TACL (Tandem Advanced Command Language) is the scripting programming language which acts as the shell in Tandem Computers. History Tandem computers were originally designed and sold by Tandem Computers, Inc., based in Cupertino, CA. These were the first commercially available parallel processing computers. Despite their ability to expand to large sizes using parallelism, later to be called clustering, they were in the category of "mini-computers". Tandem's strategy was the emerging concept of "continuous availability" for applications requiring near-total uptime, in which single points of failure were minimized through mirrored storage devices, controllers and software, and operating systems engineered specifically for fault tolerance (using the Guardian operating system, later NonStop Kernel or NSK, and later NonStop OS). In computing, a single point of failure refers to any scenario in which the failure of a single component, application or process could result in broader impacts such as data loss or the cascading failure of the system. The relative likelihood of multiple component or process failures within a short time is considered much lower than the likelihood a single failure. Tandem Computers, Inc. was acquired by Compaq Computer Corporation in 1997; and Compaq Computer Corp. was subsequently purchased by Hewlett-Packard in 2001. Today they are known as "HPE Nonstop", with products such as the HP Integrity NonStop Blade systems. In 2022, HPE NonStop platforms underpin many of the largest banks, casinos, retailers, telephone companies, email systems, and stock exchanges worldwide. TACL continues to be the scripting language used on NonStop Servers. Design philosophy Many high-uptime computer systems that were produced after the Tandem NonStop platform rely on a pair of concurrently running, parallel but independent processes. A "failover" scheme ensures that the secondary process (or hot backup) continues operating in the event of a failure in the primary process, thereby ensuring that any failure in the primary process does not result in a total failure of the function, network or system it supports. The Tandem NonStop design philosophy differs as each CPU performs its own work and may contain a dormant "backup" process in another CPU. Each pair of CPUs share hardware ownership of controllers and disk drives. Writes to the mirrored disk pair are performed on both the primary and backup drives. However, the drives are not truly redundant as reads will only be performed by the drive that can most quickly access the data, not both. If one CPU in the