BCR — Branch on Conditiion Register — Opcode 07 / Decimal 7 — 2 byte RR Instruction
Contents
FormatEdit
 BCR mask_value,branch_register (both values are 0 to 15)
 BCR 6,1
RR Instruction (2 bytes)  
Byte 1 bits (07) 
Byte 2  
Mask value (811) 
branch register (1215) 

(in hex)  Opcode 07 
(4 bits) 0..F / (dec 0..15) 
(4 bits) 0..F / (dec 0..15) 
 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.
AvailabilityEdit
The BCR instruction is available on all models, 360, 370 and z/System.
UsageEdit
This instruction provides three types of branch operations: no branch (equivalent to no operation), branch conditionally depending on a mask value, and unconditional branch.
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: Branch if High Register, 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
 If the mask or base_register is zero, the instruction is a nooperation, 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.
MaskEdit
After executing an instruction which does an arithmetic, test or comparison, the processor sets the "Condition Code" flags (bits 18 and 19) in the PSW register, allowing the condition code to be examined for a potential branch. 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 OpcodesEdit
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 nooperation/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 noop) Equivalent to GOTO in highlevel languages 
Optional formatsEdit
Mask value ignoredEdit
07FE BR 14 unconditional branch  equivalent to BC 15,14 0705 NOP 5 nooperation  BC 0,5 07F0 BC 15,0 despite the mask being 15, because the base register is 0, * this is also a noop
Used after standard comparison of a and bEdit
0785 BER 5 branch if a equal b  BCR 8,5 0725 BHR 5 branch if a high  BCR 2,5 0745 BLR 5 branch if a low  BC 4,5 0775 BNER 5 branch if a not equal b  BC 7,5 07D5 BNHR 5 branch if a not high  BC 13,5 0745 BNLR 5 branch if a not low  BC 4,5
Used after arithmetic operationsEdit
0715 BOR 5 branch on overflow  BCR 1,label 0727 BPR 7 branch on plus  BCR 2,7 0745 BMR 5 branch on minus  BCR 4,5 0785 BZR 5 branch on zero  BCR 8,5 07E5 BNOR 5 branch on not ones  BCR 14,5 07D5 BNPR 5 branch on not plus  BCR 13,5 07B5 BNMR 5 branch on not minus  BCR 11,5 0775 BNZR 5 branch on not zero  BCR 7,5
Used after Test Under Mask instructionsEdit
4717 BOR 7 branch on ones  BC 1,7 474F BMR 15 branch on mixed  BC 4,15 478E BZ 14 branch on zeroes  BC 8,14 47E5 BNO 5 branch on not ones  BC 14,5
AvailabilityEdit
The BCR instruction is available on all models of the 360, 370 and z/System.
OperationEdit
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 notequal.
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 highlevel 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 InstructionEdit
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 OperationEdit
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 BranchEdit
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 BranchEdit
A branch to another location (the equivalent of a GOTO in highlevel 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 UsageEdit
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 reissued 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 instruction LA 3,CHECKINQ could 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' languagetype 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 FaultsEdit
 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 instructionsEdit
 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 highlevel 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 12bit 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 12bit 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.
Previous Instruction BC 
360 Assembly Instructions  Next Instruction BCT 
Previous Opcode 06 
Next Opcode 08 
360 Assembly Language  
360 Family  Introduction · Basic FAQ · 360 Family · 360 Architecture 

360 Instruction Set  360 Instructions · Branch Instructions · Data Transfer Instructions · Control Flow Instructions · Arithmetic Instructions · Logic Instructions · Shift and Rotate Instructions · Other Instructions 
Syntaxes and Assemblers  360 Assemblers· Pseudo Instructions 
Instruction Extensions  Floating Point · HighLevel Languages 

This page or section is an undeveloped draft or outline. You can help to develop the work, or you can ask for assistance in the project room. 