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basic: document better

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Virgil Dupras 2019-11-21 17:03:46 -05:00
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@ -11,10 +11,6 @@ reuse those bits of code.
Integrating an existing BASIC to Collapse OS seemed a bigger challenge than
writing from scratch, so here I am, writing from scratch again...
The biggest challenge here is to extract code from zasm, adapt it to fit BASIC,
not break anything, and have the wisdom to see when copy/pasting is a better
idea.
## Design goal
The reason for including a BASIC dialect in Collapse OS is to supply some form
@ -28,3 +24,66 @@ an interpreter.
Because the goal is not to provide a foundation for complex programs, I'm
planning on intentionally crippling this BASIC dialect for the sake of
simplicity.
## Usage
Upon launch, a prompt is presented, waiting for a command. There are two types
of command invocation: direct and numbered.
A direct command is executed immediately. Example: `print 42` will print `42`
immediately.
A numbered command is added to BASIC's code listing at the specified line
number. For example, `10 print 42` will set line 10 to the string `print 42`.
Code listing can be printed with `list` and can be ran with `run`. The listing
is kept in order of lines. Line number don't need to be sequential. You can
keep leeway in between your lines and then insert a line with a middle number
later.
### Numbers, expressions and variables
Only 16-bit integers (unsigned for now) are supported in this BASIC. When
printed, they're printed in decimal form. When expressing number literals, you
can do so either in decimal (`42`), hexadecimal (`0x2a`) or binary (`0b101010`).
Expressions are accepted wherever a number is expected. For example,
`print 2+3` will print `5`. Expressions can't have whitespace inside them and
don't support (yet) parentheses. Supported operators are `+`, `-`, `*` and `/`.
Inside a `if` command, "truth" expressions are accepted (`=`, `<`, `>`, `<=`,
`>=`). A thruth expression that doesn't contain a truth operator evaluates the
number as-is: zero if false, nonzero is true.
There are 26 one-letter variables in BASIC which can be assigned a 16-bit
integer to them. You assign a value to a variable with `=`. For example,
`a=42+4` will assign 46 to `a` (case insensitive). Those variables can then
be used in expressions. For example, `print a-6` will print `40`. All variables
are initialized to zero on launch.
### Commands
There are two types of commands: normal and direct-only. The latter can only
be invoked in direct mode, not through a code listing.
**bye**. Direct-only. Quits BASIC
**list**. Direct-only. Prints all lines in the code listing, prefixing them
with their associated line number.
**run**. Direct-only. Runs code from the listing, starting with the first one.
If `goto` was previously called in direct mode, we start from that line instead.
**print**. Prints the result of the specified expression, then CR/LF. Can be
given multiple arguments, separated with `,`. In that case, all arguments are
printed separately with a space in between. For example, `print 12 13` prints
`12 13<cr><lf>`
**goto**. Make the next line to be executed the line number specified as an
argument. Errors out if line doesn't exist. Argument can be an expression. If
invoked in direct mode, `run` must be called to actually run the line (followed
by the next, and so on).
**if**. If specified condition is true, execute the rest of the line. Otherwise,
do nothing. For example, `if 2>1 print 12` prints `12` and `if 2<1 print 12`
does nothing. The argument for this command is a "thruth expression".