It’s been both - was stack, converted to register[0][1].

[0] https://www.sqlite.org/vdbe.html

[1] https://www.sqlite.org/src/info/051ec01f2799e095

This was the question I was going to ask! I've recently been diving into Lua 5.x internals and have been extremely impressed with Lua's register-based bytecode implementation. Lua has a stable byte code interface between patch releases (5.4.0 -> 5.4.1, etc) but not between major/minor revisions (5.3 -> 5.4). SQLite on the other hand does not consider this a public interface at all!

> "Remember: The VDBE opcodes are not part of the interface definition for SQLite. The number of opcodes and their names and meanings change from one release of SQLite to the next." https://www.sqlite.org/opcode.html#the_opcodes

For anyone interested in understanding how a register-based VM operates I highly recommend:

A No-Frills Introduction to Lua 5.1 VM Instructions by Kein-Hong Man. https://www.mcours.net/cours/pdf/hasclic3/hasssclic818.pdf

Does SQLite's VM have an API? I mean, one where I can build and issue opcodes into directly.