Architectures and Processors

ARM Cortex-M4 Cache Coherency Problems During DMA Transfers The Neoverse N1 microarchitecture, a high-performance ARM core designed for server and infrastructure workloads, exhibits unexpected pipeline behavior when executing specific arithmetic instructions with large shift values. Specifically, the adds instruction with a logical shift left (LSL) greater than 4, such as adds x3, x4, x5, lsl…

ARM Cortex-R5F MPU and Cache Interaction Leading to Stack Corruption The ARM Cortex-R5F processor integrates a Memory Protection Unit (MPU) and cache subsystems that are critical for ensuring memory safety and performance in real-time embedded systems. However, enabling the MPU can lead to unexpected behavior, such as stack corruption, particularly when caches are enabled. This…

Cortex-R5 MicroSCU Role in Multi-Core Coherency The Cortex-R5 processor, a member of ARM’s real-time processor family, is widely used in embedded systems requiring deterministic performance and high reliability. One of the key architectural features of the Cortex-R5 is its optional MicroSCU (Micro Snoop Control Unit), which plays a critical role in maintaining cache coherency in…

Cortex-M7 ALU Overflow Detection Challenges The ARM Cortex-M7 processor, known for its high performance and efficiency, is widely used in embedded systems requiring real-time processing capabilities. One of the critical aspects of ensuring reliable operation in such systems is the detection and handling of arithmetic logic unit (ALU) overflow. Overflow occurs when the result of…

ARM Assembly Coding Formats: Legacy AREA vs. GNU .global Directives When working with ARM assembly, developers often encounter two distinct coding formats: the legacy AREA directive used in the ARM proprietary assembler (armasm) and the .global directive commonly found in GNU assembler (as) or ARM’s armclang integrated assembler. These formats are not interchangeable and are…

ARM TLB Functionality and Its Role in Physical Address Space Management The Translation Lookaside Buffer (TLB) is a critical component in ARM architectures, primarily designed to accelerate virtual-to-physical address translation. However, in systems where virtual memory is not utilized, the necessity of the TLB comes into question. When the entire memory map fits within the…

ARM Cortex-M23 TrustZone Branch Instruction Behavior in Non-Secure State The ARM Cortex-M23 processor, which implements the ARMv8-M architecture, introduces TrustZone security extensions to enable secure and non-secure state separation. A critical aspect of this architecture is the handling of branch instructions, particularly when transitioning between secure and non-secure states. One specific issue arises when executing…

ARM-V8 PCIe Peer-to-Peer DMA Performance Drop Due to IOMMU_MMIO Attribute The core issue revolves around a significant performance degradation observed during PCIe peer-to-peer transactions between two GPU cards on an ARM-V8 server when the IOMMU is enabled. The throughput drops from an expected 28GB/s to a mere 4GB/s. This degradation is traced back to the…

ARMv9 RME Cache Coherency Problems During GPC-Protected Memory Access The ARMv9 architecture introduces Realm Management Extensions (RME), which include Granule Protection Checks (GPC) to enforce memory access permissions at a granular level. The GPC mechanism is designed to ensure that memory accesses are validated against the Granule Protection Table (GPT) before proceeding. However, a critical…

ARMv7-M Exception Handling and Stack Switching During Late Arriving Interrupts The ARMv7-M architecture, which includes popular cores like the Cortex-M3, Cortex-M4, and Cortex-M7, employs a sophisticated exception handling mechanism designed to ensure deterministic and efficient interrupt servicing. One of the key features of this architecture is the dual-stack mechanism, which utilizes the Main Stack Pointer…