Wheeler Jump facts for kids

The Wheeler Jump is a special way that some very old computers used to remember where to go back to after they finished a specific task. Imagine you're reading a book and you need to quickly check a definition in a dictionary. You'd put your finger on the page you're on, go to the dictionary, find the word, and then come back to exactly where your finger was. The Wheeler Jump helped computers do something similar!

This clever idea was created by a computer scientist named David Wheeler in the 1950s. He was working on one of the first computers ever, called EDSAC. Back then, computers weren't designed to easily jump to a small piece of code (called a subroutine) and then automatically return to the exact spot they left off. They didn't have special memory spots or "notes" to save the return address.

How the Wheeler Jump Worked

David Wheeler's solution was a smart way to write the computer's instructions. Here's the basic idea:

• Setting up the Subroutine: When a programmer wrote a subroutine, the very last instruction was a "jump" command. This command usually tells the computer to go to a specific memory location. But for the Wheeler Jump, this location was temporarily set to a fake number, like zero. Think of it as a blank space waiting to be filled.

• Calling the Subroutine: When the computer needed to use this subroutine, it first put the address of where it wanted to return (the "return address") into a special temporary storage area called the accumulator. Then, it would jump to the beginning of the subroutine.

• Inside the Subroutine: The first few instructions inside the subroutine would quickly figure out the correct return address based on what was in the accumulator. It would then write this real return address into that "blank space" at the end of the subroutine.

• Returning Home: Once the subroutine finished its work, it would reach that last "jump" instruction. Since the blank space was now filled with the correct return address, the computer would jump right back to where it left off before calling the subroutine!

Why It Was Needed

This method was very important for early computers like EDSAC because they didn't have the advanced parts that modern computers have. Today's computers have special "registers" or "stacks" that make saving return addresses super fast and easy.

However, the Wheeler Jump wasn't perfect. Writing information to memory (like filling in that blank space) was slower than using those special registers. Also, this method couldn't handle something called recursion. Recursion is when a subroutine calls itself, which is a more complex task. Newer computers, like EDSAC 2, were designed with better ways to handle these jumps and returns.