COMP is a binary storage format while COMP-3 is packed
Cobol Usage Clause:
DISPLAY Default. Number of bytes required equals to the size of the data item. COMP Binary representation of data item. PIC clause can contain S and 9 only. S9(01) S9(04) Half word. S9(05) S9(09) Full word. S9(10) - S9(18) Double word. Most significant bit is ON if the number is negative.
COMP-1 Single word floating point item. PIC Clause should not be specified.
COMP-2 Double word floating-point item. PIC Clause should not be specified.
COMP-3 Packed Decimal representation. Two digits are stored in each byte.
Last nibble is for sign. (F for unsigned positive, C for signed positive and D for signed negative) Formula for Bytes: Integer ((n/2) + 1)) => n is number of 9s.