86 lines
6.3 KiB
Markdown
86 lines
6.3 KiB
Markdown
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# i swear this is what fuzzy actually does
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## the stack
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fuzzy works on a 16-bit cell-width, zero-page data stack indexed with the x register, as documented in Garth Wilson's [stack treatise](https://wilsonminesco.com/stacks/virtualstacks.html)
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to push a byte onto the data stack, we just:
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```asm
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dex ; decrement the stack pointer
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lda some_value ; load the byte we want on the stack into a
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sta 0, x ; put the byte on the stack!
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```
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and to pop a byte off it:
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```asm
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lda 0, x ; pop the top of stack off into a
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inx ; increment the stack pointer
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```
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## types
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these are used in word definitions, and refer to the type of an individual stack cell:
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| type | desc |
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| ---------------------- | ----------------------------------------------------------- |
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| **bool** | a boolean value, represented by $0000 or $ffff |
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| **nat** | an unsigned 16-bit integer |
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| **int** | a signed 16-bit integer |
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| **char** | an 8-bit george-ascii character, padded with leading zeroes |
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| **string** | a 16-bit pointer to a string in memory |
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| **word** _`dangerous`_ | a 16-bit pointer to a fuzzy word or quotation |
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## operators
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- `!` NOT: applies NOT to tos
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- `&` AND: pops 2 off the stack and pushes the AND'ed result
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- `|` OR: pops 2 off the stack and pushes the OR'ed result
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- `+` add: pops 2 off the stack and pushes the sum
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- `-` subtract: pops 2 off the stack and pushes the difference
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- `*` multiply: pops 2 off the stack and pushes the result, truncating if it's >$FFFF
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- `/` divide: pops 2 off the stack and pushes the remainder and quotient
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- `=` equality: pushes true/false if the top 2 stack cells do/don't match
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- `>` greater than: pushes true/false if tos-1 is/isn't greater than tos
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- `<` less than: pushes true/false if tos-1 is/isn't greater than tos
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- `#` quote _`dangerous`_: pops tos and pushes a word that produces its value
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### supported types (this will need to be more clearly laid out later)
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| operator | input type | output type | notes |
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| -------- | ------------------------ | ------------------------ | ------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
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| `!` | `bool`, `nat`, `int` | `bool`, `nat`, `int` | |
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| `&` | `bool`, `nat`, `int` | `bool`, `nat`, `int` | |
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| `\|` | `bool`, `nat`, `int` | `bool`, `nat`, `int` | |
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| `+` | `nat` `nat`, `int` `int` | `nat`, `int` | |
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| `-` | `nat` `nat`, `int` `int` | `nat`, `int` | subtracting two `nat`s |
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| `*` | `nat` `nat`, `int` `int` | `nat`, `int` | most products will be truncated, since most 16 bit multiplications result in a >16 bit product, but in practice that shouldn't matter cause we're not doing science |
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| `/` | `nat` `nat`, `int` `int` | `nat` `nat`, `int` `int` | produces two cells, the quotient and remainder |
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| `=` | any any | `bool` | equality/order is checked based on stack cell value, not type (e.g. a `word` pointing to $abcd and a `nat` with the value $abcd are equivalent) |
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| `>` | any any | `bool` | see above |
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| `<` | any any | `bool` | see above |
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| `#` | any | `word` | _`dangerous`_ |
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## `danger!`
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the `danger!` keyword marks a word as being _`dangerous`_. certain language features can only be used in dangerous words, such as:
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- inline assembly
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- quotations
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- typechecking quotations is a difficult problem & probably too complex too implement on george if we ever want to fully self-host fuzzy
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- unchecked operator usage
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- applying `+` to two chars, applying `&` to two strings, etc
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- this does not mean that _dangerous_ words are untyped! just the type of the result of an operation is asserted to be the word result type
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- `danger! dangerous_word num num is char: +` can't be used on a `num char` stack, and any words used after `dangerous_word` treat the top of the stack as having a `char` and don't care that it was made with two `num`s
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the program body cannot use any _dangerous_ features. this makes it so that _dangerous_ behavior is contained to specific words.
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## memory layout
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| start | end | use |
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| ------ | ------ | ---------------------------- |
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| `$200` | `$300` | |
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| | | core language implementation |
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| | | core language implementation |
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