The Programmer's Key, or interrupt button, is a button or switch on a computer which causes an asynchronous interrupt request (IRQ) to be sent to the processor. If a debugger is installed on the machine, it is activated when the interrupt request is processed, allowing the user to view and usually modify the computer's internal memory. The interrupt is of highest priority, allowing debugging of interrupt handlers. This is quite useful in debugging software, which is why it gets its name.
The "key" need not always be a button or a switch. On some computers, it is a pinhole into which a straightened paperclip - or syringe - must be inserted (to prevent accidental activation).
This term is used chiefly among Macintosh programmers, perhaps because the Mac hardware once supported this function. On most 68000 family based Macintosh computers, an interrupt request can also be sent by holding down the command key and striking the power key on the keyboard. This effect is also simulated by the 68000 environment of the Mac OS nanokernel on PowerPC machines and the Classic environment.
Modern Mac hardware no longer includes the interrupt button, as the Mac OS X operating system has integrated debugging options. In addition, Mac OS X's protected memory blocks direct patching of system memory (in order to better secure the system).
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