Section: User Contributed Perl Documentation (3)
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Imager::regmach - documents the register virtual machine used by
The register machine is a complete rewrite of the stack machine
orginally used by Imager::transform(), written for use by
(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
colour for an image.
The machine takes as input:
An array of instructions
- numeric registers
An array of numeric registers. Some registers are initialized as
- colour registers
An array of colour registers. Currently these registers aren't
- 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 colour
registers with a single output numeric or colour 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
The final instruction must be a 'ret' instruction, which returns the
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 colour) 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 Regops.pm 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.
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
If you feed bad 'machine code' to the register machine, you have a
good chance of a SIGSEGV.
- Adding new instructions
- Future directions
linux.jgfs.net manual pages