Imager::regmach(3) documents the register virtual machine used by


The register machine is a complete rewrite of the stack machine originally used by Imager::transform(), written for use by Imager::transform2().


(This document might be a little incoherent.)

The register machine is a fast implementation of a small instruction set designed for evaluating an arithmetic expression to produce a color for an image.

The machine takes as input:

An array of instructions
numeric registers
An array of numeric registers. Some registers are initialized as literals.
color registers
An array of color registers. Currently these registers aren't initialized.
input images
An array of Imager i_img pointers. The "getpn" operators read pixels from these images.

The instructions supplied each take up to 4 input numeric or color registers with a single output numeric or color register. The machine attempts to execute instructions as safely as possible, assuming that correct instructions have been provided, eg. the machine protects against divide by zero, but doesn't check register numbers for validity.

The final instruction must be a "ret" instruction, which returns the result ;)

Adding new instructions

To add a new instruction:
Add a new opcode to the enumeration in regmach.h - make sure to add comment after the enum name giving the input registers ("rX" for numeric, "pX" for color) that the instruction takes. These must be in the order that the instruction expects to take the. Put a letter (r or p) after -> to indicate the result type.
Add a case to regmach.c that executes the instruction.

The Makefile should rebuild the file, and your new instruction will be added as a function.

If you want to add a single alternative instruction that might take different argument types (it must take the same number of parameters), create another instruction with that name followed by a p. The current expression parsers explicitly look for such instruction names.

Future directions

Conditional and non-conditional jumps to implement iteration. This will break the current optimizer in Imager::Expr (and the compilers for both expression compilers, for that matter.)

Complex arithmetic (Addi suggested this one). This would most likely be a separate machine. Otherwise we'll have a very significant performance loss.


If you feed bad 'machine code' to the register machine, you have a good chance of a "SIGSEGV".


Tony Cook <[email protected]>, Arnar M. Hrafnkelsson