What is Interleaved Memory?

It is a technique for compensating the relatively slow speed of DRAM(Dynamic RAM). In this technique, the main memory is divided into memory banks which can be accessed individually without any dependency on the other.

For example: If we have 4 memory banks(4-way Interleaved memory), with each containing 256 bytes, then, the Block Oriented scheme(no interleaving), will assign virtual address 0 to 255 to the first bank, 256 to 511 to the second bank. But in Interleaved memory, virtual address 0 will be with the first bank, 1 with the second memory bank, 2 with the third bank and 3 with the fourt, and then 4 with the first memory bank again.

Hence, CPU can access alternate sections immediately without waiting for memory to be cached. There are multiple memory banks which take turns for supply of data.

Memory interleaving is a technique for increasing memory speed. It is a process that makes the system more efficient, fast and reliable.

For example: In the above example of 4 memory banks, data with virtual address 0, 1, 2 and 3 can be accessed simultaneously as they reside in spearate memory banks, hence we do not have to wait for completion of a data fetch, to begin with the next.

An interleaved memory with n banks is said to be n-way interleaved. In an interleaved memory system, there are still two banks of DRAM but logically the system seems one bank of memory that is twice as large.

In the interleaved bank representation below with 2 memory banks, the first long word of bank 0 is floowed by that of bank 1, which is followed by the second long word of bank 0, which is followed by the second long word of bank 1 and so on.

The following figure shows the organization of two physical banks of n long words. All even long words of logical bank are located in physical bank 0 and all odd long words are located in physical bank 1.

Types of Interleaving

There are two methods for interleaving a memory:

2-Way Interleaved

Two memory blocks are accessed at same time for writing and reading operations.

4-Way Interleaved

Four memory blocks are accessed at the same time.