F18 Variables

Variables on the F18 are not like regular Forth variables. Usually a variable is a word returning the address of its value used along with fetch ( @ ) and store ( ! ) words. On the F18 the idiomatic approach is to have individual variable-specific fetch and store words.

Greg Bailey explains it nicely here:


Let's start with constants. To make a constant in colorForth we define a simple word that pushes a literal value and returns.

This is a good idea if we use the value in multiple places. It compiles to the following; taking two memory cells:

Retrieving the value is a call/jump to joe. If instead we inlined the literal 123 it would take one cell (four slots) plus one more slot for the  @p instruction. Besides consolidating the value to a single place in your source, you are saving space by avoiding multiple inline literals.


The tricky part comes when you want to set the value. It's been many years (decades!) since programmers regularly dealt with self-modifying code, but here we go. We define !joe directly preceding the constant in such a way as to reach in and manipulate the value in place.

It's easier to understand when looking at the assembly:

Remember that the P register points at the next word from which instructions will be fetched. Each fetch populates the I register from which instructions are then executed. So while !joe (at address 0000) is being executed, the P register already points at address 0001. The  @p drop fetches and discards the contents of this address, essentially incrementing P (to 0002). Then the !p stores a value from the stack at that address and we return; having stomped on the literal value in place!


Let's put the trick to use with a fun little demo. We'll have a "ball" with an x/y position (stored in a pair of variables) and a direction (another pair) as well as a cycling color. We'll paint a little block to the console to represent the ball (note that the B register initially points to the console device and there's a protocol for setting the cursor position and color). We'll check for the ball hitting the boundaries of the console and reverse the direction; bouncing off the "walls". The result is kind of interesting to watch!

Here's the source:

You may notice some little tricks in there, like dup or to produce a zero on the stack or - + - to perform subtraction. Some of these are listed in this little gem by Chuck Moore himself. We'll get into these and others in a future post.

Here's the assembly if you're curious: