of ROM. One could use a 13-input AND to detect an access to address 0xFFF0-0xFFFF (one input of the AND would be tied to an 'address valid' signal), and a 4-bit latch to grab the bottom 4 bits of such an address. Fifteen of the ROM address bits would be attached to the CPU's address bus; the other four bits would be connected to latch outputs. The net effect would be that the ROM would be divided into sixteen banks of 32KB each, all of which would be accessed from 0x8000-0xFFEF.

Note that such a system wouldn't quite have 512KB of usable ROM, since accessing any of the last 16 bytes of each 32K bank would switch to one particular 32K bank. Thus, 0xFFF0 would only be available on bank 0, 0xFFF1 from bank 1, etc. Further, in many cases, one would end up having to duplicate some code in different banks. Still, such a system would be quite practical and is in fact very close to what was done in many things like 1980's arcade games.