You can generate a "cold" boot or a "warm" boot. A cold boot is the same
 as turning the power off and on; a warm boot is the same as Ctrl-Alt-Del
 and skips the power-on self 'test.

 For a warm boot, store the hex value 1234 in the word at 0040:0072. For
 a cold boot, store 0 in that word. Then, if you want to live
 dangerously, jump to address FFFF:0000. Here's C code to do it:

   /* WARNING:  data loss possible */
   void bootme(int want_warm)  /* arg 0 = cold boot, 1 = warm */
   {
     void (far* boot)(void) = (void (far*)(void))0xFFFF0000UL;
     
     unsigned far* type = (unsigned far*)0x00400072UL;
     *type = (want_warm ? 0x1234 : 0);
     (*boot)( );
   }

 What's wrong with that method? It will boot right away, without closing
 files, flushing disk caches, etc. If you boot without flushing a
 write-behind disk cache (if one is running), you could lose data or
 trash the file allocation table in your hard drive.

 There are two methods of signaling the cache to flush its buffers:

 (1) Simulate a keyboard Ctrl-Alt-Del in the keystroke translation
 function of the BIOS (INT 15 AH=4F; but see notes below)

 (2) Issue a disk reset (DOS function 0D). Most disk-cache programs hook
 one or both of those interrupts, so if you use both methods you'll
 probably be safe.

 When user code simulates a Ctrl-Alt-Del, one or more of the programs
 that have hooked INT 15 AH=4F can ask that the key be ignored by
 clearing the carry flag. For example, HyperDisk does this when it has
 started but not finished a cache flush. So if the carry flag comes back
 cleared, the boot code has to wait a couple of clock ticks and then try
 again. (None of this matters on older machines whose BIOS can't support
 101- or 102-key keyboards; see the discussion of INT 21 AH=4F in
 <Q:04.07> [What is the SysRq key for?])

 C code that tries to signal the disk cache (if any) to flush is given
 below. Turbo Pascal code by Timo Salmi that does more or less the same
 job may be found at question 49 (as of this writing) in the Turbo Pascal
 FAQ in comp.lang.pascal, and is downloadable as file FAQPAS2.TXT, which
 is part of: <ftp://garbo.uwasa.fi/pc/link/>

 Here's C code that reboots after trying to signal the disk cache:

   #include <dos.h>
   
   void bootme(int want_warm)  /* arg 0 = cold boot, 1 = warm  */
   {
     union REGS reg;

     void (far* boot)(void) = (void (far*)(void))0xFFFF0000UL;
     unsigned far* boottype = (unsigned far*)0x00400072UL;
     char     far* shiftstate = (char far*)0x00400017UL;
     unsigned      ticks;
     int           time_to_waste;

     /* Simulate reception of Ctrl-Alt-Del: */
     for (;;)
     {
       *shiftstate |= 0x0C;    /* turn on Ctrl & Alt */
       reg.h.ah = 0x4F;        /* see notes below */
       reg.h.al = 0x53;        /* 0x53 = Del's scan code */
       reg.x.cflag = 1;        /* sentinel for ignoring key  */
       int86(0x15, &reg, &reg);

       /* If carry flag is still set, we've finished. */
       if(reg.x.cflag)
         break;

       /* Else waste some time before trying again: */
       reg.h.ah = 0;
       int86(0x1A, &reg, &reg); /* system time into CX:DX */

       ticks = reg.x.dx;
       for (time_to_waste = 3;  time_to_waste > 0;  )
       {
         reg.h.ah = 0;
         int86(0x1A, &reg, &reg);
         if (ticks != reg.x.dx)
           ticks = reg.x.dx , --time_to_waste;
       }
     }

     /* Issue a DOS disk reset request: */
     reg.h.ah = 0x0D;
     int86(0x21, &reg, &reg);

     /* Set boot type and boot: */
     *boottype = (want_warm ? 0x1234 : 0);
     (*boot)( );
   }

 Reader Timo Salmi reported (26 July 1993) that the INT 15 AH=4F call may
 not work on older PCs (below AT, XT2, XT286), according to Ralf Brown's
 interrupt list (<Q:02.03> [What and where is Ralf Brown's interrupt
 list?]).

 Reader Roger Fulton reported (1 July 1993) that INT 15 AH=4F hangs even
 a modern PC "ONLY when ANSI.SYS [is] loaded high using EMM386.EXE.
 (Other things loaded high with EMM386.EXE were OK; ANSI.SYS loaded high
 with QEMM386.SYS was OK; ANSI.SYS loaded low with EMM386.EXE installed
 was OK.)" His solution was to use only the disk reset, INT 21 AH=0D,
 which does flush SMARTDRV, then wait five seconds in hopes that any
 other disk-caching software would have time to flush its queue.

 Reader Per Bergland reported (10 Sep 1993) that the jump to FFFF:0000
 will not work in Windows or other protected-mode programs. (For example,
 when the above reboot code ran in a DOS session under Windows, a box
 with "waiting for system shutdown" appeared. The PC hung and had to be
 reset by cycling power.) His solution, which does a cold boot not a warm
 boot, is to pulse pin 0 of the 8042 keyboard controller, which is
 connected to the CPU's "reset" line.

 He has tested the following code on various Compaqs, and expects it will
 work for any AT-class machine; he cautions that you must first flush the
 disk cache as indicated above.

   cli
   @@WaitOutReady:   ;  Busy-wait until  8042 ready for new command
   in al,64h         ; read 8042 status byte
   test al,00000010b ; this bit indicates input buffer full
   jnz @@WaitOutReady
   mov al,0FEh       ; Pulse "reset" = 8042 pin 0
   out 64h,al        ; The PC will reboot now