7.3. The Little Computer - Branching

Normally, a computer works its way through the instructions in memory in order. After instruction 1, it runs instruction 2, then 3, etc… A branch instruction can change that. A branch might tell the computer “skip ahead to memory location number 20 and execute that” or “go back and execute the instruction at location 1 again”.

Decisions

Branch instructions are how a computer can make decisions. In high-level languages, we generally use “if” statements to make decisions about which instructions to execute. At a low level, those decisions are usually implemented as branches to skip certain instructions.

In the Little Computer, an instruction that starts with 6, 7, or 8 is a branch.

This video demonstrates a Little Computer program that makes a decision based on the input it gets:

You can test that program using the link below. Since it behaves differently if you enter a positive or negative number, make sure to test it with both:

Launch the Little Computer branch program (Control or Command click to open in a new window)

Looping

The target of a branch does not need to be later in the program - it is perfectly legal to branch to an earlier instruction. This is how we make a computer “loop” or repeat a series of instructions. This video demonstrates a Little Computer program that uses a branch instruction to loop through a series of instructions.

You can test that program using the link below. As you do, pay attention to each output - the program outputs more than one value, so the behavior is best described by the pattern of the output, “It counts down from 1 to 0”, than its final output. We would not say that this program just “outputs 0”.

Launch the Little Computer loop program (Control or Command click to open in a new window)

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