Function-as-a-Service (FaaS) has emerged as a popular cloud paradigm that simplifies software development and deployment by providing scalable and event-driven function execution without the burden of managing servers. FaaS was originally created with a function as a function semantics to enable standalone microservices with adequately short execution time to meet microsecond-scale service-level objectives (SLOs). Unfortunately, today's FaaS systems fundamentally suffer from millisecond-level performance bottlenecks that arise from isolating functions in separate address spaces inside containers or microVMs. Prior work has focused on optimizing FaaS performance, but these systems still fall short of meeting microsecond-level SLOs. In this paper, we present Jord, a FaaS system that revives the original function-as-a-function vision of FaaS. Jord leverages hardware/software co-design to colocate functions in a single address space with user-level in-process memory isolation, extending the capability of traditional virtual memory. By performing memory isolation and management in nanoseconds, Jord enables zero-copy cross-function communication and scalable function dispatch, thereby minimizing FaaS overheads. We demonstrate that Jord can meet microsecond-level SLOs for microservice workloads while performing within 16% of an idealized but insecure baseline and delivering over 2× higher throughput compared to enhanced state-of-the-art systems.