Awesome
basic_rs
: a BASIC Interpreter/Compiler for the Original Dartmouth Version
A BASIC language interpreter written in rust
. This project is motivated and inspired by Peter Norvig's BASIC interpreter in python, reading that notebook helped me tremendously.
Overview
The repo contains an interpreter and a couple compilers of the original Dartmouth BASIC language.
- Main crate
basic_rs
implements the frontend of BASIC (scanner, parser, ast), and a VM-based interpreter - Crate
basic2wasm
compiles BASIC to Web Assembly using binaryen (INPUT
statement only works with console output, due to lack of blocking IO in browser environment)- example: wasm Game of Life from BASIC source, see it running here
- README
- Crate
basic2js
compiles BASIC to JavaScript (using generator functions for asyncINPUT
handling) - Crate
basic2rs
, compiles BASIC torust
source code, and to subsequently native code withrustc
(click below for BASIC to rs in action)
Calling the last two "compilers" is a bit disingenuous, as they are source-to-source transpilers, and produce strings rather than any kind of runnable machine code. This is primarily a learning project, to get myself familiar with compiler constructions and optimizations. However, I will continue to add tests and bug fixes and strive to make this a solid BASIC implementation.
Features and Limitations
Matches first version of Dartmouth Basic closely: reference manual here, which means this implementation inherits all its limitations.
- No input support other than
DATA
statements in source program- [Update] Added
INPUT
statement support in #28 - Not sure what the official syntax for
INPUT
is, but statements like10 INPUT "Prompt" X, Y
works fine - This makes at least some vintage BASIC using only number inputs playable
- [Update] Added
- No string / boolean value types, only value type is
f64
- Variable names restricted to
[A-Z]\d?
- Function names restricted to
FN[A-Z]
- List and table dimension restrictions
- [Update] Restriction since has been removed
- Otherwise supports all 15 types of statements:
LET
,READ
,DATA
,PRINT
,GOTO
,IF
,FOR
,NEXT
,END
,STOP
,DEF
,GOSUB
,RETURN
,DIM
andREM
, in addition, addedINPUT
Run Program
basic_rs ./my_program.bas
Optionally, -d
flag disassembles compiled VM byte code:
basic_rs -d ./debug.bas
Note on INPUT
Original BASIC does not have INPUT
statement, and its syntax in different implementations of BASIC later varies. I have chosen the following:
inputStatement := (label | ";" | ",")* variable ("," variable)*
variable := ident ( "(" expr ")" | "(" expr "," expr ")" )?
Essentially, an input statement is some optional prompts followed by one or more of variables (allowing array subscripting) separated by commas.
At runtime, each line is consider a single value, and empty lines are treated as 0.
Implementation Details
Compared to Norvig's implementation, the flavor of this project is more of no-hack, from-scratch approach (Norvig's version leveraged many of Python's powerful, dynamic features to get things done fast and cleverly). My goal was trying to learn how to implement a simple language as principled as I could manage.
BASIC source code is lexed, parsed and compiled into a custom stack based VM byte code (consist of instructions I made up somewhat arbitrarily along the way instead of properly designed).
VM features:
- standard stack based arithmetics
- global variables and arrays
- function call with 0 or 1 argument
- the former is used for subroutine calls
- the latter is used for user defined function calls
FNA
-FNZ
- custom IO instructions to match BASIC's weird print semantics
Some sample disassembler output. (Source of this program is sample_programs/func_redefine.bas
).
10 0000 decl.loc 2
15 0002 const 0
| 0005 set.loc $0
20 0008 const 0
| 0011 set.loc $1
25 0014 bind.fn FNZ <compiled function 0>
30 0019 bind.fn FNA <compiled function 1>
| 0024 const 10
| 0027 get.fn FNA
| 0030 call_ args: 1
| 0032 set.loc $0
| 0035 prt.lab "FNA(10) ="
| 0038 prt;
40 0039 get.loc $0
| 0042 prt.expr
45 0043 prt\n
50 0044 bind.fn FNA <compiled function 2>
| 0049 const 10
| 0052 get.fn FNA
| 0055 call_ args: 1
| 0057 set.loc $1
| 0060 prt.lab "FNA(10) ="
| 0063 prt;
| 0064 get.loc $0
| 0067 get.loc $1
| 0070 eq
| 0071 not
| 0072 jmp.t 80
70 0075 prt.lab "FAILED"
| 0078 prt\n
| 0079 ret
100 0080 prt.lab "Ok"
| 0083 prt\n
| 0084 ret
Chunk: <compiled function 0>
15 0000 decl.loc 0
| 0002 get.loc $0
| 0005 ret.val
Chunk: <compiled function 2>
40 0000 decl.loc 0
| 0002 get.loc $0
| 0005 get.fn FNZ
| 0008 call_ args: 1
| 0010 const 1
| 0013 sub
| 0014 ret.val
Chunk: <compiled function 1>
20 0000 decl.loc 0
| 0002 const 1
| 0005 get.loc $0
| 0008 add
| 0009 ret.val
WASM
See README for basic2wasm
crate.
Performance
A simple BASIC program using RND
to estimate PI via Monte Carlo method is used for benchmarking (using 1000 iterations), and compared to these implementations:
python
(benches/pi.py)nodejs
(benches/pi.js)rust
(fn
embedded in benchmark file)rust
compiled from BASIC sourcejs
compiled from BASIC source
Here are the results:
interpreter:pi.bas time: [388.12 us 389.08 us 390.18 us]
extern:pi.py time: [202.06 us 203.38 us 205.06 us]
extern:pi.js time: [19.374 ns 19.605 ns 19.869 ns]
(*fastest) rust:pi time: [296.27 ps 296.78 ps 297.42 ps]
bas:compiled_rust time: [1.1920 us 1.1988 us 1.2064 us]
bas:compiled_js time: [18.245 ns 18.317 ns 18.410 ns]
It is only 2 times slower than python
. Nodejs is four magnitude (10000 times compared to python) faster, and rust
(with target-cpu = "native"
) is pretty much on a different level. There are some minor penalties to node and python versions due to communicating via stdin/stdout. However, the pattern still holds if iteration is increased from 1000 to 1000_000 and without IO barrier.
The pi.bas
program compiled to rust
, runs in 1.2us, slower than nodejs(60x slower) and handwritten rust
version (6000x slower) by quite a bit - mostly due to:
- Using
f64
for loop counter - Inefficient control flow generated by
Relooper
(using runtime variable to encode otherwise statically known branching), potentially made it harder torustc
andLLVM
to optimize
It does run much faster than than interpreter version and a non-JIT-ed interpreted Python code.
Somewhat surprisingly, the compiled js version rust the same as hand written js version. And it is faster than the compiled rust version - even though the generated control flow is identical, I guess due to the V8's JIT magic.