Usually the instructions in a program are executed in order, one after another. Often, though, we want to make decisions, as in:

         If (A is equal to B)
           Then
             do something
           Else
             do dome other thing
         End-If

How do we do this in assembly language? We make use of the Condition Code or CC, a 2-bit "special register" in the hardware. As it is 2 bits long, it can contain only 4 possible values: 0, 1, 2 and 3. Many different instructions affect the value of the CC. After one of these instructions, we use a "branch" instruction to cause the flow of execution to move to some location in the code as needed.

Which instructions set the CC?

• arithmetic (add, subtract)
• logical (AND, OR, XOR)
• comparisons
• various others

The yellow card contains a list of which instructions set the CC and how. In general:

• for comparisons, CC = 0 for "equal", CC = 1 for "left < right", CC = 2 for "Left > right" and CC = 3 is not used

• for add and subtract, CC = 0 for result = 0, CC = 1 for result < 0, CC = 2 for result > 0 and CC = 4 for overflow

Branch Instructions

A branch instruction can be understood as:

         If the CC has one of the following values 
          then
           go to the indicated address 
          else
           continue with the next instruction after the branch.

The two simplest branch instructions are:

         BCR     Branch on Condition Register

and

         BC      Branch on Condition

In either case, we use a mask. A mask is a binary 4-bit value expressed often in the form B'bbbb' with 4 bits b. For instance:

         B'1101' 
         B'0110'
         B'1111'

The idea of a mask is that we are providing a list of values of the CC. The 4 bits in the mask correspond, left to right, with the 4 values 0 to 3 of the CC. The bit is 1 if we want to include that value of the CC. Thus, in B'1101', we are indicating possible CC values of 0, 1 and 3.

By the way, some instructions other than branches also use masks, and in some cases these are 8-bit masks instead of 4-bit masks.

The formats of these instructions are:

         BCR   mask,R

and

         BC    mask,D(X,B)

When the branch instruction is executed, we do the following:

• get the value of the CC, a number 0 to 3
• look at the corresponding bit in the mask
• if that bit in the mask is 1, then execution continues at the indicated address (the value of R or D(X,B))
• if the bit is 0, execution continues with the next nstruction after the branch instruction

Example

         CR    7,8
         BCR   B'1011',5

Here the CR will set the CC:

If the values in registers 7 and 8 are equal, we will have CC = 0. As the leftmost bit in the mask is 1, we will jump to the address in register 5.

If the value in register 7 is less than the value in register 8, we will have CC = 1. As the corresponding bit in the mask is 0, we will do nothing and continue with the instruction next after the BCR.

If the value in register 7 is greater than the value in register 8, we will have CC = 2. As the corresponding bit in the mask is 1, we will jump to the address in register 5. Notice that in this example we never have CC = 3.

How are these encoded?

If you check the yellow card, you will find that BCR is a type RR instruction and BC is a type RX instruction. In each case, we insert the value of the mask into the machine instruction in place of the first register.

For instance:

         BCR   B'1010',7

is encoded as

         07 A7

Special Cases

Suppose the mask is B'1111', as in:

         BCR   B'1111',14

As the CC can only be 0 to 3 and all four bits in the mask are 1, we will certainly jump to the address in register 14. This is an unconditional branch. (Modern high-level languages usually do not allow these any more.)

Suppose the mask is B'0000' as in:

         BCR   B'0000',0

In this case, all the bits in the mask are 0 and we will certainly not jump anywhere. This is a "do nothing" or "no operation" instruction. It is actually irrelevant which register we provide in it.

It is not actually essential to write the mask in binary. We could write it in decimal or in hexadecimal:

         BC    8,ENDLOOP

or

         BC    X'A',THERE

Most of us probably find it easier to understand the meaning of the mask in binary, but it is more characters to type. To make life easier, we have a set of abbreviations called extended mnemonics. You can find a list of these in the yellow card. For instance:

• BE means "BC with a mask of B'1000'"
• BNL means "BC with a mask of B'1011'"
• BOR means "BCR with a mask of B'0001'"

We actually use one of these to end our programs:

         BR    14

means

         BCR   B'1111',14

Other Branch Instructions

There are several other branch instructions which are much more specialized. We will sooner or later run into some of these:

• BCT Branch on Count

• BCTR Branch on Count Register

• BAL Branch and Link

• BALR Branch and Link Register