Interleaved memory

In computing, interleaved memory is a design made to compensate for the relatively slow speed of dynamic random-access memory (DRAM) or core memory, by spreading memory addresses evenly across memory banks. That way, contiguous memory reads and writes are using each memory bank in turn, resulting in higher memory throughputs due to reduced waiting for memory banks to become ready for desired operations.

It is different from multi-channel memory architectures, primarily as interleaved memory is not adding more channels between the main memory and the memory controller. However, channel interleaving is also possible, for example in freescale i.MX6 processors, which allow interleaving to be done between two channels.

Overview

With interleaved memory, memory addresses are allocated to each memory bank in turn. For example, in an interleaved system with two memory banks (assuming word-addressable memory), if logical address 32 belongs to bank 0, then logical address 33 would belong to bank 1, logical address 34 would belong to bank 0, and so on. An interleaved memory is said to be n-way interleaved when there are n banks and memory location i resides in bank i mod n.

Memory interleaving example with 4 banks. Red banks are refreshing and can't be used.

Interleaved memory results in contiguous reads (which are common both in multimedia and execution of programs) and contiguous writes (which are used frequently when filling storage or communication buffers) actually using each memory bank in turn, instead of using the same one repeatedly. This results in significantly higher memory throughput as each bank has a minimum waiting time between reads and writes.

Advantages

Main memory (random-access memory, RAM) is usually composed of a collection of DRAM memory chips, where a number of chips can be grouped together to form a memory bank. It is then possible, with a memory controller that supports interleaving, to lay out these memory banks so that the memory banks will be interleaved.

In traditional (flat) layouts, memory banks can be allocated a continuous block of memory addresses, which is very simple for the memory controller and gives equal performance in completely random access scenarios, when compared to performance levels achieved through interleaving. However, in reality memory reads are rarely random due to locality of reference, and optimizing for close together access gives far better performance in interleaved layouts.

Note that the way memory is addressed has no effect on the access time for memory locations which are already cached, having an impact only on memory locations which need to be retrieved from DRAM.

History

Early research into interleaved memory was performed at IBM in the 60s and 70s in relation to the IBM 7030 Stretch computer,^[1] but development went on for decades improving design, flexibility and performance to produce modern implementations.

See also

• Multi-channel memory architecture
• DIMM (dual in-line memory module)

References

External links

• Dale Adams on Interleaved Memory on Centris 650 & Quadra 800
• Memory Systems and Pipelined Processors by Harvey G Cragon