Raymond Chen from Microsoft explained, why pressing and holding the Shift key while rebooting helped the Windows 95 system start faster than a full computer reboot would have.
When the command EW_RESTARTWINDOWS was issued while holding down the Shift key, the 16-bit ExitWindows function would terminate the 16-bit Windows kernel, then the 32-bit virtual memory manager would shut down, after which the processor would switch to 'real mode' and hand over control to the executable file 'win.com'. 'Real mode' is an outdated operating mode of the processor offering direct access to hardware; it precedes the protected mode used by modern operating systems.
When running in real mode, as typically seen in a DOS environment, the win.com file received control, prompting the processor to signal it to launch Windows in protected mode. At startup, .com files were allocated all the remaining memory left after the standard DOS 640 KB. 'In the case of 'win.com', it releases all memory beyond its image for the system, ensuring that there is one large contiguous block of memory available for loading Windows in protected mode,' Mr. Chen explained.
If any program occupied memory in the space that 'win.com' allocated for Windows in protected mode, the regular memory would be fragmented, and attempting to revert the system to the state it was in when 'win.com' first launched would fail, as the requirement for 'one large contiguous block of memory' would not have been met. In this case, a full reboot was required, taking the time needed for BIOS and DOS; while holding down the Shift key and with a successful 'clean' restart of Windows, this would not have been needed.
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Source: 3dnews.ru
