|
In computers, a terminate and stay resident program (commonly referred to by the initialism TSR) is a computer program that uses a system call in DOS operating systems to return control of the computer to the operating system, as though the program has quit, but stays resident in computer memory so it can be reactivated by a hardware or software interrupt. This technique partially overcame DOS operating systems' limitation of executing only one program, or task, at a time. TSR is unique to DOS and not used in Windows. Some terminate and stay resident programs were utility programs that a computer user might call up several times a day, while working in another program, using a hotkey. Borland Sidekick was an early and popular example of this type. Other TSRs serve as device drivers for hardware that the operating system did not directly support. == Using TSRs == Normally in DOS operating systems, only one program can run at any given time. To stop running, it gives control back to the DOS shell program, COMMAND.COM, using the system call INT 21h/4Ch.〔() HelpPC reference: INT 21,0 – Program Terminate〕 The memory and system resources that were used by the program are then marked as unused. This in effect makes it impossible to restart parts of it again without reloading it from scratch. However, if a program ends with the system call INT 27h or INT 21h/31h, the operating system does not reuse a certain specified part of the program's memory. The original call, INT 27h, is called "terminate but stay resident", hence the name "TSR". Using this call, a program can make up to 64 KB of its memory resident. MS-DOS version 2.0 introduced an improved call, INT 21h/function 31h ('Keep Process'), which removed this limitation and let the program return an exit code. Before making this call, the program can install one or several interrupt handlers pointing into itself, so that it can be called again. Installing a hardware interrupt vector allows such a program to react to hardware events. Installing a software interrupt vector allows it to be called by the currently running program. Installing a timer interrupt handler allows a TSR to run periodically (see ISA and programmable interval timer, especially the section "IBM PC compatible"). The typical method of utilizing an interrupt vector involves reading its present value (the address), storing it within the memory space of the TSR, and installing a pointer to its own code. The stored address is called before or after the TSR has received the interrupt and has finished its processing, in effect forming a singly linked list of interrupt handlers, also called ''interrupt service routines'', or ISRs. This procedure of installing ISRs is called ''chaining'' or ''hooking'' an interrupt or an interrupt vector. By chaining the interrupt vectors TSR programs could take complete control of the computer. A TSR could have one of two behaviors: * Take complete control of an interrupt by not calling other TSRs that had previously altered the same interrupt vector. * Cascade with other TSRs by calling the old interrupt vector. This could be done before or after they executed their actual code. This way TSRs could form a chain of programs where each one calls the next one. The "terminate and stay resident" method was used by most DOS viruses which could either take control of the PC or stay in the background. Viruses would react to disk I/O or execution events by infecting executable (.EXE or .COM) files when they were run and data files when they were opened. Parts of DOS itself, especially in DOS versions 5.0 and later, used this same technique to perform useful functions, such as the DOSKEY command-line editor and various other installable utilities which were installed by running them at the command line (manually, from AUTOEXEC.BAT or through INSTALL from within CONFIG.SYS) rather than loading them as device drivers through DEVICE statements in CONFIG.SYS.A TSR program can be loaded at any time; sometimes, they are loaded immediately after the operating system's boot, by being explicitly loaded in the AUTOEXEC.BAT batch program, or alternatively at the user's request (for example, Borland's Sidekick and Turbo Debugger, Quicken's QuickPay, or FunStuff Software's Personal Calendar). These programs will, as "TSR" implies, stay resident in memory while other programs are executing. Some of them do not have an option for unloading themselves from memory, so calling TSR means the program will remain in memory until a reboot. However unloading is possible externally, using utilities like the MARK.EXE/RELEASE.EXE combo by TurboPower Software or ''soft reboot'' TSRs which will catch a specific key combination and release all TSRs loaded after them. As the chain of ISRs is singly linked, there is no provision for discovering the previous handler's address (other than attempting to trace back the interrupt chain), or to inform its predecessor that it needs to update its "next address to which to jump" not to point to the TSR which desires to remove itself, so that in order to safely unload TSRs in the middle of a chain, stubs had to be left in memory in most cases, thereby causing memory fragmentation. This gave rise to TSR cooperation frameworks such as TesSeRact and AMIS.〔(a list of TSR libraries ) also known as frameworks.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Terminate and stay resident program」の詳細全文を読む スポンサード リンク
|