Copyright Goodheart-Willcox Co., Inc. 42 Computer Service and Repair Beginning with Windows 95, all newer versions of Windows OS, including Windows 10 and Windows Server, use preemptive multitasking. Most other modern operating systems, including OS/2, macOS (previously named Mac OS X), Unix, and Linux also use a preemptive kernel. Cooperative Multitasking Cooperative multitasking allows one program to dominate the CPU, but will allow another program to run while it is idle. Th is is a legacy multitasking classifi cation. It was common to early macOS up to version nine. It was also used in DOS and Windows pre-Win98. Although it is rarely used in commercial operating systems to- day, it can still often be found in embedded systems with memory constraints used for specifi c applications. Multithreading You may have heard the term thread or threads used when referring to computer pro- cessing. Th ese terms refer to passing small portions of a computer program through the core of the processor. Multithreading refers to the sharing of data and parts of a program between two or more CPUs or between the two cores of a single CPU. Note the diff erence between multithreading and multitasking, which is the ability of an operating system to support two or more programs running at the same time. 32-Bit and 64-Bit Operating systems, hardware devices, and software applications are identifi ed using the terms 32-bit and 64-bit. Th ese terms are used to describe the amount of data that can be stored or processed. In other words, 32-bit and 64-bit refer to how much data can be processed at once by the CPU or how much total memory the operating sys- tem can access. All modern PCs run 64-bit because it is faster and more effi cient as well as backward compatible with 32-bit programs. Th e only reason to run a 32-bit OS is on legacy PC hardware with less than 2 GB of RAM. Th e maximum amount of memory a 32-bit Windows operating system can ac- cess is 4 GB. Even if the computer contains more than 4 GB of RAM, only 4 GB will be accessible by the 32-bit operating system, and the amount of memory dedicated for video will further reduce the actual usable RAM to the 2.6–3.5 GB range. A 64-bit Windows desktop operating system recognizes a maximum from 8 GB to 512 GB of memory depending on the version and operating system edition. When upgrading an operating system from one edition to another without a full reinstall, you are usually required to stay at the same bit level. For example, you can only directly upgrade a 32-bit operating system to another 32-bit operating system or a 64-bit operating system to another 64-bit operating system. Upgrading directly from a 32-bit operating system to a 64-bit operating system will delete all data fi les (documents, photos, music, etc.) stored on the computer. You need to back up all fi les, then perform the upgrade from 32-bit to 64-bit, and then restore the software applications and user data fi les. Th is is the recommended approach unless the sys- tem has less than 2 GB of RAM. Since all 64-bit versions of Windows come with DOS support via NT DOS Virtual Machine (NTDVM), there is no reason to run 32-bit OS solely for the sake of software compatibility. User Interface Th ere are two dominant user interfaces used to issue commands on a computer sys- tem: graphical user interface and command-line interface. Graphical user interface 1002: 1.1, 1.3 1002: 1.2