Chain loading is a method used by

computer program A computer program is a sequence or set of instructions in a programming language for a computer to execute. Computer programs are one component of software, which also includes documentation and other intangible components. A computer program ...

s to replace the currently executing program with a new program, using a common data area to pass information from the current program to the new program. It occurs in several areas of computing. Chain loading is similar to the use of

overlay Overlay may refer to: Computers *Overlay network, a computer network which is built on top of another network *Hardware overlay, one type of video overlay that uses memory dedicated to the application *Another term for exec, replacing one process ...

s. Unlike overlays, however, chain loading replaces the currently executing program in its entirety. Overlays usually replace only a portion of the running program. Like the use of overlays, the use of chain loading increases the I/O load of an application.

Chain loading in boot manager programs

In operating system

boot manager A bootloader, also spelled as boot loader or called boot manager and bootstrap loader, is a computer program that is responsible for booting a computer. When a computer is turned off, its softwareincluding operating systems, application code, an ...

programs, chain loading is used to pass control from the boot manager to a boot sector. The target boot sector is loaded in from

disk Disk or disc may refer to: * Disk (mathematics), a geometric shape * Disk storage Music * Disc (band), an American experimental music band * ''Disk'' (album), a 1995 EP by Moby Other uses * Disk (functional analysis), a subset of a vector sp ...

, replacing the in-memory boot sector from which the boot manager itself was bootstrapped, and executed.

Chain loading in Unix

Unix Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, and ot ...

(and in

Unix-like A Unix-like (sometimes referred to as UN*X or *nix) operating system is one that behaves in a manner similar to a Unix system, although not necessarily conforming to or being certified to any version of the Single UNIX Specification. A Unix-li ...

operating systems), the exec() system call is used to perform chain loading. The program image of the current

process A process is a series or set of activities that interact to produce a result; it may occur once-only or be recurrent or periodic. Things called a process include: Business and management *Business process, activities that produce a specific se ...

is replaced with an entirely new image, and the current thread begins execution of that image. The common data area comprises the process' environment variables, which are preserved across the system call.

Chain loading in Linux

In addition to the process level chain loading

Linux Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, which ...

supports the system call to replace the entire operating system kernel with a different version. The new kernel boots as if it were started from power up and no running processes are preserved.

Chain loading in BASIC programs

BASIC BASIC (Beginners' All-purpose Symbolic Instruction Code) is a family of general-purpose, high-level programming languages designed for ease of use. The original version was created by John G. Kemeny and Thomas E. Kurtz at Dartmouth College ...

programs, chain loading is the purview of the CHAIN statement (or, in Commodore BASIC, the LOAD statement), which causes the current program to be terminated and the chained-to program to be loaded and invoked (with, on those dialects of BASIC that support it, an optional parameter specifying the line number from which execution is to commence, rather than the default of the first line of the new program). The common data area varies according to the particular dialect of BASIC that is in use. On BBC BASIC, for example, only a specific subset of all variables are preserved across a CHAIN. On other BASICs, the COM statement can be used in conjunction with CHAIN to specify which variables are to be preserved as common data across a chain operation. Chain loading permits BASIC programs to execute more program code than could fit into available program and variable memory. Applications written in BASIC could thus be far larger than the size of working memory, via a set of cooperating programs that CHAIN back and forth amongst themselves as program flow moves within the overall application.

Chain loading in FORTRAN programs

Many versions of Fortran include a CALL CHAIN or CALL LINK statement that performs chain loading, preserving the contents of COMMON storage. This is not the same as the unrelated LINK subroutine in GNU Fortran.

Chain loading in OS/360

OS/360 and successors use the XCTL (for "transfer control") macro for chain loading. Because of the operating system's memory management this may or may not result in replacement of the code of the calling program in memory.

References

* * * {{citation , mode=cs1 , chapter-url=http://www.faqs.org/docs/artu/ch07s02.html , title= The Art of Unix Programming , first=Eric Steven , last=Raymond , chapter=Multiprogramming: Taxonomy of Unix IPC Methods: Wrappers and Bernstein Chaining Operating system technology Process (computing)