A Fork in the Road

A computer processor will just execute instruction after instruction, all in a row as its program counter (PC) increases, unless it encounters a "jump" or "branch" instruction. A jump or branch instruction directly changes the value of the PC, and therefore it changes where the processor's next instruction will come from. Last time we talked about jump instructions, which unconditionally change the PC to whatever value the programmer wants. This allows the programmer to do things like loop back to execute again instructions that the processor has already executed. In this post we'll talk about branch instructions.

Branch instructions are forks in the road. A branch is a jump instruction with a condition attached. When the processor encounters a branch instruction one of two things will happen. Either the "branch condition" will be met (or in other words, it will be true), or it will not be true. If the branch condition is true, then the branch is "taken," meaning that the jump is performed. In this case the PC gets changed to whatever the branch instruction says it should be. If the branch condition is false, and therefore fails, the branch is "not taken," and the PC is just incremented like normal to the next instruction (PC+1).

There are many kinds of branches, but they all involve comparing two things against each other, and then seeing if some property about that comparison is true. Here are some common types of branches, their associated computer instruction shorthand version, and what they mean in English. The shorthand all starts with "b" for "branch," and the remaining letters give you a hint about what kind of comparison the branch instruction is performing. I'm also going to write two variable names, x and y. Treat them the same as you did in algebra. They're just two numbers, ANY two numbers. Their value isn't important (just like algebra).

beq x,y - branch when x is equal to y
bez x - branch when x equals zero
bgt x,y - branch when x is greater than y
bge x,y - branch when x is greater than or equal to y
blt x,y - branch when x is less than y
ble x,y - branch when x is less than or equal to y
bgtz x - branch when x is greater than or equal to zero
bltz x - branch when x is less than or equal to zero

A lot of those are redundant if you think about it. For example, asking if "x is greater than y" is the same as asking if "y is less than or equal to x." There's no real reason why we need both versions, but having both might make it so that reading computer instructions is easier if you need to debug the code after writing it.

The next post will have a brief example of branching in action, showing a loop from a program that I just made up!