Manipulating Bits

When you program in a COBOL environment, you deal with data that is composed of characters, fields, records, or files. However, there are occasions when you must deal with individual data bits. For example, bit manipulation is an important feature in applications that deal with:

  • Screen and graphics interfaces
  • Encryption techniques
  • System-level routines
  • Table lookup, that is, converting an incoming code to an expanded value
  • Packing and unpacking data

Manipulating Bits with Logic Routines

There is a set of library routines for bit manipulation. These routines are:

CBL_NOT logical NOT
CBL_AND logical AND
CBL_OR logical OR
CBL_EQ logical EQuivalence
CBL_XOR logical eXclusive OR
CBL_IMP logical IMPlies

These logical operations give the results shown in the following truth tables.

A   0 0 1 1
CBL_NOT (not A) 1 1 0 0
A   0 0 1 1
B   0 1 0 1
CBL_AND (A AND B) 0 0 0 1
CBL_OR (A OR B) 0 1 1 1
CBL_EQ (A EQ B) 1 0 0 1
CBL_XOR (A XOR B) 0 1 1 0
CBL_IMP (A IMP B) 1 1 0 1

For example, when the CBL_EQ routine compares bits A and B, the resulting bit is set to 1 if both A and B are 0 or if both A and B are 1. Otherwise the resulting bit is set to 0.

Manipulating Bits as Boolean Data Items

You can also define data items as boolean using picture character 1, and use the boolean operators B-AND, B-OR, B-XOR, and B-NOT to perform "and", "or", "exclusive or", and "negate" operations. These operators are used in boolean expressions; for example:

 01 My-flag PIC 1111 USAGE BIT VALUE B"0000".
01 My-flag-2 PIC 1111 USAGE BIT.
MOVE B"0011" to My-flag-2 *> Initialize My-flag-2
COMPUTE My-flag = B-NOT My-flag-2 *> set the bits in My-flag to the
*> reverse of My-flag-2, 1100.
COMPUTE My-flag = My-flag B-AND B"0000" *> turn off all the bits in My-flag.
COMPUTE My-flag-2 = My-flag-2 B-OR B"1000" *> set bit 1 ON in My-flag-2, keeping
*> other bits unchanged.

Boolean items can be tested in two ways:

  • as a boolean condition, if the length of an item is 1 bit position or 1 character position; for example:
    01 Single-bit-item pic 1 usage bit.
    01 Multiple-bit-item pic 1(7) usage bit.
    IF Single-bit-item THEN CALL a-program *> calls if Single-bit-item is true (1)
  • as a relation condition, testing for equal or not equal; for example:
    IF My-flag-2 EQUAL B"1000" THEN CALL a-program

Converting boolean data items to integers

Boolean items can be converted to integer with the INTEGER-OF-BOOLEAN intrinsic function, and integer items can be converted to boolean with the BOOLEAN-OF-INTEGER intrinsic function. For example:

01 bit-item PIC 1(6) usage BIT.

01 integer-item PIC 9(5) VALUE 544. *>  1000100000 in binary
01 integer-item-2 PIC 9(3).
MOVE FUNCTION BOOLEAN-OF-INTEGER ( integer-item , 6) TO bit-item.
*> the function returns the low order 6 bits of the binary representation of the
*> integer value, and MOVE stores it in bit-item, padding on the right with 0’s
COMPUTE integer-item-2 = FUNCTION INTEGER-OF-BOOLEAN (bit-item).
*> the function returns the numeric value of the leading 6 bits (100000 in binary), 32, and
*> COMPUTE puts 032 in integer-item-2