CLC is used to compare two fields that are both in storage. The fields are compared, one byte at a time beginning with the bytes specified in addresses B1D1D1D1 and B2D2D2D2 , and moving to higher addresses in the source and target fields. Each byte in the source is compared to a byte in the target according to the ordering specified in the EBCDIC encoding sequence. Executing a compare instruction sets the condition code (a two bit field in the PSW) to indicate how operand 1 (target field) compares with operand 2 (source field). The condition code is set as follows,

 

Comparison Condition Code Value Test With

 

Operand 1 equals Operand 2 0 (equal) BE (Branch Equal)

BNE (Branch Not equal)

Operand 1 is less than Operand 2 1 (low) BL (Branch Low)

BNL (Branch Not Low)

Operand 1 is greater than Operand 2 2 (high) BH (Branch High)

BNH (Branch Not High)

 

The table above also indicates the appropriate branch instructions for testing the condition code. When comparing two fields, a CLC instruction should be followed immediately by one or more branch instructions for testing the contents of the condition code:

CLC FIELDA,FIELDB

BH AHIGH BRANCH IF FIELDA IS HIGH

BL BHIGH BRANCH IF FIELDA IS LOW

Bytes are compared until the number of bytes specified (implicitly or explicitly) in operand 1 have been exhausted or until two unequal bytes are found - whichever occurs first.

As you can see from the instruction format above, the instruction carries with it the maximum number of bytes to be compared, as well as the beginning addresses of the source and target fields. Notice that the instruction does not specify the ending addresses of either field - the instruction is no respecter of fields. If a longer field is compared to a shorter field, the bytes following the shorter field may be used in the comparison operation.

 

The length (LL1) determines the maximum number of bytes which will be compared. The length is usually determined implicitly from the length of operand 1 but the programmer can provide an explicit length. Consider the two example CLCs below,

 

 

 

 

 

 

 

Object code Assembler code

 

FIELDA DC CL4ABCD

FIELDB DC CABE

...

D502C00CC008 CLC FIELDB,FIELDA Implicit length = 3,

COND CODE = HIGH

D501C00CC008 CLC FIELDB(2),FIELDA Explicit length = 2,

COND CODE = EQUAL

 

In the first CLC, A in FIELDB is compared with A in FIELDA, then B in FIELDB is compared with B in FIELDA, finally, E in FIELDB is compared with C in FIELDA. At this point, the condition code is set to HIGH since E follows C in the EBCDIC encoding sequence. In the second example, A in FIELDB is compared with A in FIELDA, then B in FIELDB is compared with B in FIELDA. The condition code is set to EQUAL since an explicit length of 2 was coded.

 

Some Unrelated CLCs:

A DC CPQR

B DC CABCD

C DC CPQ

D DC P12 D = X012C

... Result:

CLC A,B Condition Code = High, one byte compared.

CLC A(2),C Condition Code = Equal, two bytes compared.

CLC C,A Condition Code = Equal, two bytes compared.

CLC A,=C Condition Code = High, one byte compared. This

coding is unwise since it sets up the

possibility that bytes following the blank

literal in the literal pool might become part

of the comparison.

CLC A,=CL3 Condition Code = High, this is a better

version of the previous comparison. Should

length of A change, an error may occur.

CLC B,=XC1C2C3C4 Condition Code = Equal,

4 bytes were compared.

CLC D,=P12 This is a dangerous compare since the data

involved is in packed format. If the

fields are unchanged from their assembly

time format the condition code would be

equal.

CLC A(500),B Assembly Error - max length is 256

CLC A,B(20) Assembly Error - operand 1 determines the

length

 

1. As with any storage to storage instruction, you must pay careful attention to lengths of the two operands. Generally, you should be comparing fields that are the same size.

 

2. The instruction was designed to compare fields that are in character format. It can be used to compare fields with non-character data, but this takes special consideration to make sure the comparison will produce the desired results. Packed decimal data and binary data are supported with their own special comparison instructions.

 

3. The condition code can be changed by any other type of comparison instruction as well as by a variety of arithmetic instructions. Dont rely on the condition code to remain set - after you have issued a CLC , you should follow it up immediately with a branch instruction.