104 lines
4.6 KiB
Markdown
104 lines
4.6 KiB
Markdown
# basic
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**Work in progress, not finished.**
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This is a BASIC interpreter which is being written from scratch for Collapse OS.
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There are many existing z80 implementations around, some of them open source
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and most of them good and efficient, but because a lot of that code overlaps
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with code that has already been written for zasm, I believe that it's better to
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reuse those bits of code.
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Integrating an existing BASIC to Collapse OS seemed a bigger challenge than
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writing from scratch, so here I am, writing from scratch again...
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## Design goal
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The reason for including a BASIC dialect in Collapse OS is to supply some form
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of system administration swiss knife. zasm, ed and the shell can do
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theoretically anything, but some tasks (which are difficult to predict) can
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possibly be overly tedious. One can think, for example, about hardware
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debugging. Poking and peeking around when not sure what we're looking for can
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be a lot more effective with the help of variables, conditions and for-loops in
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an interpreter.
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Because the goal is not to provide a foundation for complex programs, I'm
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planning on intentionally crippling this BASIC dialect for the sake of
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simplicity.
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## Usage
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Upon launch, a prompt is presented, waiting for a command. There are two types
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of command invocation: direct and numbered.
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A direct command is executed immediately. Example: `print 42` will print `42`
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immediately.
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A numbered command is added to BASIC's code listing at the specified line
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number. For example, `10 print 42` will set line 10 to the string `print 42`.
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Code listing can be printed with `list` and can be ran with `run`. The listing
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is kept in order of lines. Line number don't need to be sequential. You can
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keep leeway in between your lines and then insert a line with a middle number
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later.
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Some commands take arguments. Those are given by typing a whitespace after the
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command name and then the argument. Additional arguments are given the same way,
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by typing a whitespace.
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### Numbers, expressions and variables
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Only 16-bit integers (unsigned for now) are supported in this BASIC. When
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printed, they're printed in decimal form. When expressing number literals, you
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can do so either in multiple forms. . See "Number literals" in `apps/README.md`
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for details.
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Expressions are accepted wherever a number is expected. For example,
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`print 2+3` will print `5`. See "Expressions" in `apps/README.md`.
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Inside a `if` command, "truth" expressions are accepted (`=`, `<`, `>`, `<=`,
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`>=`). A thruth expression that doesn't contain a truth operator evaluates the
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number as-is: zero if false, nonzero is true.
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There are 26 one-letter variables in BASIC which can be assigned a 16-bit
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integer to them. You assign a value to a variable with `=`. For example,
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`a=42+4` will assign 46 to `a` (case insensitive). Those variables can then
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be used in expressions. For example, `print a-6` will print `40`. All variables
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are initialized to zero on launch.
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### Commands
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There are two types of commands: normal and direct-only. The latter can only
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be invoked in direct mode, not through a code listing.
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**bye**. Direct-only. Quits BASIC
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**list**. Direct-only. Prints all lines in the code listing, prefixing them
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with their associated line number.
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**run**. Direct-only. Runs code from the listing, starting with the first one.
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If `goto` was previously called in direct mode, we start from that line instead.
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**print**. Prints the result of the specified expression, then CR/LF. Can be
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given multiple arguments. In that case, all arguments are printed separately
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with a space in between. For example, `print 12 13` prints `12 13<cr><lf>`
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Unlike anywhere else, the `print` command can take a string inside a double
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quote. That string will be printed as-is. For example, `print "foo" 40+2` will
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print `foo 42`.
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**goto**. Make the next line to be executed the line number specified as an
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argument. Errors out if line doesn't exist. Argument can be an expression. If
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invoked in direct mode, `run` must be called to actually run the line (followed
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by the next, and so on).
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**if**. If specified condition is true, execute the rest of the line. Otherwise,
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do nothing. For example, `if 2>1 print 12` prints `12` and `if 2<1 print 12`
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does nothing. The argument for this command is a "thruth expression".
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**input**. Prompts the user for a numerical value and puts that value in the
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specified variable. The prompted value is evaluated as an expression and then
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stored where specified. For example, `input x` stores the result of the
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evaluation in variable `x`. Before the variable name, a quoted string literal
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can be specified. In that case, that string will be printed as-is just before
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the prompt.
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