Q59071: Changing a Compaq’s CPU Speed

Article: Q59071
Product(s): See article
Version(s): 5.10
Operating System(s): MS-DOS
Keyword(s): ENDUSER | s_quickc s_quickasm | mspl13_c
Last Modified: 15-MAR-1990

There are no C run-time functions that change the CPU speed, but there
is an interrupt that will change the speed of the CPU. This interrupt,
shown below, works only on Compaq computers.

INT 16h:

     AH = F0h
     AL = speed
          00h  equivalent to 6 MHz
          01h  equivalent to 8 MHz
          02h  full 16 MHz
          03h  toggles between 8 MHz -- equivalent and speed set by
               system board switch (AUTO or HIGH)
          08h  full 16 MHz except 8 MHz during floppy disk access
          09h  specify speed directly
               CX = speed value, 1 (slowest) to 50 (full), 3 ~= 8088

To read the current CPU speed, set AH = F1h and call interrupt 16h.

INT 16h:
     AH = F1h
RETURN:
     AL = speed code (see function F0h for speed code)
          if AL = 09h, CX = speed code