Virtual memory in the context of SDS 940

An instruction set architecture (ISA) is an abstract model that defines the programmable interface of the CPU of a computer; how software can control a computer. A device (i.e. CPU) that interprets instructions described by an ISA is an implementation of that ISA. Generally, the same ISA is used for a family of related CPU devices.

In general, an ISA defines the instructions, data types, registers, and the programming interface for managing main memory such as addressing modes, virtual memory, and memory consistency mechanisms. The ISA also includes the input/output model of the programmable interface.

Computer memory stores information, such as data and programs, for immediate use in the computer. The term memory is often synonymous with the terms RAM, main memory, or primary storage. Archaic synonyms for main memory include core (for magnetic core memory) and store.

Main memory operates at a high speed compared to mass storage which is slower but less expensive per bit and higher in capacity. Besides storing opened programs and data being actively processed, computer memory serves as a mass storage cache and write buffer to improve both reading and writing performance. Operating systems typically borrow RAM capacity for caching so long as it is not needed by running software. If needed, contents of the computer memory can be transferred to storage; a common way of doing this is through a memory management technique called virtual memory.

The THE multiprogramming system (THE OS) was a computer operating system designed by a team led by Edsger W. Dijkstra, described in monographs in 1965-66 and published in 1968.Dijkstra never named the system; "THE" is simply the abbreviation of "Technische Hogeschool Eindhoven", then the name (in Dutch) of the Eindhoven University of Technology of the Netherlands. The THE system was primarily a batch system that supported multitasking; it was not designed as a multi-user operating system. It was much like the SDS 940, but "the set of processes in the THE system was static".

The THE system apparently introduced the first forms of software-based paged virtual memory (the Electrologica X8 did not support hardware-based memory management), freeing programs from being forced to use physical locations on the drum memory. It did this by using a modified ALGOL compiler (the only programming language supported by Dijkstra's system) to "automatically generate calls to system routines, which made sure the requested information was in memory, swapping if necessary". Paged virtual memory was also used for buffering input/output (I/O) device data, and for a significant portion of the operating system code, and nearly all the ALGOL 60 compiler. In this system, semaphores were used as a programming construct for the first time.

Memory management (also dynamic memory management, dynamic storage allocation, or dynamic memory allocation) is a form of resource management applied to computer memory. The essential requirement of memory management is to provide ways to dynamically allocate portions of memory to programs at their request, and free it for reuse when no longer needed. This is critical to any advanced computer system where more than a single process might be underway (multitasking) at any time.

Several methods have been devised that increase the effectiveness of memory management. Virtual memory systems separate the memory addresses used by a process from actual physical addresses, allowing separation of processes and increasing the size of the virtual address space beyond the available amount of RAM using paging or swapping to secondary storage. The quality of the virtual memory manager can have an extensive effect on overall system performance. The system allows a computer to appear as if it may have more memory available than physically present, thereby allowing multiple processes to share it.

In software development, a breakpoint is an intentional stopping or pausing place in a program, put in place for debugging purposes. It is also sometimes simply referred to as a pause.

More generally, a breakpoint is a means of acquiring knowledge about a program during its execution. During the interruption, the programmer inspects the test environment (general-purpose registers, memory, logs, files, etc.) to find out whether the program is functioning as expected. In practice, a breakpoint consists of one or more conditions that determine when a program's execution should be interrupted.

The Berkeley Software Distribution (BSD), also known as Berkeley Unix, is a discontinued Unix operating system developed and distributed by the Computer Systems Research Group (CSRG) at the University of California, Berkeley. First released in 1978, it began as an improved derivative of AT&T's original Unix developed at Bell Labs, based on the source code. Over time, BSD evolved into a distinct operating system and played a significant role in computing and the development and dissemination of Unix-like systems.

BSD development was initially led by Bill Joy, who added virtual memory capability to Unix running on a VAX-11 computer. During the 1980s, BSD gained widespread adoption by workstation vendors in the form of proprietary Unix distributions—such as DEC with Ultrix and Sun Microsystems with SunOS—due to its permissive licensing and familiarity among engineers. BSD also became the most widely used Unix variant in academic institutions, where it was used for the study of operating systems. The BSD project received funding from DARPA until 1988, during which time BSD incorporated ARPANET support and later implemented the TCP/IP protocol suite, released as part of BSD NET/1 in 1988. By that time, the codebase had diverged significantly from the original AT&T Unix, with estimates suggesting that less than 5% of the code remained from AT&T. As a result, NET/1 was distributed without requiring an AT&T source license.

In computing, an address space defines a range of discrete addresses, each of which may correspond to a network host, peripheral device, disk sector, a memory cell or other logical or physical entity.

For software programs to save and retrieve stored data, each datum must have an address where it can be located. The number of address spaces available depends on the underlying address structure, which is usually limited by the computer architecture being used. Often an address space in a system with virtual memory corresponds to a highest level translation table, e.g., a segment table in IBM System/370.

