Daily Forth Lesson. Edition 1

[Mitch Bradley]

== Review ==

Yesterday we learned that:

# The top-level parser only looks for whitespace, so Forth words must be 
separated by one or more spaces
# Execution proceeds from left to right.  Get a word, execute it.

Today we will start trying some commands.

== Getting a Forth Interpreter ==

If you have an OLPC board or system, power it up and interrupt the boot 
countdown to get to an ok prompt.
Otherwise, get http://firmwork.com/linux/forth.zip , extract it on an 
x86 Linux system, and type "./forth".  It is better to use an OLPC 
system if possible, because you will be able to access hardware 
directly, which you can do easily under an operating system.  That won't 
matter for the first few lessons, though.

== The Stack ==

Forth has an explicitly visible stack that is used to pass numbers 
between words (commands).  Using Forth effectively requires you to think 
in terms of the stack.  That can be hard at first, but as with anything, 
it becomes much easier with practice.

Forth also has a secondary stack - the "return stack" - that is used for 
return addresses and a few other things.  The Forth system automatically 
manages the return stack so you don't have to think about it for the 
most part (but you can get at it if you need to; Forth doesn't block you 
from doing anything).

== Displaying the Stack ==

  ok  showstack

will cause Forth to show you what is on the stack before every "ok" 
prompt.  "noshowstack" turns it off.  With showstack mode on, if the 
stack contained the numbers 5678 and  112233, you would see:

   5678 112233 ok

as the prompt.  The number to the right is the top of the stack.

I recommend running with showstack mode on while you are learning.  (If 
showstack is off, and you want to display the stack contents once, type 
".s".)

== Putting Numbers on the Stack ==

If you type a number, it gets pushed onto the stack:

  ok 12345 998877
  12345 998877 ok

What we see here is that the two numbers did get pushed onto the stack, 
and since showstack mode was on, the stack contents were displayed to 
the left of the ok prompt.

== First Words ==

  12345 998877 ok +
  9aabbc ok

The "+" word pops two numbers from the stack, adds them, and pushes them 
back on the stack.  Note that:
# + doesn't display the result, it just pushes the result back on the 
stack.  In this case, however, we are assuming that "showstack" mode is 
on, so the interpreter displayed the stack for us automatically before 
the next prompt.
# The numbers are in hexadecimal.  Forth can operate in any base up to 
36.  The default base in Open Firmware is hex, whereas when Forth is 
running under an OS, the default is decimal.  You can change the base at 
any time by typing "hex" or "decimal".  All of the examples in this 
tutorial series will use hex as the default base.
# The current number base applies to both number input and number output.

== Displaying Numbers ==

  9aabbc ok noshowstack
  ok

Now the stack display is gone, but the number is still there.

  ok .s
  9aabbc
  ok .s
  9aabbc
  ok

".s" displays the entire stack, non-destructively.

  ok .
  9aabbc
  ok .s
  Empty
  ok showstack
  ok

"." pops the top of the stack and displays that number.  Afterwards, the 
number is no longer there.  There are variations of "." (with different 
spellings) that display in specific bases, with specific formatting, 
signed/unsigned, etc.  Those will be covered later.

== How the Interpreter Works ==

The Forth interpret is very simple.  I just does this over and over:

# Read a line of input
# While there is more data in the line:
## Parse a whitespace-delimited word
## Lookup the word in a list of defined words. If found, execute the 
code for that word.
## Otherwise, try to interpret the word as a number in the current 
number base.  If so, push it on the stack.
## Otherwise display an error message

Thus endeth the lesson.