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Your compiler will produce x86 assembly. We won’t transform the assembly into an executable ourselves - that’s the job of the assembler and linker, which are separate programs 2 . To see how this program looks in assembly, let’s compile it with gcc 3 :

Now, let’s look at the assembly itself. We can ignore the .section , .align and .subsections_via_symbols directives - if you delete them, you can still assemble and run return_2.s 4 . .globl _main indicates that the _main symbol should be visible to the linker; otherwise it can’t find the entry point to the program. (If you’re on a Unix-like system other than OS X, this symbol will just be main , no underscore.)

Finally, we have our actual assembly instructions:

The most important point here is that when a function returns, the EAX register 5 will contain its return value. The main function’s return value will be the program’s exit code.

An important side note: throughout this tutorial, I’ll use ATT assembly syntax, because that’s what GCC uses by default. Some online resources might use Intel syntax, which has operands in the reverse order from ATT syntax. Whenever you’re reading assembly, make sure you know what syntax it’s using!

The only thing that can change in the snippet of assembly above is the return value. So one very simple approach would be to use a regular expression to extract the return value from the source code, then plug it into the assembly. Here’s a 20-line Python script to do that:

But parsing the whole program with one big regular expression isn’t a viable long-term strategy. Instead, we’ll split up the compiler into three stages: lexing, parsing, and code generation. As far as I know, this is a pretty standard compiler architecture, except you’d normally want a bunch of optimization passes between parsing and code generation.

Lexing

The lexer (also called the scanner or tokenizer) is the phase of the compiler that breaks up a string (the source code) into a list of tokens. A token is the smallest unit the parser can understand - if a program is like a paragraph, tokens are like individual words. (Many tokens are individual words, separated by whitespace.) Variable names, keywords, and constants, and punctuation like braces are all examples of tokens. Here’s a list of all the tokens in return_2.c:

Note that some tokens have a value (e.g. the constant token has value “2”) and some don’t (like parentheses and braces). Also note that there are no whitespace tokens. (In some languages, like Python, whitespace is significant and you do need tokens to represent it.)

Here are all the tokens your lexer needs to recognize, and the regular expression defining each of them:

NRICH
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Age 7 to 18

Published May 2008,February 2011.

As the sun moves across the sky, shadows change in direction and length, so a simple sundial can measure the length of a day. It was quickly noticed that the length of the day varies at different times of the year. The reasons for this difference were not discovered until after astronomers accepted the fact that the earth travels round the sun in an elliptic orbit, and that the earth's axis is tilted at about 26 degrees. This variation from a circular orbit leads to the Equation of Time (see 'Note 2' below) which allows us to work out the difference between 'clock' time and 'sundial time'.
Another discovery was that sundials had to be specially made for different latitudes because the Sun's altitude in the sky decreases at higher latitudes, producing longer shadows than at lower latitudes. Today, artists and astronomers find many ways of creating modern sundials.
An Egyptian sundial from about 1,500 BCE is the earliest evidence of the division of the day into equal parts, but the sundial was no use at night. The passage of time was extremely important for astronomers and priests who were responsible for determining the exact hour for the daily rituals and for the important religious festivals, so a water clock was invented.
When you think about the problem - we can find due South easily from the sun at midday. Looking at the night sky, we eventually deduce that there is a fixed point in the heavens around which all the stars rotate once every day (24 hours). This is where we find the 'Pole Star' (from the Great Bear or Ursa Major, measure the distance of about four lengths of the two stars at the end, 'the pointers' to find Polaris). This is the Celestial Pole - which was different for the Egyptians from today because of the phenomenon of Precession.

Up to about 1,900 BCE the Celestial Pole was Thuban a star in the 'tail' of the constellation Draco. By 1,000 BCE it was Thuban in the constellation Ursa Minor. Today Polaris is the last star in the 'tail' of Ursa Minor.

Note 2

'Sun time' and 'clock time' are different. Sun time is based on the fact that the sun reaches its highest point (the meridian), in the middle of the day, and on the next day at its highest point, it will have completed a full cycle. However, the time between the sun reaching successive meridians is often different from clock time. According to clock time, from May to August, the day is close to 24 hours, but in late October the days are about 15 minutes shorter, while in mid February the days are about 14 minutes longer. For our daily routines, it is important to have a constant 'clock time' of 24 hours. This variation is called the 'Equation of Time' and shows the relationship between sun time and clock time. The variation has two causes; the plane of the Earth's equator is inclined to the Earth's orbit around the Sun, and the orbit of the Earth around the sun is an ellipse and not a circle. The National Maritime Museum website shows two separate graphs for these causes, and a third graph where they are combined to give the full correction.

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