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Mansa Musa was the ruler of the Mali Empire in West Africa. Details recorded here should be sourced; unknown information is left blank.
MORE →Reflects the personal views, recollections, and perspective of the author, Mike Davis.
This is a personal recollection on the Move fire on May 13, 1985
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Chain loading is a method used by computer programs 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 overlays. 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.
In operating system boot manager 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, replacing the in-memory boot sector from which the boot manager itself was bootstrapped, and executed.
In Unix (and in Unix-like operating systems), the exec() system call is used to perform chain loading. The program image of the current process 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.
In addition to the exec() process level chain loading Linux supports the kexec() 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.
In BASIC 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[1] 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.
Many versions of Fortran include a CALL CHAIN or CALL LINK statement that performs chain loading, preserving the contents of COMMON storage.[2] This is not the same as the unrelated LINK subroutine in GNU Fortran.[3]
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.
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Chain loading is a method used by computer programs 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 overlays. 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.
A boot sector is the sector of a persistent data storage device (e.g., hard disk, floppy disk, optical disc, etc.) which contains machine code to be loaded into random-access memory (RAM) and then executed by a computer system's built-in firmware (e.g., the BIOS). Usually, the first sector of the hard disk is the boot sector, regardless of sector size (512 or 4096 bytes) and partitioning flavor (MBR or GPT). The purpose of defining one particular sector as the boot sector is inter-operability between firmware and various operating systems. The purpose of chain-loading, first firmware (e.g., the BIOS), then code in the boot sector, and then, for example, an operating system, is maximal flexibility.
A volume boot record (VBR) (also known as a volume boot sector, a partition boot record or a partition boot sector) is a type of boot sector introduced by the IBM Personal Computer. It may be found on a partitioned data storage device, such as a hard disk, or an unpartitioned device, such as a floppy disk, and contains machine code for bootstrapping programs (usually, but not necessarily, operating systems) stored in other parts of the device. On non-partitioned storage devices, it is the first sector of the device. On partitioned devices, it is the first sector of an individual partition on the device, with the first sector of the entire device being a Master Boot Record (MBR) containing the partition table. The code in volume boot records is invoked either directly by the machine's firmware or indirectly by code in the master boot record or a boot manager. Code in the MBR and VBR is in essence loaded the same way. Invoking a VBR via a boot manager is known as chain loading. Some dual-boot systems, such as NTLDR (the boot loader for all releases of Microsoft's Windows NT-derived operating systems up to and including Windows XP and Windows Server 2003), take copies of the bootstrap code that individual operating systems install into a single partition's VBR and store them in disc files, loading the relevant VBR content from file after the boot loader has asked the user which operating system to bootstrap. In Windows Vista, Windows Server 2008 and newer versions, NTLDR was replaced; the boot-loader functionality is instead provided by two new components: WINLOAD.EXE and the Windows Boot Manager. In file systems such as FAT12 (except for in DOS 1.x), FAT16, FAT32, HPFS and NTFS, the VBR also contains a BIOS Parameter Block (BPB) that specifies the location and layout of the principal on-disk data structures for the file system. (A detailed discussion of the sector layout of FAT VBRs, the various FAT BPB versions and their entries can be found in the FAT article.)
In artillery, chain shot is a type of cannon projectile formed of two sub-calibre balls, or half-balls, chained together. Bar shot is similar, but joined by a solid bar. They were used in the age of sailing ships and black powder cannon to shoot masts, or to cut the shrouds and any other rigging of a target ship. When fired, after leaving the muzzle, the shot's components tumble in the air, and the connecting chain fully extends. In past use, as much as 1.8 m (6 ft) of chain would sweep through the target. However, the tumbling made both bar and chain shot less accurate, so they were used at shorter ranges. Chain shot was sometimes used on land as an anti-personnel load. It was used by the defenders of Magdeburg in May 1631 as an anti-personnel load, which, according to counselor Otto von Guericke, was one reason for the extreme violence of the victorious attackers. It was also used against Parliamentarians in the first English Civil War, and against Cromwell in Ireland at the siege of Clonmel in 1650, against the 76th Regiment of Foot in India in 1803, and by the French against the Dutch at the Battle of Waterloo. The military usefulness of chain shot died out as wooden sail-powered ships were replaced with armored steam ships—first among navies, and then among commercial fleets—which do not have rigging to serve as proper targets for chain-shot. Additionally, the conversion of naval armament from smoothbore, muzzle loaded, black powder cannon to rifled, breech-loaded guns further slowed the production of new chain shot ammunition; the chain would damage barrels (degrading maximum range, and further degrading effective range by degrading accuracy), and the new breech loading guns and their ammunition were meant to be effective against armored vessels as well as wooden sailing vessels. In modern times, the effect is replicated in shotguns with the use of bolo shells, a pair of slugs connected by a strong wire. They are banned in several jurisdictions, including Florida and Illinois.
Before the 1921 destruction of Tulsa’s Greenwood District, Black residents had created a remarkable center of business and community life. The district included stores, professional offices, entertainment venues and homes owned by Black citizens. Understanding Greenwood means learning what was built—not only what was burned.
MORE →Brown v. Board of Education in 1954.