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Delta Forth .NETWorld’s first Forth compiler for the .NET platform
Valer BOCAN, PhD
Agenda What is Forth anyway? Reverse Polish Notation Stack-based programming Delta Forth .NET Demonstration
What is Forth anyway? Procedural, stack-based language that
is compiled and interpreted at the same time
Developed by Chuck Moore at the US National Radio Astronomy Observatory in the early 1970s
Procedures are called words that are grouped in vocabularies.
Forth is a meta-language, as new words can form new specialized Forth-like languages
Where is Forth used? Popular for developing embedded
systems. Implemented on RISC processors Open Firmware boot ROMs used by
Apple, IBM, and Sun Microsystems First stage boot controller for FreeBSD Software that controlled the robotic arm
that repaired the Hubble Space Telescope
Implementing Forth 50-70% of a typical Forth
implementation is written in… Forth Implementation effort is small; a single
developer can create a Forth system on a new platform in months
Delta Forth .NET was implemented in just 5 weeks
Agenda What is Forth anyway? Reverse Polish Notation Stack-based programming Delta Forth .NET Demonstration
Reverse Polish Notation (RPN) Invented by Charles Hamblin in 1950’s
to enable zero-address memory stores In postfix notation, the operators follow
their operands, i.e. 3 + 5 becomes 3 5 +
RPN obviates the needs for parentheses otherwise required by infix notation
Reading from left to right, calculations can occur as soon as an operator is read
RPN Sample
2
5
+
7
10
4
-
*
6
42
Take infix expression (2 + 5) * (10 – 4) This translates to postfix expression 2 5
+ 10 4 - * See how the postfix expression is
evaluated using a stack-based algorithm
Agenda What is Forth anyway? Reverse Polish Notation Stack-based programming Delta Forth .NET Demonstration
Stack-based programming Unusual programming style, close to
machine Forth uses two stacks:
Parameter stack, used to transmit parameters between words
Return stack, used to hold the return address of the word caller and some counter for loop functions
There are some powerful primitives to handle the stacks
Forth primitives (1/5) DUP – duplicates the topmost element
on the stack
33
20
9
33
33
20
9
DUP
Forth primitives (2/5) DROP – drops the topmost element on
the stack
33
20
9
20
9
DROP
Forth primitives (3/5) SWAP – swaps the two topmost
elements on the stack
33
20
9
20
33
9
SWAP
Forth primitives (4/5) OVER – duplicates the second topmost
element on the stack
33
20
9
20
33
20
9
OVER
Forth primitives (5/5) ROT – rotates the three topmost
elements on the stack
33
20
9
33
20
10
9
ROT10
Agenda What is Forth anyway? Reverse Polish Notation Stack-based programming Delta Forth .NET Demonstration
Delta Forth .NET Sequel of the Delta Forth for Java project
(1997) C# command-driven application This dialect is not interpreted, the
generated code is fully managed Supports Forth at the .NET consumer level
(i.e. no new .NET classes can be derived) Supports interoperability with other .NET
libraries
Hello World in Forth Perhaps the most odd looking Hello
World out there: main \ Entry point
."Hello World"
;
Forth Sample (1/2) Finding prime numbers (up to 400 in
this case)400 constant limit
: isprime \ Returns 1 if the number on top of stack is a prime number 2 begin over over mod 0= 0= rot rot dup >r over 2 / > 0= rot and r> swap while 1+ repeat over 2 / >;
: main \ Entry point
."Prime numbers up to " limit . .": " limit 1 do i isprime if i . space then
loop;
Forth Sample (2/2) Euclid’s algorithm
4330 constant num1 \ The first number8235 constant num2 \ The second number
( Word to test if value on top of stack is equal to or less than 0 ): ZeroLessEqual dup 0= swap 0< or;
: gcd (num1 num2 - - -) over ZeroLessEqual if (num1 is <= 0) drop drop else dup ZeroLessEqual if (num2 is <= 0) drop drop else begin over over = if (We've got the result) . else over over > if
swap then over - 0 then until then then;
: main \ Entry point ."GCD of " num1 . ." and " num2 . ." is " num1 num2 gcd cr;
Agenda What is Forth anyway? Reverse Polish Notation Stack-based programming Delta Forth .NET Demonstration
Questions?
www.bocan.ro/[email protected]