Controller and Virtual Machine Module

This document is the first draft of an description of Bardolph’s core module that interprets a script and accesses the lights. It serves as documentation of some of the program logic, as well as design notes for certain parts of the code.

If you are trying to use Bardolph, this document won’t be of much use to you. It covers only the internals of the code.

The module builds on a very simple virtual machine that executes a program built out of a narrow set of instructions, described below.

VM Architecture

The VM has a set of special-purpose registers. They are:

  • hue

  • saturation

  • brightness

  • kelvin

  • duration

  • first_zone

  • last_zone

  • power

  • name

  • operand

  • pc (program counter)

  • result

  • time

  • unit_mode

Although each register is expected to have a specific type, in practice each one is a Python variable and can reference any object.

Units Mode

Both the parser and VM maintain a current units mode. All of the numbers that appear in the LS code also appear in the generated instructions.

Any numerical values that move from register to register are untouched as they are copied. Movement between two variables is also done with no processing.

If a value is moved between a variable and a register, a conversion may take place. Registers always contain raw values, but variables can contain either type. If the parser or VM is in logical units mode, then any value moved out of or into a register must be converted.

Here is an example that illustrates this behavior:

units logical
assign x 50
brightness x          # converted: brightness is 32767

units raw
assign x brightness   # x = 32767
units logical         # no change: x = 32767
assign x brightness   # x = 50

Operand Register

Many instructions can be applied to either a light, group, or location. By setting the operand register, you specify what any subsequent command should be applied to. Defined values for this register are light, group, and location.

The content of this register often specifies the meaning of the contents of the “name” register, which could be a name of a light, a group, or a location.


Although no assembler is available, it’s convenient to think of a VM’s program as a set of machine instructions. An instruction contains an op-code and maybe parameters. The list of instructions, which would be considered mnemonics in an assembly language, is represented by Enum bardolph.vm.OpCode.

This section covers some of the instructions that I needed to document for myself while working on the VM.

Move - move and moveq

These are the basic instructions for moving data between registers and variables. In a move instruction, the source and destination can each be a variable or a register. The VM determines the appropriate action by examining the Python type information for param0 and param1.

In a moveq instruction, param0 always contains a literal value that the VM will copy directly from the program code to the destination.

The destination in param1 can be either a string or an instance of Register. If the destination is a string, it is interpreted as the name of a variable, and the value is assigned to that variable. If the destination is a Register object (which is an Enum), the destination will be the VM’s corresponding register.

In the case of a move instruction, param0 is assumed to contain a reference to a value, as either a string or an instance of Register. If the source is a Register, the VM copies the content of the corresponding register to the destination. If the source is a string, it is treated as the name of a variable, and the variable is dereferenced to get the value.

With respect to the destination in param1, the move instruction has the same behavior as moveq: a string is treated as a variable name, and a Register object refers to a VM register.

In any case, the VM’s CallStack resolves variable names. If a variable is the destination, the CallStack checks to see if that variable is in the current stack frame. If so, the value in the stack frame gets replaced.

If a destination variable name is not in the top stack frame, a value is added to it, effectively creating a local variable. If the name is present in the top stack frame, its value is replaced.

If a variable is a source, the VM first looks for it in the top stack frame. If that symbol is not there, the VM then checks the globals. If the symbol isn’t in either dictionary, an error has occurred. The parser should catch that error and report it; if it doesn’t, there’s a bug in the parse code.

Set Color - color

To execute the color command, the VM reads the values from its hue, saturation, brightness, and kelvin registers to assemble a color for the target device. If the operand register contains light, the name register is assumed to contain the name of a light. Correspondingly, if operand contains “group” or “location”, the name register will be treated as the name of a group or location. Lastly, if operand contains “all”, the VM will set all known lights to that color.

Get Color - get_color

This command retrieves current color information from lights themselves and sets the registers accordingly. The affected registers are hug, saturation, brightness, and kelvin.

The “operand” register determines the source of the color data. If it contains light, the “name” register is assumed to contain the light’s name, and the colors are retrieved from light with that name. If the “name” register is empty, all lights are examined, and the arithmetic mean of each setting is stored in the registers.

If the “operand” register contains group or location, then the registers receive the arithmetic mean of the lights belonging to that group or location.

Jump to Address - jump

In order to keep the code relocatable, all jump instructions have relative addresses. To effect the jump, the VM adds an offset, which may be negative, to the pc register.

Each instruction has a condition that controls the behavior of the jump. Those conditions are defined in the JumpCondition enum in vm/ In this context, the term “truthy” describes an object for which the Python bool() function would True.

  • ALWAYS: jump unconditionally. Add param1 from the instruction to the pc


  • IF_FALSE: if the result register is not truthy, add param1 to the pc


  • IF_TRUE: if the result register is truthy, add param1 to the pc


  • INDIRECT: jump unconditionally, but treat param1 as the name of a variable

    and get the offset by dereferencing that variable.

