Why does Forth reserve two cells per variable?

Question

While trying to find out how Forth manages the dictionary (and memory in general), I came across this page. Being familiar with C, I have no problem with the concept of pointers, and I assume I understood everything correctly. However, at the end of the page are several exercises, and here I noticed something strange.

Exercise 9.4, assuming DATE has been defined as a VARIABLE, asks what the difference is between

DATE .

and

' DATE .

and exercise 9.5 does the same using the user variable BASE.

According to the supplied answers, both phrases will give the same result (also with BASE). Trying this with Win32Forth however, gives results with a difference of 4 bytes (1 cell). Here is what I did:

here . 4494668  ok
variable x  ok
x . 4494672  ok
' x . 4494668  ok

Creating another variable gives a similar result:

variable y  ok
y . 4494680  ok
' y . 4494676  ok

Thus, it looks like each variable gets not just one cell (for the value), but two cells. The variable itself points to where the actual value is stored, and retrieving the contents at the execution token (using ' x ?) gives 0040101F for both variables.

For exercise 9.5, my results are:

base . 195F90  ok
' base . 40B418  ok

These are not even close to each other. The answer for this exercise does however mention that the results can depend on how BASE is defined.

Returning to normal variables, my main question thus is: why are two cells reserved per variable?

Additionally:

Since only one cell contains the actual value, what do the contents of the other cell mean?
Is this specific to Win32Forth? What happens in other implementations?
Is this different for run-time and compile-time variables?
How do answers to the above questions apply to user variables (such as BASE)?

EDIT1: Okay, so Forth also stores a header for each variable, and using the ' gives you the address of this header. From my tests I would then conclude the header uses just one cell, which does not correspond to all the information the header should contain. Secondly, according to the exercise retrieving the address of a variable should for both cases give the same result, which appears to contradict the existence of a header altogether.

My gut feeling is that this is all very implementation-specific. If so, what happens in Win32Forth, and what should happen according to the exercise?

There is much more reserved. Most Forth words (including variables) have a complete header, with fields pointing to the previous word, to the name of the variable, (and usually that name is also stored in the header), to the code that is executed when the variable is referenced, and to the data cell. And then there is the data cell itself (or more than one, depending on what was allotted when the variable was created). Tick (') returns the address of the code that is executed for the word that follows. This can even be different for interpreted code and for compiled code... — Rudy Velthuis
... if you reference the word, like DATE itself, the code for the variable puts the data address on the stack. So there is a big difference between ' DATEand DATE itself. — Rudy Velthuis
@RudyVelthuis Then where is the header data stored? One cell certainly isn't enough. Also, why does the exercise then say the addresses are the same? — Mattenii
Depends on implementation. The Starting Forth you are referring to describes just one exact Forth implementation. — Phil Krylov

Lars Brinkhoff Lars Brinkhoff · Accepted Answer · 2017-08-01T17:33:17

This is roughly how a definition looks like in the dictionary using a traditional memory layout. Note that implementations may well diverge from this, sometimes a lot. In particular, the order of the fields may be different.

Link to previous word (one cell)
Flags (a few bits)
Name length (one byte, less a few bits)
Name string (variable)
Code field (one cell)
Parameter field (variable)

Everything except the code and parameter fields is considered the header. The code field usually comes right before the parameter field.

Ticking a word with ' gives you an XT, or execution token. This can be anything the implementation fancies, but in many cases it's the address of the code field.

Executing a word created with CREATE or VARIABLE gives you the address of the parameter field.

This is probably why in Win32Forth, the two addresses differ by 4 bytes, or one cell. I don't know why the answers to the exercises state there should be no difference.

Assuming BASE is a user variable, it probably works like this: Every task has its own user area in which user variables are allocated. All user variables know their specific offset inside this area. Ticking BASE gives you its XT, which is the same for all tasks. Executing BASE computes an address by adding its offset to the base of the user area.

Why does Forth reserve two cells per variable?

1 Answers