Binary-Coded Decimal
BCD is a way to represent decimal numbers in a more
compact way than ASCII text. One byte represents two

01H = 1 decimal
20H = 20 decimal (not 32)
99H = 99 decimal (not 153)
  BCD is used rarely on PCs and in PC programming.
  Note that hex digits A-F are never used, so numbers
such as 0aH or b1H are invalid as BCD values.