Starting Forth by Leo Brodie

• I disregarded much conventional wisdom in order to include exactly the capabilities needed by a productive programmer. The most important of these is the ability to add whatever capabilities later become necessary.
• One principle that guided the evolution of Forth, and continues to guide its application, is blunty: Keep it simple.
• Simplicity provides confidence, reliability, compactness, and speed.
• Forth is a very unusual language. It violates many cardinal rules of programming.
• Actually a computer language should not be difficult to understand. Its purpose is simply to serve as a convenient compromise for communication between a person and a computer.
• A high-level language looks the same to a programmer regardless of which make or model of computer it’s running on.
• A language should be designed for the convenience of its human users, but at the same time for compatibility with the operation of the computer.
• Forth begins with a powerful set of standard commands, then provides the mechanism by which you can define your own commands.
• The structural process of building definitions upon previous definitions is Forth’s equivalent of high-level coding.
• Forth offers a simple means to maximize a processor’s efficiency.
• In Forth, a defined command is called a “word”.
• The ability to define a word in terms of other words is called “extensibility”.
• Extensibility leads to a style of programming that is extremely simple, naturally well organized, and as powerful as you want it to be.
• One of Forth’s many unique features is that it lets you “execute” a word by simply naming the word.
• Forth is called an “interactive” language because it carries out your commands the instant that you enter them.
• Words must be separated by at least one space for Forth to be able to recognize them as words and/or numbers.
• A Forth application, when listed, consists of a series of increasingly powerful definitions rather than a sequence of instructions to be executed in order.
• Each word and its definition are entered into Forth’s “dictionary”.
• In general, computers perform their operations by breaking everything they do not  ridiculously tiny pieces of information and ridiculously easy things to do.
• Forth uses “postfix” notation rather than “infix” notation so that all words which “need” numbers can get them from the stack.
• When all operators are defined to work on the values that are already on the stack, interaction between many operations remains simple even when the program gets complex.
• In general, the only accessible value at any given time is the top value.
• To convert to postfix, simply move the operator to the end of the expression.
• The word SWAP is defined to switch the order of the top two stack items.
• When writing equations in Forth, it’s best to “factor them out” first.
• You can define the same word more than once in different ways--only the most recent definition will be executed.
• The word FORGET looks up a given word in the dictionary and, in effect, removes it from the dictionary along with anything you may have defined since that word.
• Only the compiled form of your definition is saved in the dictionary.
• All Forth systems use disk memory.
• The compiler compiles all dictionary entries into computer memory so that the definitions will be quickly accessible.
• Disk memory is divided into units called “blocks”. Each block holds 1024 characters of source, or binary data, traditionally organized as 16 lines of 64 characters.
• The words that follow IF are executed if the condition is true. The words that follow THEN are always executed.
• The words 0=, 0< and 0> expect only one value on the stack. The value is compared with zero.
• Forth allows you to provide an alternate phrase in an IF statement with the word ELSE.
• IF will take any non-zero value to mean true.
• All comparison operators return well formed flags.
• As your applications become more sophisticated, you will be able to write statements in Forth that look like postfix English and are very easy to read.
• The parameter stack holds parameters (or “arguments”) that are being passed from word to word.
• The return stack, however, holds any number of “pointers” which the Forth system uses to make its merry way through the maze of words that are executing other words.
• “Floating point representation” is a way to store numbers in computer memory using a form of scientific notation.
• You should note carefully that when a processor supports hardware floating-point, it is almost always much faster and more compact than the fixed-point equivalent.
• It turns out that you can approximate nearly any constant by many different pairs of integers, all numbers less than 32768, with an error less than 10^-8.
• The user may keep values on the return stack temporarily, under certain circumstances.
• Conditional branching is one of the things that make computers as useful as they are.
• The ability to perform loops is probably one of the most significant things that make computers as powerful as they are.
• LEAVE causes the loop to end immediately.
• This bizarre-seeming method for representing negative values makes it possible for the computer to use the same procedures for subtraction as well as addition.
• The stack can be used to hold either signed or unsigned numbers. Whether a binary value is interpreted as signed or unsigned depends on the operators that you apply to it.
• In Forth, variables are appropriate for any value that is used inside a definition that may need to change at any time after the definition has already been compiled.
• One use for constants is to name a hardware address.
• Forth programmers tend to follow this convention: when possible, words should destroy their own parameters.
• In general, it’s better to put DUP inside the “calling definition” then the “called” definition.
• Forth systems provide ways to manage the dictionary by organizing words into vocabularies.
• Execution of a definition will stop when KEY is encountered until an input character is received.
• Programming in Forth is more of an “art” than programming in any other language.