Given this simple view of a system’s hardware organization and operation, we can begin to understand what happens when we run our example program. We must omit a lot of details here that will be filled in later, but for now we will be content with the big picture.

Initially, the shell program is executing its instructions, waiting for us to type a command. As we type the characters “./hello” at the keyboard, the shell program reads each one into a register, and then stores it in memory, as shown in this Figure.

"Reading the hello command from the keyboard".

When we hit the enter key on the keyboard, the shell knows that we have finished typing the command. The shell then loads the executable hello file by executing a sequence of instructions that copies the code and data in the hello object file from disk to main memory. The data include the string of characters “hello, world\n” that will eventually be printed out.

Using a technique known as direct memory access (DMA), the data travels directly from disk to main memory, without passing through the processor. This step is shown in this Figure.

"Loading the executable from disk into main memory."

Once the code and data in the hello object file are loaded into memory, the processor begins executing the machine-language instructions in the hello program’s main routine. These instructions copy the bytes in the “hello, world\n” string from memory to the register file, and from there to the display device, where they are displayed on the screen. This step is shown in this Figure.

"Writing the output string from memory to the display."

In Nutshell : When you type the command ./hello into the shell, the shell program reads each character from the keyboard, stores it in memory, and waits for you to press enter. Once you do, the shell uses direct memory access (DMA) to load the executable hello file from the disk directly into main memory without involving the processor. After the code and data are loaded into memory, the processor executes the instructions in the hello program, copying the "hello, world\n" string from memory to the register file and then to the display device, where it appears on the screen.