Increaseing memory size and width
 

  • It is not necessary to construct very large memory circuits

  • There are 2 techniques that can be used to expand the "size" of a memory circuit:

      • One technique can increase the width (= # bits in a row of memory) of memory circuits

      • One technique can increase the size (= # rows in a memory circuit) of memory circuits

  • You also can combine the 2 techniques to increase both the width and the size of any memory circuit

Increasing the width of a memory circuit

Previously, we constructed a memory that has 4 bits in each row (i.e.: width=4 bits):

(Actually, I used 4 bits to keep the design simple)

Increasing the width of a memory circuit

Suppose, we constructed a memory that only has 1 bit in each row (i.e.: width=1 bit):

The input and output of the D-latch are both connected to the data bus wire - redraw it

Increasing the width of a memory circuit

There is actually only 1 connection to the data bus wire:

Increasing the width of a memory circuit

We can represent this as follows - it has N address lines and 1 data line:

This memory circuit has 2N rows and 1 bit in each row.
Is such memory circuit useful ???

Increasing the width of a memory circuit

Problem:   use 8 of these 2N×1 memory circuits

construct a 2N×8 memory circuits:

A 2N×8 memory circuit has 2N rows and 8 bits in each row.

Increasing the width of a memory circuit

We start with drawing out all the input and output signals:

 

Increasing the width of a memory circuit

We use 1 (one) 2N×1 memory circuit to store the data transfered over 1 data bus wire:

 

Increasing the width of a memory circuit

We use another 2N×1 memory circuit to store the data transfered over another 1 data bus wire:

 

Increasing the width of a memory circuit

And so on: connect the output of each 2N×1 memory circuit to a different data bus wire:

The resulting circuit is a 2N×8 memory circuit !

Increasing the width of a memory circuit - example application
 

  • Memory chip manufacturers can easily make 1,2,4 G×1 bit memory chips

  • They would use 32 of such 1,2,4 G×1 bit to make 4,8,16 Gbyte DDR memory circuits :

                

Increasing the size of a memory circuit

Suppose you have 4 (four) 2N×8 bits memory circuits:

Construct a (large size) 2N+2×8 bits memory circuits:

Increasing the size of a memory circuit

For simplicity (of the presentation), I will use a concrete example with 10 address pins (wires):

  • Suppose we have 4 (four) 210×8 bits memory circuits:
                

  • Construct a (large size) 212×8 bits memory circuits:
                              

Increasing the size of a memory circuit - background info
 

  • The 212×8 bits memory circuit has 12 address pins

  • The 12 bits binary numbers are listed below in 4 groups: 

       000000000000  010000000000  100000000000  110000000000 
 000000000001  010000000001  100000000001  110000000001 
 000000000010  010000000010  100000000010  110000000010 
 000000000011  010000000011  100000000011  110000000011 
 ...
 ...
 001111111111  011111111111  101111111111  111111111111

  • Each one of a 12 bits binary number can be used to select a (8 bits) memory cell in a 212×8 memory circuit

Increasing the size of a memory circuit - background info

How to construct a 212×8 memory circuit using 4 (four) 210×8 memory circuit:

  • We use the first 2 bits of the (12 bits) address to select (activate) one of the 4 (four) 210×8 memory circuits 

       000000000000  010000000000  100000000000  110000000000 
 000000000001  010000000001  100000000001  110000000001 
 000000000010  010000000010  100000000010  110000000010 
 000000000011  010000000011  100000000011  110000000011 
 ...
 ...
 001111111111  011111111111  101111111111  111111111111
  |             |             |             |
  V             V             V             V
 Activate #1   Activate #2   Activate #3   Activate #4

Increasing the size of a memory circuit - background info

How to construct a 212×8 memory circuit using 4 (four) 210×8 memory circuit:

  • We use the last 10 bits of the (12 bits) address to select row of memory cells inside the actiavte 210×8 memory circuit 

       000000000000  010000000000  100000000000  110000000000 
 000000000001  010000000001  100000000001  110000000001 
 000000000010  010000000010  100000000010  110000000010 
 000000000011  010000000011  100000000011  110000000011 
 ...
 ...
 001111111111  011111111111  101111111111  111111111111
      |             |             |             |
      V             V             V             V
      Select row (of memory cells) when  activated

Increasing the size of a memory circuit

We start with the input and the output signals:

Increasing the size of a memory circuit

Use a decoder to translate the first 2 bits of the address into a selection signal:

Increasing the size of a memory circuit

Activate one of the 4 (four) 210×8 memory circuit when first 2 addr bits = 00:

Increasing the size of a memory circuit

Activate another of the 4 (four) 210×8 memory circuit when first 2 addr bits = 01:

Increasing the size of a memory circuit

Activate another of the 4 (four) 210×8 memory circuit when first 2 addr bits = 10:

Increasing the size of a memory circuit

Activate another of the 4 (four) 210×8 memory circuit when first 2 addr bits = 11:

Increasing the size of a memory circuit

This is how to increase the size (= more rows) of a memory circuit: