This is a tiny project-exercise about parsers, compilers and interpreters. It implements a simple descent recursive parser and a compiler for the teaching programming language https://en.wikipedia.org/wiki/PL/0. It emits assembly code directly while parsing for a fantasy computer, the [https://github.com/uri-nyx/ultima](Taleä Computer System). It is highly unoptimized, but it works!
A version for a somewhat standard PL/0 compiler can be found in the standard branch. This version implements extra features (arrays and strings) to allow self-hosting, as described by [https://briancallahan.net/blog/20210822.html](Brian Callahan), and implements a self hosted PL/0 for the Taleä Computer System (Just adding another backend to [https://github.com/ibara/pl0c](Brian Callahan's compiler)).
It takes the input from stdin, and prints it to stdout, but is easily redirectable to files. To assemble, use https://github.com/hlorenzi/customasm (though I am currently using the fork by [https://github.com/JosephAbbey/customasm](Joseph Abbey), it won't assemble with the original).
The programs rely in a minimal runtime, crt0.asm, that provides the intrinsics for input and output, some interrupt and exception handling, and initialization. All this is assembled statically to a binary file, that can be used as rom in the emulator for the Taleä System.
The grammar for PL/0 with extensions, from [https://briancallahan.net/blog/20210906.html](Brian Callahan's blog), this:
program = block "." .
block = [ "const" ident "=" number { "," ident "=" number } ";" ]
[ "var" ident [ array ] { "," ident [ array ] } ";" ]
{ "forward" ident ";" }
{ "procedure" ident ";" block ";" } statement .
statement = [ ident ":=" expression
| "call" ident
| "begin" statement { ";" statement } "end"
| "if" condition "then" statement [ "else" statement ]
| "while" condition "do" statement
| input_int [ "into" ] ident
| "readchar" [ "into" ] ident
| output_int expression
| output_char expression
| "writeStr" ( ident | string )
| "exit" expression ] .
condition = "odd" expression
| expression ( comparator ) expression .
expression = [ "+" | "-" | "not" ] term { ( "+" | "-" | "or" ) term } .
term = factor { ( "*" | "/" | "mod" | "and" ) factor } .
factor = ident
| number
| "(" expression ")" .
comparator = "=" | "#" | "<" | ">" | "<=" | ">=" | "<>" .
array = "size" number .
input_int = "read" | "?".
output_int = "write" | "!".
output_char = "writechar" | "echo".
All keywords are case insensitive, but identifiers are case sensitive.
This version is a bit different from the specification: it needs a main procedure, that will be the entry point to the program. Here is a sample:
procedure main;
begin
call factorial
end;
.end with te sequence end; . if the procedure has a begin statement. Alternatively to ? and ! for I/O, read and readchar can be used instead to read integers and characters respectively. write and writechar can be used as well to output. Comments are introduced by // and last until the end of the line.