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P01.1 · How computers run programs

A free, illustrated module of 4 lessons and one interactive in P01 Python from Zero: A program is a list of instructions; From source text to running program; Running Python yourself; Bits, bytes and memory.

P01.1 L0 · Foundation 4 lessons · about 100 min Live · in review

Part of P01 Python from Zero in the Programming and Data track.

What actually happens when a computer runs a program. These four lessons name the parts that do the work and trace a four-line program by hand, follow Python’s text through translation to bytecode and on to a running program, set up Python to run at its prompt and as a script, and open up memory to show that numbers, letters and the pixels of a picture are all stored as bits.

What you need first. School arithmetic and a computer on which you may install a program. No programming is assumed: this is the first module of P01 Python from Zero.

Lessons

1. A program is a list of instructions P01.1.1 25 min

   A program is a text of instructions that a computer carries out one at a time, top to bottom, exactly as written. Meet the parts that do the work, trace a four-line program by hand in a trace table, and see why swapping two lines changes what it prints.

2. From source text to running program P01.1.2 25 min

   A processor understands only its own machine instructions, so Python translates your text first: it compiles the whole file to bytecode, then its virtual machine runs that one step at a time. Worked on two broken files, one that never starts and one that stops halfway.

3. Running Python yourself P01.1.3 25 min

   Install Python, check its version and run it two ways: at the interactive prompt, a read, evaluate, print loop that shows the value of whatever you type, and as a script, which shows only what it prints. Then read a first error message from its last line upwards.

4. Bits, bytes and memory P01.1.4 25 min

   Everything in a computer’s memory is a pattern of 0s and 1s. Work binary place value by hand, see why a byte counts from 0 to 255, store letters as numbers and a grey image as a grid of numbers, and picture memory as a long row of numbered cells.

The interactive

P01.1-X1 · Line-by-line stepper. Step forward and back through a four-line program and watch the current-line marker move and the output grow one line at a time. Move a line to reorder the output, or plant a missing bracket or a misspelt name and see how far the program gets. It lives in From source text to running program.