Up to this point, the RISC-V tutorial has focused on single applications running on a single hardware thread. The application's environment was composed of the processor's state and memory map. The processor state was controlled via assembly instructions, and the memory map was defined at build time via a linker script. However, modern systems are almost always multiprogrammed and will execute many applications concurrently (by interleaving their instructions in a single stream). Moreover, multiple hardware threads are common, allowing application instructions to execute simultaneously. This workflow requires a lot more care to ensure correctness and proper separation of memory. This idea was touched upon briefly in chapter 4 while discussing the A extension which provides atomic memory operations that were used to define synchronization primitives. In addition to memory synchronization, the application must control the execution environment of all of the active hardware threads, this chapter will explore the mechanisms available for this purpose.