Data Alignment on Embedded CPUs for Programmable Control Devices

Abstract

This work aims to investigate the impact of memory access limitations in microcontrollers and microprocessors on the performance of software that deals with binary data. The research area covers control systems that process data from the IEC 61131-3 standard using a software-implemented virtual machine. Three methods of memory access are considered, namely byte access, memory copying, and direct pointer. Tests of these methods are performed on several CPUs with ARM architecture (with variants), MIPS, RISC-V, Quark, and others, often used as hardware platforms for control devices. The tests cover 1-, 2-, 4-, and 8-byte data sizes, which correspond to the integer types of the IEC 61131-3 standard. By analyzing the results covering both unaligned and aligned data, the goal of this paper is to indicate which of the memory access methods is the most efficient for a particular platform. The research is supplemented with an evaluation of power and memory requirements for a group of STM32 microcontrollers. Therefore, the contribution of this paper rests in indicating the most efficient memory access method for each of more than a dozen CPUs intended for control applications, with consideration of power and memory requirements.