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Welcome

About Us

About You

  • Experience Levels
  • Backgrounds
  • Goals

What Are We Doing Today?

  • Use Forth to understand low level computer concepts.
  • Focus will be Arduino, but the concepts apply to all computers and languages.
    • Intentionally focusing on small / old comuters to reduce conceptual load
  • There will be A few lab experiments.

Dry Run Timer

Let's start that now.

History

  • Chuck Moore
  • First Forth projects
  • Rise of popularity in the early 80's
  • Characteristics (procedural, concatenative, untyped)
  • Forth today

Common Use Cases for Forth

  • Firmware
  • Resource constrained applications
  • Firmware
  • And occasionally Firmware

Advantages

  • Even the tiniest devices can run Forth- easily ported to new systems.
  • Very resource efficient (minimal memory requirements)
  • "Low Floor, High Ceiling"
  • Neat: Jones Forth

Real World Forth

Why Bother?

  • It's fun!
  • Less tooling and setup than C
  • The only language a developer can completely understand quickly.
  • Makes you a better programmer
  • Influences your understanding of other languages.
  • Low level concepts are easier to understand.
  • If you can dodge a wrench, you can dodge a ball.

How Does Forth Compare to X?

This

Forth Minimalism

  • No Garbage Collector
  • No Floating Point Numbers (sometimes)
  • No Type System or Type Checker (everything is an integer)
  • Compiled and interpreted!

The Computer We Will Use Today

http://www.microchip.com/wwwproducts/en/ATmega328P

Detour: What is A Computer, Really?

  • Processor
  • Memory
  • Input lines
  • Output lines
  • Other lines (power, reset, clock, etc.)

http://cs-alb-pc3.massey.ac.nz/notes/59304/l15.html

What Is Memory?

A computer must "remember" the data it is working with.

Memory is like a spreadsheet for integers. Memory has many addresses that hold values.

Address Value (decimal)
0 24
1 13
2 27
3 255
4 37
5 0
6 42
7 86

Arduino Memory

Kind Size
Flash 32 kB
RAM 2 kB
ROM 1 kB

Symbols and Signals

Computers require symbolic thinking.

Instead of... Let's say...
A 65
65 1011
1011 high, low, high, high
high, low, high, high A

At a low level, Computers only understand numbers!

All concepts must be symbolized as numbers or sequences of numbers to be useful.

What Is a Processor?

Performs calculations and actions using contents of memory.

Example: Read address, write address, execute address.

We will come back to this in a moment.

What About Input / Output Lines?

Convert voltage fluctuations to numbers which are usually stored in memory.

That's it. Keyboards, monitors, WiFi cards all work like this.

https://sites.google.com/site/lofturj/cwreceive

What's going on inside the chip?

What Just Happened?

  1. Computer senses an incoming signal
  2. Computer stores signal value in a memory address
  3. Computer processes the value, often storing the result in a new address
  4. Computer writes to certain special addresses, that cause signals to be sent back out (output)

More Examples of I/O Operations

Memory operations in the old days: https://www.youtube.com/watch?v=vAhp_LzvSWk

Programming Languages

  • Now that we understand the hardware, let's focus on human computer interaction.
  • Any questions on what we've covered so far?
  • We will cover machine language and assembly, but only to the extent needed for Forth.

If you are a programmer, but have never worked in a language with manual memory management (C, C++, Forth, Rust, Fortran, etc.), some of these concepts will be new to you.

Computer Languages and the "Instruction Cycle"

Remember when we said that computers can only operate on numeric symbols?

The processor follows this sequence, over and over:

  1. Fetch: Get a value from a memory address.
  2. Decode: Determine what the number symbolizes (Read, Write, Jump, etc.)
  3. Execute: Perform the operation. Ex: "Add contents of cell 1 and cell 2".
  4. Repeat: This cycle is repeated continually. Some architectures feature a HALT instruction.

The lowest level computer language is Machine Language.

Machine Language Example

Not (easily) human readable. Can be directly placed into memory without any changes.

Address Instruction Description
0 00000001 Turn pin 1 (connected to LED bulb) on
1 00000010 sleep for 2 seconds
2 00000001 Turn pin 1 off
3 00000010 If pin2 is off, skip next line
4 00000000 Halt
5 01010101 Jump to address 0

Real world example: https://www.computerhope.com/jargon/p/punccard.htm

Credit: https://chortle.ccsu.edu/java5/Notes/chap04/ch04_4.html

Assembly Language

One up from machine language is assembly language.

Like machine language, it has a one-to-one correspondence to the computer's instruction codes.

Unlike machine language, it uses pneumonic based (human readable) codes to represent each instruction.

An assembler is required to translate a pneumonic code to machine language.

NEXT: See an arcade game written in assembly.

Snake Game Written in Assembly

  • Someone wrote a snake arcade game using 6502 assembly.
  • Move the snake via W, A, S, D keys
  • Be sure to see the "notes" section.

