From ffcd93699efdfe339187d642ee7a37e350c7d2e9 Mon Sep 17 00:00:00 2001 From: Virgil Dupras Date: Mon, 10 Aug 2020 09:44:47 -0400 Subject: [PATCH] doc: out-of-system documentation I've changed my mind about having documentation in-system. It doesn't serve much of a purpose and make blkfs significantly heavier. This commit is the first step in writing a documentation outside of the blkfs. --- doc/intro.txt | 33 ++++++++ doc/primer.txt | 201 +++++++++++++++++++++++++++++++++++++++++++++++++ 2 files changed, 234 insertions(+) create mode 100644 doc/intro.txt create mode 100644 doc/primer.txt diff --git a/doc/intro.txt b/doc/intro.txt new file mode 100644 index 0000000..ffca087 --- /dev/null +++ b/doc/intro.txt @@ -0,0 +1,33 @@ +# Introduction to Collapse OS + +Collapse OS is a minimal operating system created to preserve +the ability to program microcontrollers through civilizational +collapse. Its author expect the collapse of the global supply +chain means the loss of our computer production capability. Many +microcontrollers require a computer to program them. + +Collapse OS innovates by self-hosting on extremely tight resour- +ces and is thus (theoretically thus far) able to operate and be +improved in a world without modern computers. + +# Forth + +This OS is a Forth. It doesn't adhere to any pre-collapse stand- +ard, but is pretty close to the Forth described by Starting +Forth by Leo Brodie. It is therefore the recommended introduct- +ory material to learn Forth in the context of Collapse OS. + +If you don't have access to this book and don't know anything +about Forth, learning Collapse OS could be a rough ride, but +don't despair. There's a Forth primer in primer.txt. + +# Documentation and self-hosting + +Collapse OS is self-hosting, its documentation is not, that is, +Collapse OS cannot read this document you're reading. Text +blocks could, of course, be part of Collapse OS' blocks, but +doing so needlessly uses blocks and make the system heavier than +it should. + +This documentation is expected to be printed before the last +modern computer of your community dies. diff --git a/doc/primer.txt b/doc/primer.txt new file mode 100644 index 0000000..eb50d56 --- /dev/null +++ b/doc/primer.txt @@ -0,0 +1,201 @@ +# Forth Primer + +# First steps + +Before you read this primer, let's try a few commands, just for +fun. + +42 . + +This will push the number 42 to the stack, then print the number +at the top of the stack. + +4 2 + . + +This pushes 4, then 2 to the stack, then adds the 2 numbers on +the top of the stack, then prints the result. + +42 0x8000 C! 0x8000 C@ . + +This writes the byte "42" at address 0x8000, and then reads +back that bytes from the same address and print it. + +# Interpreter loop + +Forth's main interpeter loop is very simple: + +1. Read a word from input +2. Look it up in the dictionary +3. Found? Execute. +4. Not found? +4.1. Is it a number? +4.2. Yes? Parse and push on the Parameter Stack. +4.3. No? Error. +5. Repeat + +# Word + +A word is a string of non-whitepace characters. We consider that +we're finished reading a word when we encounter a whitespace +after having read at least one non-whitespace character + +# Character encoding + +Collapse OS doesn't support any other encoding than 7bit ASCII. +A character smaller than 0x21 is considered a whitespace, +others are considered non-whitespace. + +Characters above 0x7f have no special meaning and can be used in +words (if your system has glyphs for them). + +# Dictionary + +Forth's dictionary link words to code. On boot, this dictionary +contains the system's words (look in B30 for a list of them), +but you can define new words with the ":" word. For example: + +: FOO 42 . ; + +defines a new word "FOO" with the code "42 ." linked to it. The +word ";" closes the definition. Once defined, a word can be +executed like any other word. + +You can define a word that already exists. In that case, the new +definition will overshadow the old one. However, any word def- +ined *before* the overshadowing took place will still use the +old word. + +# Cell size + +The cell size in Collapse OS is 16 bit, that is, each item in +stacks is 16 bit, @ and ! read and write 16 bit numbers. +Whenever we refer to a number, a pointer, we speak of 16 bit. + +To read and write bytes, use C@ and C!. + +# Number literals + +Traditional