Set Power - power

Similar to the color instruction, power relies on the operand and name registers to determine which lights to turn on or off. The content of the power determines whether to turn the lights on or off. Technically, to remain consistent with the LIFX API, this should be either 0 or 65535. However, the VM will interpret any non-zero or non-False value to mean turn the lights on, and will send 65535 to the lights. As with the set command, the targetd lights are specified by the content of the operand register.

Pause for Keypress - pause

Display a message on the console, and wait for the user to press a key. If they press !, the script will continue to run and ignore any subsequent pause instructions. Pressing ‘q’ stops the execution and exits. Any other key resumes normal execution of the script.

Discover Lights - disc, discm

Discover the lights on the network. The operand register determines what to look for: lights, groups, or locations. Each mnemonic has a different purpose:

  • disc - start discover. Takes no parameters, and uses the content of the operand register to choose lights, groups, or locations.

  • dnext - get next element in whatever list is being traversed, again determined by the content of the operand register. The first parameter contains the current element.

  • discm - start discovering members within a group or location. Takes one parameter, which is the name of a group or a location, as specified by the operand parameter.

  • dnextm - get the next element within a group or location. The first parameter is the name of the group/location, and the second parameter is the current member.

All Lights

To perform some process on all lights:

  1. set operand register to lights.

  2. disc command.

  3. The result register now contains the name of the current light in the iteration.

  4. dnext instruction with the current light name as the first parameter.

  5. Repeat until the result register contains None.

Iterate Groups and Locations

To iterate over all of the groups:

  1. set operand register to group

  2. disc command.

  3. The result register now contains the name of the current group in the iteration.

  4. dnext instruction with the current group as the first parameter. The next group gets put into the result register.

  5. Repeat until the result register contains None.

To iterate within a group:

  1. set operand register to group

  2. discm with the name of a group in the first parameter.

  3. The result register contains the name of the current light in the iteration.

  4. dnextm instruction with the group as the first parameter and the current light in the second parameter. The next group gets put into the result register.

  5. This iteration process continues until the result register contains None.

To access locations: to iterate locations, use a process similar to the one above, but put location into the operand register.


Wait for the given delay to expire. The time register can contain the delay, expressed in milliseconds. If the time register contains a time pattern, then the VM idles until the system time matches the pattern.


A variable can exist in local or global scope. When one is created inside a routine definition, it exists in local scope and hides any global variable of the same name.

Parsing Variable Usage

In all cases, param0 is a string containing the name of the variable.


  1. In source code, reach an “assign” command.

  2. Get the name of the variable, in the next token.

  3. Add the variable to the current call context.

  4. and use it as param1. Note that in all cases, the name of the variable is in param1.

  5. Get the next token, which contains the value for the variable.

  6. If the value is a macro or literal, generate a moveq instruction with the actual value in param0. If the value is a register, generate a move instruction with an instance of Register in param0. If the value resolves to variable, generate a move instruction where param0 is a string containing the name.

Executing Variable Usage

In a moveq instruction, param0 is aways considered to be a literal value, including when it is a string.

With this instruction, the VM examines the Python type of param1. If it’s a string, param1 is considered to be the name of the destination variable. If it is of type Register, the destination is the VM’s associted register.

In a move instruction, either parameter can be a string. In all cases with this instruction, a string is considered a variable name. Either parameter can also be an instance of Register. Because param0 and param1 can both be either a Register or a string, there are 4 permutations of source/destination types.

When a variable is assigned a value, it is added to the dictionary of variables at the top of the call stack. This means that any existing value gets replaced, and new variables are created automatically.

If the currently executing code is not within a routine, the top of the call stack will effectively point to the root frame, which contains the global variables.


A macro is distinguished from a variable as it is resolved at compile time. Its value is embedded directly into the instruction. At this point, variables are unimplemented.

Sequence: #. In source code, reach define statement for value, which can be a string, number, or time pattern. #. Save the value of the macro in the call context’s globals.


Although other names are available, such as “method” or “function”, for this project, the term “routine” refers to a chunk of code that can be invoked.

A routine definition contains a list of parameter names that also defines their order. Because call instructions use the name of a routine, the loader in the VM bears the responsibility of transforming that name to the entry point of the routine.

Call Context

The call context is used by the parser. The purpose of the call context is to provide information about symbols at compile time. This includes a Symbol’s name, its type and possibly its value.

The global section of the context contains routine and macro definitions. These values can be resolved at compile time. The context also has a stack, which handles parameters and their scope.

Within a routine’s code, occurances of name tokens yield symbol look-ups. Given an arbitrary string, the call context can tell whether that symbol exists, and if it does, what its type and possibly its value are.

