# 360 Assembly/360 Instructions/BCR

**BCR** - Branch on Condition Register - RR type instruction - Opcode 07

This instruction provides three types of branch operations: no branch (equivalent to no operation), branch conditionally depending on a mask value, and unconditional branch.

## Format[edit]

BCR 0,0 No branch

Note: Since it may be difficult to understand the difference between the mask and register values, to distinguish between the mask value and the register, registers will be referenced by using the label R followed by the register number, e.g. R5 for Register 5, R6 for Register 6, R14 for Register 14, etc.

BCR 2,R5 BHR R6 - Special opcode, same as BCR 2,R6 BCR 1,R1 BOR R1 - Same as BC 1,R1 BCR 15,R10 BR R10 - BR opcode is unconditional branch and is same as BCR 15,R10 BCR 3,R6

The specific syntax is

- BCR
*mask*,*base_register*

RR Instruction (2 bytes) | |||

Byte 1 bits (0-7) |
Byte 2 | ||

Mask value(8-11) |
base register(12-15) |
||

(in hex) | Opcode07 |
(4 bits)0..F |
(4 bits)0..F |

- The first argument is a
*mask*which the condition code is compared against. - The second argument is the
*source register*whose contents are to be used as the location to transfer to if the mask matches the currently set condition code. - The
*Mask value*and*base register*values are 0 to 15.

- If the
*mask*or*base_register*is zero, the instruction is a no-operation, and does not branch. - If the
*mask*is 15 the branch is unconditional (unless the*base_register*is 0, in which case it does not branch). So in either of the following cases:

BCR X,0 BCR 0,X

Where X is any value from 0 to 15, the branch is not taken.

## Mask[edit]

After executing any instruction, IBM sets some flag called "Condition Code" in some register called "PSW". For example suppose we are using an arithmetic instruction e.g. arithmetic add called "AR". Then in following conditions the value of "Condition Code" will be as following:

Condition | Symbol | Condition code in PSW |
---|---|---|

Result is zero | Z | 0 |

Result is negative | N | 1 |

Result is positive | P | 2 |

Result overflows | O | 3 |

Then consider for example instruction "BNZR". It's mask is 7. What does that mean? This instruction branches if the result is not zero, which means it branches if the value of condition code is not zero! So it is 1,2 or 3. Consider the following table

Z | N | P | O |
---|---|---|---|

0 | 1 | 1 | 1 |

The example above gives us mask (0111) = 7. For each of following opcodes you can construct the table ZNPO and find the corresponding mask!

## Other Opcodes[edit]

The assembler provides the mask as part of several optional opcodes. These simply require the base register which contains the target address to branch to. These opcodes are:

Opcode | Mask | Usage | Purpose | Equivalent to | Used after |
---|---|---|---|---|---|

NOPR | 0 | NOPR R6 |
No Operation | BCR 0,R6 |
Any place where a no-operation/filler is wanted. The value contained in the register is ignored. |

BOR | 1 | BOR R6 |
Branch on Overflow/Ones | BCR 1,R6 |
After an arithmetic operation or arithmetic comparison, if arithmetic overflow or result is all ones occurred |

BHR | 2 | BHR R6 |
Branch (a High) | BCR 2,R6 |
After any comparison, branch if first value in comparison is higher than second value (A > B) |

BPR | 2 | BPR R6 |
Branch on Plus | BC 2,R6 |
After an arithmetic operation or arithmetic comparison, branch if result is positive |

BLR | 4 | BLR R6 |
Branch (a Low) | BCR 4,R6 |
After any comparison, branch if first value is lower than second (A < B) |

BMR | 4 | BMR R6 |
Branch on Minus/Mixed | BCR 4,R6 |
After an arithmetic operation or arithmetic comparison, branch if the result is negative or is ones and zeroes |

BNER | 7 | BNER R6 |
Branch Not Equal | BCR 7,R6 |
After any comparison, branch if first value is not equal to the second value (A <> B or A ~= B or A != B) |

