A Managed Memory System for Micro Controllers with NOR Flash Memory

Abstract

This paper presents a managed memory system for micro controllers with only a small amount of memory but with NOR flash memory. This system is targeted at a device such as Raspberry Pi Pico, which is equipped with ARM Coretex M0+, on-chip 264KB SRAM, and 2MB flash memory. To extend an available memory space for user programs, this system provides virtual memory by using NOR flash memory as a backing store. Since writing data to the flash memory is slow and the number of writes is limited during its lifetime, this system cooperates language-level memory management to mitigate these drawbacks. It runs a garbage collector that may move objects aggressively when a memory page is paged-out to flash memory. This paper also proposes a technique named a forwarding bit to efficiently implement the movement of objects stored in flash memory. According to experiments using a prototype of this system implemented for the mruby language on Raspberry Pi Pico, when the available SRAM size is small, this system successfully reduces the number of erasures in flash memory to an average of less than 10% and even improves execution speed by an average of 4 times faster despite overhead of moving objects.