A symbol of type macro has a concrete value at compile time, which can be put directly into param0 of the VM param instruction. If a name resolves to a symbol of type param, then param0 gets a Symbol, also of type param, with a name but no value.

Upon exit, the stack is popped and the routine’s parameters go out of scope.

Stack Frame

The stack frame is used by the virtual machine. It tracks return addresses for when routines exit, and manages parameters.

Within the code, various move instructions copy data from parameters into VM registers. In these instructions, the “source” in param0 contains a Symbol of type var. The value for this parameter is available from the currrent routine’s stack frame, at the top of the stack, or in the global symbol table.

That stack frame is populated by zero or more param instructions, each with a name and a value. Prior to the routine call, those instructions cause parameters to be accumulated in a dictionary, which serves as an activation record. The param instructions are immediately followed by a call command. A new stack frame with that activation record gets pushed on top of the stack, where it can be accessed by move instructions in the current routine’s code. The VM then creates a new staging dictionary for any nested routine calls.

Upon exit, the stack frame is popped. The dictionary representing the activation record should be empty at this point. The stack should never be empty; in all cases, at least the root frame must be present.

Before any routines are called, the stack has a single stack frame which represents the root, or global frame. Any effort to resolve a variable name first checks the top of the stack. If the name isn’t found, the call stack then checks the root frame.

Parsing a Routine

Because nested routine definitions will not be allowed (at first), the call context should never have a stack longer than one, which means it’s not really a stack. It’s just toggling between main code and routine definitions.


  1. In the source, reach a define statment with name and optional parameter list. If parameters are present, put their names into the current call context. The order in which they are added determines their order in calls to the routine.

  2. Push the call context.

  3. Add routine instruction with name.

  4. Code - For data access, the top call context tells whether a name is a parameter or macro. If’s a parameter, then use a move instruction with the parameter’s name. Otherwise, use moveq and put the macro’s literal value into the instruction. Obtain that constant value from the call context.

  5. Generate end instruction.

  6. Pop call context.

  7. Store Routine object in call context globals.

Parsing a Call to a Routine

Each parameter to a routine call can be a literal (number, string, or time stamp), a reference to a macro, or a Symbol.

  1. For each parameter in the routine definition’s list, generate a param instruction.

  2. Generate a call instruction containing the routine’s name in a string.

To set the value of a parameter, a param instruction holds the name of the parameter in param0, and param1 contains the parameter itself. In the case of a literal, the value can be put directly into param0 in the instruction. For a macro, the name can be resolved through the call context and its value put into param0.

If the parameter is of type var, then param1 in the generated param instruction is an instance of Symbol. During execution, upon detecting that param1 contains a Symbol, the VM will attempt to resolve it, first in the call stack, then in its globals.

Running Code

The output of the parser contains code that is executed immediately, with routine definitions mixed in. The loader puts the immediate code the main segment while collecting the routine code in a routine segment.

Layout of a program after it has been loaded:

  1. jump instruction to main segment.

  2. Routine code.

  3. Main code.

With this layout, the program terminates when the VM finishes executing the last instruction.


  1. Read instructions into main code segment.

  2. routine instruction.

    1. New Routine object.

    2. Zero or more param instructions - add them to Routine.

    3. Save entry point to Routine

    4. Copy instructions into routines segment.

    5. Add Routine object to symbol table for globals.

  3. end instruction - continue with copy into main segment.

  4. Combine segments into a jump instruction followed by a single list. Because the main segment follows the routine segment, the address for the jump instruction is equal to the length of the routine segment.


  1. Initialize by creating staging stack frame.

  2. Start at entry point. Interpret until at end.

  3. param instruction: put value into staging stack frame.

  4. call instruction

    1. Push staging frame onto stack (creates a new one as current).

    2. Retrieve Routine object from globals.

    3. Jump to routine address.

    4. Continue executing.

    5. move instruction: param0 contains the name of a variable. Use the stack frame to find the value of that variable and move it into the register specified by param1.

    6. moveq instruction: param0 contains the actual value. Put that value directly into the register specified by param1.

  5. end instruction

    1. Get return address from top context.

    2. Pop context off stack.

    3. Jump to return address.


A counting loop has the syntax:

repeat <iteration model>
with <variable>
from <starting value> to <ending value>

Note that

If iteration_model is omitted, the loop is considered infinite, and repeats until the VM stops executing the code:

# Repeat until the VM is told to stop executing the code.
repeat begin
   hue 120 set all
   hue 180 set all

The iteration_model can be a symbol, constant, or arithmetic expression, indicating a discreet number of iterations. The generated VM code evaluates the limit once, before beginning the loop:

# Execute the code 5 times.
repeat 5 begin

define five 5
repeat five begin
    # ...