Forth often uses HEX/DEC switches to go from deci- +mal to hexadecimal parsing. Collapse OS parses literals in a +way that is closer to C. + +Straight numbers are decimals, numbers starting with "0x" +are hexadecimals (example "0x12ef"), "0b" prefixes indicate +binary (example "0b1010"), char literals are single characters +surrounded by ' (example 'X'). Char literals can't be used for +whitespaces. + +# Parameter Stack + +Unlike most programming languages, Forth execute words directly, +without arguments. The Parameter Stack (PS) replaces them. There +is only one, and we're constantly pushing to and popping from +it. All the time. + +For example, the word "+" pops the 2 number on the Top Of Stack +(TOS), adds them, then pushes back the result on the same stack. +It thus has the "stack signature" of "a b -- n". Every word in +a dictionary specifies that signature because stack balance, as +you can guess, is paramount. It's easy to get confused so you +need to know the stack signature of words you use very well. + +# Return Stack + +There's a second stack, the Return Stack (RS), which is used to +keep track of execution, that is, to know where to go back after +we've executed a word. It is also used in other contexts, but +this is outside of the scope of this primer. + +# Conditional execution + +Code can be executed conditionally with IF/ELSE/THEN. IF pops +PS and checks whether its nonzero. If it is, it does nothing. +If it's zero, it jumps to the following ELSE or the following +THEN. Similarly, when ELSE is encountered in the context of a +nonzero IF, we jump to the following THEN. + +Because IFs involve jumping, they only work inside word defin- +itions. You can't use IF directly in the interpreter loop. + +Example usage: + +: FOO IF 42 ELSE 43 THEN . ; +0 FOO --> 42 +1 FOO --> 43 + +# Loops + +Loops work a bit like conditionals, and there's 3 forms: + +BEGIN..AGAIN --> Loop forever +BEGIN..UNTIL --> Loop conditionally +DO..LOOP --> Loop X times + +UNTIL works exactly like IF, but instead of jumping forward to +THEN, it jumps backward to BEGIN. + +DO pops the lower, then the higher bounds of the loop to be +executed, then pushes them on RS. Then, each time LOOP is +encountered, RS' TOS is increased. As long as the 2 numbers at +RS' TOS aren't equal, we jump back to DO. + +The word "I" copies RS' TOS to PS, which can be used to get our +"loop counter". + +Beware: the bounds arguments for DO are unintuitive. We begin +with the upper bound. Example: + +42 0 DO I . SPC LOOP + +Will print numbers 0 to 41, separated by a space. + +# Variables + +We can read and write to arbitrary memory address with @ and ! +(C@ and C! for bytes). For example, "1234 0x8000 !" writes the +word 1234 to address 0x8000. + +The word "VARIABLE" link a name to an address. For example, +"VARIABLE FOO" defines the word "FOO" and "reserves" 2 bytes of +memory. Then, when FOO is executed, it pushes the address of the +"reserved" area to PS. + +For example, "1234 FOO !" writes 1234 to memory address reserved +for FOO. + +# IMMEDIATE + +We approach the end of our primer. So far, we've covered the +"cute and cuddly" parts of the language. However, that's not +what makes Forth powerful. Forth becomes mind-bending when we +throw IMMEDIATE into the mix. + +A word can be declared immediate thus: + +: FOO ; IMMEDIATE + +That is, when the IMMEDIATE word is executed, it makes the +latest defined word immediate. + +An immediate word, when used in a definition, is executed +immediately instead of being compiled. This seemingly simple +mechanism (and it *is* simple) has very wide implications. + +For example, The words "(" and ")" are comment indicators. In +the definition: + +: FOO 42 ( this is a comment ) . ; + +The word "(" is read like any other word. What prevents us from +trying to compile "this" and generate an error because the word +doesn't exist? Because "(" is immediate. Then, that word reads +from input stream until a ")" is met, and then returns to word +compilation. + +Words like "IF", "DO", ";" are all regular Forth words, but +their "power" come from the fact that they're immediate. + +Starting Forth by Leo Brodie explain all of this in details. +Read this if you can. If you can't, well, let this sink in for +a while, browse the dictionary (B30) and try to understand why +this or that word is immediate. Good luck!