Real-time systems must simultaneously deliver predictable timing, fault isolation, and memory safety, yet current operating systems expose only low-level primitives that force developers to manually balance concurrency, isolation, and performance. This paper presents LARTS, a language-aided runtime system that elevates these requirements into language abstractions with enforceable semantics. LARTS introduces execution domain, a unified process–thread abstraction that combines thread-level responsiveness with process-level isolation. Memory is managed through deterministic memory contracts, which bind allocation at load time to eliminate runtime failures and unpredictable latencies. Domain interactions are expressed via deterministic communication channels that integrate efficient transfer, type safety, and priority inheritance, ensuring analyzable end-to-end bounds. Moreover, LARTS enforces secure-by-construction semantics, making classes of bugs such as double fetch and use-after-free unrepresentable in the programming model. We formalize the core semantics of LARTS and show how they guarantee determinism and safety by design. A prototype built on RTEMS demonstrates that LARTS preserves competitive real-time performance while substantially reducing programming complexity and eliminating vulnerabilities in realistic case studies. Our results suggest that high-assurance real-time programming can be treated not as an ad-hoc engineering problem, but as a first-class abstraction with verifiable semantics.