With the advent of faster 286 and 386 CPUs, many manufacturers settled on a 8MHz slow speed "DeTurbo" mode which was clocked off the 8MHz ISA bus. Many 486s and later CPUs lack a DeTurbo mode, but many still provide it. It does not work on some motherboards that claim to implement it. In some cases, the reduced speed is created by disabling the CPU cache or by loading down memory accesses with unneeded wait states rather than by clocking the machine more slowly.
Deturbo is primarily useful for running older programs that run too quickly for comfortable use on modern fast computers. It is also useful for debugging computer problems. A few programs will not run in DeTurbo mode on some motherboards, and a few others will only run in DeTurbo mode. Problems have been reported with DOS windows crashing when exiting DeTurbo mode and with OSs crashing when the mode is reentered. Inadvertent activation of DeTurbo mode was a major cause of "Slow PC" complaints.
On older motherboards, Turbo is controlled by a normally open or normally closed switch on the motherboard. There is often a separate connector on the motherboard that powers a Turbo LED indicator on the front panel. Many 486 and later motherboards allow DeTurbo to be controlled from the keyboard. The usual control keystrokes are Ctrl-Alt-Plus and Control-Alt-Minus. Some BIOSes are fussy about whether the keypad or keyboard plus/minus keys are used. Some software captures the Ctrl-Alt-Plus/Minus keystrokes and inadvertently disables Deturbo.
Several programs are available to slow a PC down that lacks DeTurbo. Some examples : SLO486; WHOA!; ICD and VARISLOW.
Return To Index Copyright 1994-2008 by Donald Kenney.