BNZR | 7 | BNZR R6 |
Branch Not Zero | BCR 7,R6 |
After an arithmetic operation or arithmetic comparison, branch if result is not zero |

BER | 8 | BER R6 |
Branch (a Equal b) | BCR 8,R6 |
After any comparison, branch if first value equals the second value (A = B or A == B) |

BZR | 8 | BZR R6 |
Branch on Zero | BCR 8,R6 |
After an arithmetic operation or arithmetic comparison, branch if result is zero |

BNLR | 11 | BNLR R6 |
Branch (a Not Low) | BCR 11,R6 |
After any comparison, branch if first value is not lower than the second value (A >= B) |

BNMR | 11 | BNMR R6 |
Branch Not Minus | BCR 11,R6 |
After an arithmetic operation or arithmetic comparison, branch if result is zero |

BNHR | 13 | BNHR R6 |
Branch Not High | BCR 13,R6 |
After any comparison, branch if first value is not higher than the second value {A<=B) |

BNPR | 13 | BNPR R6 |
Branch Not Plus | BCR 13,R6 |
After an arithmetic operation or arithmetic comparison, branch if result is not positive |

BNOR | 14 | BNOR R6 |
Branch Not Ones | BCR 14,R6 |
After an arithmetic operation or arithmetic comparison, branch if result is not all ones |

BR | 15 | BR R6 |
Branch (unconditional) | BCR 15,R6 |
Branch in all cases (unless index register is 0; then treat as no-op) Equivalent to GOTO in high-level languages |

## Optional formats[edit]

### Mask value ignored[edit]

07FE BR 14 unconditional branch - equivalent toBC 15,140705 NOP 5 no-operation -BC 0,507F0 BC 15,0 despite the mask being 15, because the base register is 0, * this is also a no-op

### Used after standard comparison of a and b[edit]

0785 BER 5 branch if a equal b -BCR 8,50725 BHR 5 branch if a high -BCR 2,50745 BLR 5 branch if a low -BC 4,50775 BNER 5 branch if a not equal b -BC 7,507D5 BNHR 5 branch if a not high -BC 13,50745 BNLR 5 branch if a not low -BC 4,5

### Used after arithmetic operations[edit]

0715 BOR 5 branch on overflow -BCR 1,label0727 BPR 7 branch on plus -BCR 2,70745 BMR 5 branch on minus -BCR 4,50785 BZR 5 branch on zero -BCR 8,507E5 BNOR 5 branch on not ones -BCR 14,507D5 BNPR 5 branch on not plus -BCR 13,507B5 BNMR 5 branch on not minus -BCR 11,50775 BNZR 5 branch on not zero -BCR 7,5

### Used after Test Under Mask instructions[edit]

4717 BOR 7 branch on ones -BC 1,7474F BMR 15 branch on mixed -BC 4,15478E BZ 14 branch on zeroes -BC 8,1447E5 BNO 5 branch on not ones -BC 14,5

## Availability[edit]

The BCR instruction is available on all models of the 360, 370 and z/System.

## Operation[edit]

Upon performing an arithmetic operation or a comparison, certain bits in the Program Status Word called the Condition Code are set or cleared. In the case of comparison of two fields, the left value is considered the "A" value, and the right value is considered the "B" value, and the result of the comparison of A to B tests how A compares to B, either low, high, equal or not-equal.

In the case of an arithmetic operation, the result being plus, minus, zero or having had an overflow is tested.

In the case of the test under mask instructions, the result of the test being all ones, all zeroes, or mixed ones and zeroes.

The unconditional branch (BCR 15,*base_register*) is the equivalent of a **return** statement or other exit from a procedure or function to the calling procedure or program in a high-level language.

The *Branch on Condition Register* instruction is used following such a test to compare the condition code bits to the mask value. If bits set in the mask match bits which are set in the condition code (or all of the bits in the mask are set), and the base register of the target address is not 0, the target address contained in the base register is placed into the PSW as the new address of the current instruction and the branch is taken. Otherwise execution continues with the next instruction following the branch on condition register instruction.