High Level Languages (HLLs)

  • With the exception of firmware developers, this is what everyone uses today (C, Java, etc.)
  • Still need to exist as machine language, but do so without a 1-to-1 correspondence to machine language.
  • Can use concepts that are easier for humans to understand.
    • Can use words instead of numbers.
    • Don't always need to think in terms of memory addresses and data lines.
set myUrl to "http://google.com"
display dialog myUrl

Recap

  • Numbers can represent real data (values) or a symbol representing an instruction.
  • The real world interacts with software by sending signals to input lines, causing changes to values in memory.
  • Software interacts with the real world by sending signals to output lines.

Start Learning Forth Now

We now have enough understanding of the hardware to move forward.

Any questions about hardware before we continue?

Lab Setup

  • Terminal
  • Text editor
  • Check that all the expected words are there
  • Install a "whoops" marker
  • Talk about lecture/lab format

Funny Calculators

Let's do some math

  • Actually called the "data stack", since it is not the only stack.
  • You never need parenthesis.
  • Copy/pasting code between locations is easier.
  • The Stack is not an abstraction on most architectures. It exists on "bare metal".
  • It's easy to go backwards or "backtrack" your work.

NEXT: Let's do math in Forth

Do Basic Math

  • Use the enter key to "push" a new number on the stack. Use spaces as a delimiter.
    • 1 2 3 <enter>
  • Use the period character to "pop" the top off the stack and view it. (.)
    • 4 5 6 . . . prints 6 5 4
  • Perform math via +, -, *, and / (integer division only)
    • 2 + 2 is 2 2 + . in forth

About (FIFO) Stacks

diagram

  • First in, first out.
  • The primary data structure in Forth.

The Serial Line

  • Interactive programming and debugging
  • Easier setup because you don't need complicated "tool chains" or cross compilers.
  • Everything resides on the host.
  • Most systems are case insensitive (not ours).

Mandatory "Hello, World!"

In forth, functions and methods are called "words"- reusable sequences of code. Let's make our Hello, world! code more DRY*

  • The space between ." and Hello is significant!
\ Backslash is a comment in Forth
\ Defines a new "dictionary entry" of `hello`.
: hello
  \ Print to serial line
  ." Hello, world!"
;

How many words are there?

See a list of words by using the words word.

You can enter a search term after words. Ex: words " returns ."

Markers and The Dictionary

  • Dictionary: A linked list of word definitions.
  • Markers: Set a "bookmark" in the dictionary when you need to reqind.

Naming Rules

  • All characters except space and carriage return are legal in Forth word names.
  • You will see words with symbols in them frequently. Eg: ., !, @.

Lab: Add6 Word

SCENARIO: You find yourself adding the number 7 to other numbers very often.


: add7 7 + ;
12 add7 \ Pushes `19` on stack.

Stack Juggling

  • Sometimes, the stack is not in the order you want.
  • Some words can "rearrange" the stack:
  • NEXT: All the stack re-arranging words
word effect hint
SWAP A B -> B A
ROT A B C -> B C A (rotate)
OVER A B -> A B A
TUCK A B -> B A B
DUP A -> A A

Let's try these now.

Comparisons and Flags

  • =, <, >, <>
  • 0 is true
  • Nonzero (usually -1) is false

...IF...ELSE...THEN...

Forth has flow control, like every other language. What is flow control, really?

NEXT: Example

: 7?
  7 = IF
    ." IT IS 7!"
  ELSE
    ." NOPE!"
  THEN ;

LAB: What do 7 7? and 8 7? print? TODO Age calculator app

Stack Comments

  • Without documentation, things get hard to read.
  • Luckily, we have standards for this stuff.

More About Words

  • Words can be redefined
  • Words must be defined before they can be used
  • FUN FACT: You could theoretically create words for numbers like 7, but don't do that.

Loops


  begin <CODE_TO_EXECUTE> <TEST_CONDITION> until

Example: Launch Timer

: second 1000 * ms ;
: speak ( n -- n ) dup 1 second . ;
: decrement 1 - ;
: less_than_zero? 0 < ;
: blastoff! ." Blastoff!" ;
: countdown ( n -- )
  begin
    speak
    decrement
    dup             \ Copy value for next loop
    less_than_zero?
  until ;

: launch ( n --- ) countdown blastoff! ;

Not Covered

There are other ways to loop:

RECURSE, BEGIN, WHILE, AGAIN, UNTIL, REPEAT

Variables

TODO

Next steps (part II?)

  • "big" Forths
  • Execution tokens
  • Vectored execution
  • Compile vs. interpret
  • inner vs. outter interpreter
  • Defining words
  • Tasks
  • Vocabularies
  • Dictionary structure
  • recursion
  • Stephen's Book
  • Pictured Numeric Input

I/O

  • LAB: "Hello, {{ name }}"
  • LAB: "It's over {{ 9000 }}"
  • LAB: Create a new marker
  • LAB: Create a constant
  • LAB: Create a variable
  • LAB: Read a variable
  • LAB: Create a FOO+! word
  • LAB: Create a FOO@ word
  • LAB: Can/Can't drink beer
  • LAB: 3 2 1 blastoff
  • LAB: Blink
  • LAB: Push to blink
  • LAB: PWM write
  • LAB PWM read
  • LAB: Fire alarm