Espruino Interpreter Internals

Please see the Performance section first for a rough overview, and for the practical implications of the implementation.

Note: There's also a forum thread with some more in-depth answers to questions about the interpreter.

Compilation

Espruino's Makefile calls a few different Python scripts that precompute various things:

Parsing

There is no bytecode, so execution happens inside the parser in jsparse.c

The parser is a hand-written recursive-descent parser, and code to be executed is executed directly from variables (see below), not from a C-style string or flat buffer.

Variable Storage

Variables are usually stored in fixed-size 16 byte blocks. In small devices this can get down to 12 bytes (10 bit addresses) and on PCs it's 32 bytes (32 bit addresses).

This has several implications:

Each block has optional links to children and siblings, which allows a tree structure to be built. However in many cases these references aren't needed and can be used to store other data.

What follows are the basic variable bytes. Note that NAME_INT_INT, NAME_INT_BOOL, and NAME_STRING_INT follow the same pattern as NAME_STR/NAME_INT - they just use child to store a value rather than a reference:

16 byte JsVars

( > 1024 variables, JsVars for 32 bit refs are similar )

Offset Name STRING STR_EXT NAME_STR NAME_INT INT DOUBLE OBJ/FUNC/ARRAY ARRAYBUFFER
0 - 3 varData data data data data data data nativePtr size
4 - 5 next data data next next - data argTypes format
6 - 7 prev data data prev prev - data argTypes format
8 - 9 first data data child child - - first stringPtr
10-11 refs refs data refs refs refs refs refs refs
12-13 last nextPtr nextPtr nextPtr - - - last -
14-15 Flags Flags Flags Flags Flags Flags Flags Flags Flags

12 byte JsVars

( where < 1024 variables )

10 bit addresses are used, with the extra bits being stored in a field called pack and the flags variable

Offset Name STRING STR_EXT NAME_STR NAME_INT INT DOUBLE OBJ/FUNC/ARRAY ARRAYBUFFER
0 - 3 varData data data data data data data nativePtr size
4 next data data next next - data argTypes format
5 prev data data prev prev - data argTypes format
6 first data data child child - data first stringPtr
7 pack pack data pack pack pack data pack pack
8 refs refs data refs refs refs refs refs refs
9 last nextPtr nextPtr nextPtr - - - last -
10-11 Flags Flags Flags Flags Flags Flags Flags Flags Flags

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