Explain 2D and 2 1/2 D memory organization. Write their advantages and disadvantages.

2D organization

 Address (n bits) --> [ n to 2^n decoder ] --> word lines
                                                 |
                                       [ cell array: 2^n words ]
                                                 |
                                             data bits

Advantages: the structure is simple, the design is easy, and a whole word is accessed at once, so it is fast for small memories. Disadvantages: the decoder needs $2^n$ output lines. For $64\text{K}$ words, that is 65536 lines and gates. The decoder is large, costly and slow, so it is unsuitable for big memories.

2 1/2D organization

In 2 1/2D organization the cells are arranged in a square-like array. The address is split into a row part and a column part. A row decoder selects one row and a column decoder (with a multiplexer) selects one column. The cell at the crossing is accessed.

 Row address (n/2)  --> [ row decoder ] --> rows
                                              |
                                        [ cell array ]
                                              |
 Col address (n/2)  --> [ column decoder / mux ] --> data

For $64\text{K} \times 1$ bits, the array is $256 \times 256$. Each decoder has 256 outputs, so the total is

$$256 + 256 = 512$$

instead of 65536 in 2D.

Advantages: the decoder size is very small, so the cost and area are low. It suits large chips, and refresh of a whole row is easy for DRAM. Disadvantages: two decoding steps are needed, so the access control is a little more complex. One cell is selected at a time per array, so several arrays must be used in parallel to get a wider word.