PRODUCT  :  Borland C++                                    NUMBER  :  1157
  VERSION  :  3.1
       OS  :  DOS
     DATE  :  October 19, 1993                                  PAGE  :  1/3

    TITLE  :  Determining Extended Memory Size from CMOS.





  /* -------------------[  ACCESSING CMOS DATA ]------------------------ *\

  The AT CMOS RAM can be accessed through ports 70H ( Control/Address )
  and 71h( CMOS Data ).   When reading CMOS RAM, one must:

      1.  Disable interrupts.
          ( This ensures that no interruption takes place between
            items 2. and 4. listed below ).

      2.  Write the offset of the CMOS RAM data to be fetched to I/O
          port 70h.

      3.  Given the increasing speed of CPUs, it may be necessary to
          create a delay between items 2. and 4.   The simplest way to
          achieve the delay is via a 'jmp' instruction:

                          asm jmp short $+2

          Include the '#pragma inline' directive in the source code when
          using the above so that the assembler ( TASM by default ) may
          process the 'jmp'.   Alternatively a 'goto/Label:' combination
          or an '__emit__()' statement may also be used.

      4.  Read the Data ( a byte ) from port 71h.
      5.  ReEnable Interrupts.

  Writing to CMOS RAM is similar to the process above:

       1.  Disable Interrupts.
       2.  Write the CMOS RAM offset which is to be written to port 70h.
       3.  Create a delay.
       4.  Write the data  byte to port 71h.

  Note that step 3. may not be necessary on some older machines.  Below
  is a function which given two offsets for a low and high order byte
  will return the word value stored in the CMOS.  This can be easily
  modified to write a word to the CMOS RAM.
  \* -------------------[  ACCESSING CMOS DATA ]------------------------ */

















  PRODUCT  :  Borland C++                                    NUMBER  :  1157
  VERSION  :  3.1
       OS  :  DOS
     DATE  :  October 19, 1993                                  PAGE  :  2/3

    TITLE  :  Determining Extended Memory Size from CMOS.




  /* ----------------------------------------------------------------- *\
  |                                                                     |
  |   This function will fetch a word from the cmos given the low byte  |
  |   offset and high byte offset in the cmos.                          |
  |                                                                     |
  \* ----------------------------------------------------------------- */

  #include <dos.h>
  #include <stdio.h>
  #pragma inline

  const int CMOS_PORT_WRITE = 0x70;
  const int CMOS_PORT_READ  = 0x71;

  unsigned get_cmos( unsigned char low, unsigned char high );


  int main()
  {
       unsigned ext;                /* Extended memory in Kilobytes      */
       ext = get_cmos( 0x30, 0x31 );/* Get extended detected during POST */
       printf( "Installed extended memory detected by POST:  %8uk\n", ext );
       return( 0 );
  }


  unsigned get_cmos( unsigned char low, unsigned char high )
  {
       unsigned word;
       unsigned char *p;
       unsigned char  l,h;

       p = ( unsigned char *) &word;

       disable();

       outportb( CMOS_PORT_WRITE, high );
       asm jmp short $+2
       h = inportb( CMOS_PORT_READ );

       outportb( CMOS_PORT_WRITE, low );
       asm jmp short $+2
       l = inportb( CMOS_PORT_READ );













  PRODUCT  :  Borland C++                                    NUMBER  :  1157
  VERSION  :  3.1
       OS  :  DOS
     DATE  :  October 19, 1993                                  PAGE  :  3/3

    TITLE  :  Determining Extended Memory Size from CMOS.




       *p = l;
       *(p+1) = h;

       enable();
       return word;
  }



  DISCLAIMER: You have the right to use this technical information subject
  to the terms of the No-Nonsense License Statement that you received with
  the Borland product to which this information pertains.