collapseos/apps/basic

basic

Work in progress, not finished.

This is a BASIC interpreter which is being written from scratch for Collapse OS. There are many existing z80 implementations around, some of them open source and most of them good and efficient, but because a lot of that code overlaps with code that has already been written for zasm, I believe that it's better to 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...

Design goal

The reason for including a BASIC dialect in Collapse OS is to supply some form of system administration swiss knife. zasm, ed and the shell can do theoretically anything, but some tasks (which are difficult to predict) can possibly be overly tedious. One can think, for example, about hardware debugging. Poking and peeking around when not sure what we're looking for can be a lot more effective with the help of variables, conditions and for-loops in 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.

Some commands take arguments. Those are given by typing a whitespace after the command name and then the argument. Additional arguments are given the same way, by typing a whitespace.

Numbers, expressions and variables

Numbers are stored in memory as 16-bit integers (little endian) and numbers being represented by BASIC are expressed as signed integers, in decimal form. Line numbers, however, are expressed and treated as unsigned integers: You can, if you want, put something on line "-1", but it will be the equivalent of line 65535. When expressing number literals, you can do so either in multiple forms. See "Number literals" in apps/README.md for details.

Expressions are accepted wherever a number is expected. For example, print 2+3 will print 5. See "Expressions" in apps/README.md.

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. In that case, all arguments are printed separately with a space in between. For example, print 12 13 prints 12 13<cr><lf>

Unlike anywhere else, the print command can take a string inside a double quote. That string will be printed as-is. For example, print "foo" 40+2 will print foo 42.

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".

input. Prompts the user for a numerical value and puts that value in the specified variable. The prompted value is evaluated as an expression and then stored where specified. For example, input x stores the result of the evaluation in variable x. Before the variable name, a quoted string literal can be specified. In that case, that string will be printed as-is just before the prompt.