In computer programming, housekeeping can refer to either a standard entry or exit routine appended to a user-written block of code (such as a subroutine or function, sometimes as a function prologue and epilogue) at its entry and exit or to any other automated or manual software process whereby a computer is cleaned up after usage (e.g. freeing resources such as virtual memory). This might include such activities as removing or archiving logs that the system has made as a result of the users activities, or deletion of temporary files which may otherwise simply take up space. Housekeeping can be described as a necessary chore, required to perform a particular computer's normal activity but not necessarily part of the algorithm. For cleaning up computer disk storage, utility software usually exists for this purpose such as data compression software - to "shrink" files and release disk space and defragmentation programs - to improve disk performance.

In computing, a loader is the part of an operating system that is responsible for loading programs and libraries. It is one of the essential stages in the process of starting a program, as it places programs into memory and prepares them for execution. Loading a program involves either memory-mapping or copying the contents of the executable file containing the program instructions into memory, and then carrying out other required preparatory tasks to prepare the executable for running. Once loading is complete, the operating system starts the program by passing control to the loaded program code.

All operating systems that support program loading have loaders, apart from highly specialized computer systems that only have a fixed set of specialized programs. Embedded systems typically do not have loaders, and instead, the code executes directly from ROM or similar. In order to load the operating system itself, as part of booting, a specialized boot loader is used. In many operating systems, the loader resides permanently in memory, though some operating systems that support virtual memory may allow the loader to be located in a region of memory that is pageable.

In computer architecture, 64-bit integers, memory addresses, or other data units are those that are 64 bits wide. Also, 64-bit central processing units (CPU) and arithmetic logic units (ALU) are those that are based on processor registers, address buses, or data buses of that size. A computer that uses such a processor is a 64-bit computer.

From the software perspective, 64-bit computing means the use of machine code with 64-bit virtual memory addresses. However, not all 64-bit instruction sets support full 64-bit virtual memory addresses; x86-64 and AArch64, for example, support only 48 bits of virtual address, with the remaining 16 bits of the virtual address required to be all zeros (000...) or all ones (111...), and several 64-bit instruction sets support fewer than 64 bits of physical memory address.

In computing, a memory address is a reference to a specific memory location in memory used by both software and hardware. These addresses are fixed-length sequences of digits, typically displayed and handled as unsigned integers. This numerical representation is based on the features of CPU (such as the instruction pointer and incremental address registers). Programming language constructs often treat the memory like an array.

An instruction set architecture (ISA) is an abstract model that defines the programmable interface of the CPU of a computer, defining how software interacts with hardware. A device (i.e. CPU) that interprets instructions described by an ISA is an implementation of that ISA. Generally, the same ISA is used for a family of related CPU devices.

In general, an ISA defines the instructions, data types, registers, and the programming interface for managing main memory such as addressing modes, virtual memory, and memory consistency mechanisms. The ISA also includes the input/output model of the programmable interface.

x86-64 (also known as x64, x86_64, AMD64, and Intel 64) is a 64-bit extension of the x86 instruction set. It was announced in 1999 and first available in the AMD Opteron family in 2003. It introduces two new operating modes: 64-bit mode and compatibility mode, along with a new four-level paging mechanism.

In 64-bit mode, x86-64 supports significantly larger amounts of virtual memory and physical memory compared to its 32-bit predecessors, allowing programs to utilize more memory for data storage. The architecture expands the number of general-purpose registers from 8 to 16, all fully general-purpose, and extends their width to 64 bits.

A modern computer operating system usually uses virtual memory to provide separate address spaces or regions of a single address space, called user space and kernel space. This separation primarily provides memory protection and hardware protection from malicious or errant software behaviour.

Kernel space is strictly reserved for running a privileged operating system kernel, kernel extensions, and most device drivers. In contrast, user space is the memory area where application software, daemons, and some drivers execute, typically with one address space per process.

A page, memory page, or virtual page is a fixed-length contiguous block of virtual memory, described by a single entry in a page table. It is the smallest unit of data for memory management in an operating system that uses virtual memory. Similarly, a page frame is the smallest fixed-length contiguous block of physical memory into which memory pages are mapped by the operating system.

A transfer of pages between main memory and an auxiliary store, such as a hard disk drive, is referred to as paging or swapping.

A superminicomputer, colloquially supermini, is a high-end minicomputer. The term is used to distinguish the emerging 32-bit architecture midrange computers introduced in the mid to late 1970s from the classical 16-bit systems that preceded them. The development of these computers was driven by the need of applications to address larger memory. The term midicomputer had been used earlier to refer to these systems. Virtual memory was often an additional criteria that was considered for inclusion in this class of system. The computational speed of these machines was significantly greater than the 16-bit minicomputers and approached the performance of small mainframe computers. The name has at times been described as a "frivolous" term created by "marketeers" that lacks a specific definition. Describing a class of system has historically been seen as problematic: "In the computer kingdom, taxonomic classification of equipment is more of a black art than a science." There is some disagreement about which systems should be included in this class. The origin of the name is uncertain.

As technology improved rapidly the distinction between minicomputer and superminicomputer performance blurred. Companies that sold mainframe computers began to offer machines in the same price and performance range as superminicomputers. By the mid-1980s microprocessors with the hardware architecture of superminicomputers were used to produce scientific and engineering workstations. The minicomputer industry then declined through the early 1990s. The term is now considered obsolete but still remains of interest for students/researchers of computer history.