Typically, the Branch on Condition Register instruction is most often used in module to module transfers or branch from one CSECT to another. In the case of a conditional branch being used within the same module or CSECT, the Branch on Condition (BC) instruction (or the optional formats, BE, BNE etc.) is more often used.

## Purpose of Instruction[edit]

The *Branch on Condition Register* instruction is used to branch within a program. It has three variations: no branch or No Operation, conditional branch depending upon a test, or unconditional branch.

### No Operation[edit]

The *no branch* - **BCR 0** - or **NOPR** is typically used to create a label which is not tied to an existing instruction. It may be used by a macro for alignment to force an instruction or data onto a specific boundary but without causing a program exception if the instruction is branched to. It can also be used to provide 'slack' space to allow later patching of the binary without having to reassemble the program. A NOPR will also occur, regardless of the mask value, if the base register of the branch is zero.

### Conditional Branch[edit]

In a *conditional branch* the bits in the mask are compared to the bits in the condition code. If the bits in the mask match the bits in the condition code, (and the base register of the target address is not zero) the branch is taken.

### Unconditional Branch[edit]

A branch to another location (the equivalent of a GOTO in high-level languages or a RETURN if inside of a procedure or subroutine) is performed by setting all the bits in the mask, e.g. **BCR 15**, or using the **BR** instruction. As long as the base register is not zero, the branch will always be taken; if the base register is zero, the branch is never taken.

## Typical Usage[edit]

The branch on condition register and optional formats are typically used after performing a comparison or arithmetic operation. In the following code, a question is asked, the response is compared to yes or no, and the question is re-issued if not either. The 'CALL' macro is used to create a standard subroutine linkage. In this example, a no response is handled internally (at label **NO**) while a yes response transfers control to another routine (called **YES**)

CHECKINQ NOP 0(0) * Optionally, the instruction 'CHECKINQ EQU *' could have been used. This would have * generated no instructions. BALR 3,0 Come back here if neither Y nor N * optionally, the instructionLA 3,CHECKINQcould have been used. L 4,YESRTN Place to go for Yes response LA 5,NO Place to go for No response CALL INQUIRE,(QUES,1,RESP) Call an external module called INQUIRE CLC RESP(1),QY1 Compare one byte for 'Y' BER 4 "Resp" is the A value in an A:B comparison CLC RESP(1),QY2 Now test for 'y' BER 4 CLC RESP(1),QN1 BER 5 Answer was 'N' CLC RESP(1),QN2 Is it 'n'? BNER 3 Something else, try again BR 5 Answer was 'n' QUES DC C'Are you ready to start?' Construct a 'C' language-type DC X'00' string, zero terminated RESP DS C One byte response QY1 DC C'Y' Available responses QY2 DC C'y' QN1 DC C'N' QN2 DC C'n' YESRTN DC V(YES) External module YES * * This will handle a 'no' answer NO NOP 0(0) Handle a no

## Exceptions and Faults[edit]

- The target address contained within the base register must not be odd, or an operation exception occurs
- The target address contained within the base register must be within the range of valid memory or an operation exception occurs.
- The storage key for the target address contained within the base register must be the same as the current process (or this process must have a key of 0) or a memory protect violate exception occurs.

## Alternative branch instructions[edit]

- The BALR instruction is used to branch to the address in a register and save the current address as a return address, similar to a procedure or function call in a high-level language
- The BC instruction is used in the same manner as the BCR instruction, but is used to branch to the address contained in a register plus a potential index register and a 12-bit displacement as opposed to the address contained in a register.
- The BCT and the BCTR instructions are used to decrement the value in the first argument, a register, store the result back in that register, then branch to an address in a register (for the BCTR instruction) or to the address contained in a register plus a potential index register and a 12-bit displacement, if the resulting value in the first argument register was not zero.
- The BRC instruction is used in the same manner as the BCR instruction, but is used to branch to the address relative to the current program counter, contained in an immediate (16 bit) value.
- The BRCL instruction is used in the same manner as the BCR instruction, but is used to branch to the address relative to the current program counter, contained in an immediate (32 bit) value.

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