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  Some memory locations in the BIOS data area are particularly interesting.
  Most of them contain data vital to the operation of various ROM BIOS and
  DOS service routines. In many instances, your programs can obtain
  information stored in these locations by invoking a ROM BIOS interrupt; in
  all cases, they can access the information directly. You can easily check
  out the values at these locations on your own computer, using either DEBUG
  or BASIC. To use DEBUG, type a command of this form:

  DEBUG
  D XXXX:YYYY L 1

  XXXX represents the segment part of address you want to examine. (This
  would be either 0040H or 0050H, depending on the data area that interests
  you.) YYYY represents the offset part of the address. The L 1 tells DEBUG
  to display one byte. To see two or more bytes, type the number of bytes
  (in hex) you want to see after the L instruction. For example, the BIOS
  keeps track of the current video mode number in the byte at 0040:0049H. To
  inspect this byte with DEBUG, you would type

  DEBUG
  D 0040:0049 L 1

  To display the data with BASIC, use a program of the following form,
  making the necessary substitutions for segment (&H0040 or &H0050),
  number.of.bytes, and offset (the offset part of the address you want to
  inspect):

  10 DEF SEG = segment
  20 FOR I = 0 TO number.of.bytes - 1
  30   VALUE = PEEK(offset + I)
  40   IF VALUE < 16 THEN PRINT "0";    ' needed for leading zero
  50   PRINT HEX$ (VALUE);" ";
  60 NEXT I