Is this address decoding circuit correct?

Question

The problem is to design an address decoding circuit for two 4Kx8 RAM chips at the 2050H . We have 16 address lines. So, for 4Kx8 RAM we need 12 address lines to address the memory. Remaining can be used for deriving CS' (chip select) signal.

Here, For RAM 1 we have

Starting address 2050H : 0010000001010000

Ending address 304FH : 0011000001001111

For RAM 2 we have

Starting Address 3050H: 0011000001010000

Ending Address 404FH: 0100000001001111

Bold = A12

So, if we use a 1x2 decoder as in the picture, and A12 is the input. The chips will be selected depending on the state of A12. It will work fine until ending address is used. What happens when someone tries to access the ending address?

Answer 1

Your circuit will not work as intended. This is not a simple, or typical, memory decoding problem since the starting and ending addresses are not integral multiples of the RAM capacities - 4K (4,096 decimal) or 1000 Hex.

You must account for the 50 hex offset in the supplied addresses so that the bottom supplied address (2050h or 3050h) aligns with address 0000 in the RAM(s). This will require some sort of adder logic.

If the RAMs were larger capacity, you could likely get away with this type of simple decoding because the required space of X050 to Y04F would all fit within the larger RAM space. Must there be two separate RAMs in the solution? Must the write and read action of the RAMs be strictly combined to the 2050h to 404Fh area of the RAM? In other words are there other devices in the adjacent memory space (e.g. at 204Fh and 4050h) that would collide with RAM operations made beyond the specified source address space? These are all considerations that would perhaps make the design of the decoder.

Answer 2

According to the schematic the listed starting and ending addresses are not correct. In fact the last address of the 4 listed is not mapped to either of the memory chips.

According to the schematic, one of the starting address might be 2000H and the ending might be 2FFFH. And the starting address of the other chip might be 3000H and the ending might be 3FFFF. The answer has to be somewhat ambiguous as there are no chip part numbers.