# Execute the code 3 times.
repeat {five - 2} begin

Iterating over Lights

(Not implemented, yet. Currently undergoing development) Lastly, the iteration can occur over a set of lights, locations, or groups. This type of iteration has one of the following syntaxes:

repeat <name> in all
    # do something

repeat name in groups
    # do something

repeat name in locations
    # do something

repeat name in <light set>
    # do something

In the last case, the <light_set> placeholder can be replaced with one or more lights, groups, and locations, connected by and.

repeat the_light in "Top" and "Middle"
    on the_light

repeat the_light in "Middle" and "Top" and group "Furniture"
    on the_light

The lights are traversed using the order in which they appear in the code. For example, the top repeat first turns on the light “Top”, and then “Middle”. In the lower loop, they are turned on in the opposite order.

Within each group, the lights are traversed in alphabetical order of their names. This guarantees that the order will always be the same.

As an example, to reduce the brightness of all lights by 10%:

repeat light in all
    get light
    brightness {brightness * 0.9}
    if {brightness < 0.1}
        brightness 0
    set light

Range of Values

The addition of with sets up a kind of index variable that is updated with each loop. The limits given indicate what the first and last desired values are. Using that and the number of repetitions, the VM evenly divides the range and sets the varaiable to the interpolated values. For example, to evenly bring up all the lights from 0% to 100%,

# Do 10 iterations and distribute the values of brt so that they
# are spread evenly between 0 and 100.
repeat 10 with brt from 0 to 100 begin
   brightness brt
   set all

The term cycle indicates that the index variable will start at the given point, and go through one complete rotation of 360 degrees.

repeat 10 with the_hue cycle 180
   hue the_hue
   set all

In this exmple, the_hue starts with a value of 180. It is then incremented 10 times. At the end of the last iteration, the_hue contains the value that comes immediately before 180.

Loop Frame

A LoopFrame is a specialized StackFrame that is used with loops. Inside a loop, some variables go into scope, but none become hidden. Therefore, a LoopFrame inherits all of the variables contained in its parent frame. This is done by making a copy of the dictionary containing the parent frame’s variables. When the loop frame exits, no variables go out of scope.

The index variable remains in scope after the loop exits. At that point, it contains the value it had during the final iteration. As such, it exists as a local variable in the current CallContext. The index variable is handled by the generated code, with no specific VM support.

The loop counter and its limit are not visible to the script code after they have been initialized. They are attributes of the top LoopFrame.

Job Scheduling

The controller maintains an internal queue of scripts to execute. When a script completes, the job scheduler moves on to the next one and launches it. The process executing the script runs in a separate thread.

By default, when script finishes, the sceduler discards it. When the queue is empty, the scheduler effectively becomes idle. However, if “repeat” mode is active, completed scripts are immediately added to the end of the queue. The effect of this is to repeatedly execute all the scripts indefinitely until a stop is requested.


Aside from access to lights, I/O has been deliberatley absent. A small VmIo module enables simple output to stdout.


Output is accomplished with one of the following commands:

  • print: print a single value, followed by a space.

  • println: print a single value, followed by a newline.

  • printf: formatted output with zero or more parameters.

For example:

print brightness
print {saturation / 100}
printf "Currently: {} {} {} {}\n" hue saturation brightness kelvin

Note that print can take only one value, while printf takes an arbitrary number. Because the number of parameters depends on the format string, that string must be either a literal or macro, known at compile time.

Support for the format specifier should be fairly complete, including placement by name:

printf "Currently: {kelvin} {} {} {}\n" hue saturation brightness kelvin
printf "Currently: {1} {0} {3} {2}\n" hue saturation brightness kelvin

In terms of data format, all numeric values are floats in RGB and logical mode, and integers in raw mode. Light names are strings. Any variables and register names can appear within the format specifier, and expressions are anonymous:

printf "Currently: {kelvin} {} \n" {brightness / 100.0} kelvin

VM Implementation

All access to the I/O module is done via the out instruction. If I ever add input, it will likely be with an in instruction. The format of an out instruction is out <target> <payload>. The <target> parameter can be one of:

  • IoOp.UNNAMED: the payload is a chunk of data to be output as an unnamed


  • IoOp.NAMED: the payload is the value to associate with the name contained

    in that string. For example, if the string is “x”, then the value of the variable x is to be output. This may include register names, such as kelvin or brightness.

  • IoOp.PRINT: the payload is sent to stdout via Python’s print() function.

  • IoOp.PRINTF: the payload is a string containing a format specfier that will

    be passed to the str.format() method. The accumulated data is output.

For example, this script:

print kelvin
printf "Data: {x} {kelvin} {} {}" x kelvin 5 saturation

could yield the following instructions:

out IoOp.PRINT Register.KELVIN

out IoOp.NAMED "x"
out IoOp.NAMED Register.KELVIN
out IoOp.UNNAMED 5
out IoOp.PRINTF "Data: {kelvin} {} {}"