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Process and State

What is a process?

A process object stores the state of some in-game object and tells the GOAL kernel how to update this object on each frame.

For example, there is a process for Jak, a process for each orb, and a process for each enemy. There is also a process for the time-of-day system and the pause menu.

In most cases, process is used as a parent type for a specific game object. For example, money (orb) is a child of process-drawable, which is a child of process. A process-drawable is a process that can be drawn as part of the drawable system.

What does a process store?

Each process stores a small amount (112 bytes) of metadata, fields from child classes, some unknown stuff, and a process heap. The process heap will automatically contain the "main thread" of the process, which contains space to back up the stack and registers when the thread suspends. You may also allocate objects on the process heap yourself (not supported in OpenGOAL yet).

How is a process run?

The process class is a child of process-tree, which is a left-child right-sibling binary tree. On each frame, the kernel iterates through the *active-pool* and runs each process. Each run consists of three steps:

  • Run the trans-hook of the process in a temporary stack.
  • Resume the main thread of the process.
  • After the main thread suspends, run the post-hook.

How do I create a process?

Setting up a process requires three steps:

  • Getting an actual process object
  • "Activating" a process so it will be run by the kernel
  • Setting up the code for the process to run

There are a few "dead pools" which contain process objects that are not in use. The *4k-dead-pool* contains processes that are 4kb each. There is also a dynamic pool called the *nk-dead-pool* that allows you to create dynamically sized processes. You must do all allocations during initialization with these processes because they automatically "shrink" their heap as small as possible. Also, *nk-dead-pool* processes will be relocated in memory as part of the process GC system, so you must make sure that all objects on the process heap support relocation, and you must use a handle to safely refer to the process, not just a normal process reference.

For example, to get a process:

gc> (define *test-proc* (get-process *nk-dead-pool* process 1024))
#<process process dead :state #f :stack -1904/1441188 :heap 0/1024 @ #x193454>

This shows that:

  • The process name is process (just a temporary name, until we activate)
  • The status is dead
  • The process is not in a state.
  • The stack is bogus because we don't have a main thread yet.
  • We have used 0 out of 1024 bytes of our process heap.

Next, we need to activate it:

(activate *test-proc* *active-pool* 'hello *kernel-dram-stack*)

This means:

  • We put it in the *active-pool*. We could specify another process in the *active-pool* if we wanted this to be a child process of an existing process.
  • Our name is 'hello.
  • When we run code, it will run on the *kernel-dram-stack*.

Now, if we (print *test-proc*) we will see:

#<process hello ready :state #f :stack 0/256 :heap 384/1024 @ #x193454>

Indicating that we are "ready" to be initialized, and that we now have a correctly set up main thread/stack.

If we run inspect, it will print out all objects on the process heap, including our main thread:

[001934c4] cpu-thread
name: code
process: #<process hello ready :state #f :stack 0/256 :heap 384/1024 @ #x193454>
previous: #f
suspend-hook: #<compiled function @ #x1679c4>
resume-hook: #<compiled function @ #x167b24>
pc: #x0
sp: #x170b30
stack-top: #x170b30
stack-size: 256
rreg[7] @ #x1934e8
freg[8] @ #x193520
stack[0] @ #x193540

If we want a reference to this process, we must create a handle. For example:

gc> (process->handle *test-proc*)
#<handle :process #<process hello ready :state #f :stack 0/256 :heap 384/1024 @ #x192fe4> :pid 2>

this is now a safe reference to this process, even if it is relocated or deactivated.

How do I make a process do something?

The state system is used to control a process. Each process can be in a state, which specifies what functions should run. To switch states in the current process, use go.

For example, we can create a simple test state like this:

(defstate test-state (process)
:enter (lambda () (format #t "enter!~%"))
:exit (lambda () (format #t "exit!~%"))
:trans (lambda () (format #t "trans!~%"))
:post (lambda () (format #t "post!~%"))
:code (lambda ()
(dotimes (i 5)
(format #t "Code ~D~%" i)

The code is the function to run in the main thread. This code should suspend itself, and the kernel will resume it after the suspend on each frame. Once the process is done, it can call process-deactivate. This will cause it to exit the current state, immediately exit the code, and clean up the process, returning it to the dead pool.

To switch the process to this state, you can use the run-now-in-process to switch to the test process and run the given code.

(run-now-in-process *test-proc* (lambda () (go test-state)))

And you will see:

Code 0

Code 1

Code 2


Note 1: After deactivation, the handle is no longer valid as the process is dead and it will print like this:

#<handle :process #f :pid 2>

Note 2: There is also a run-next-time-in-process that sets up the process to run your initialization stub function as the code on the next time the kernel iterates through the process tree.

Some notes on "the current process"

When the kernel runs a process, it sets (-> *kernel-context* current-process) and the pp register to that process. This process is called the "current kernel process".

This process may then "run code in another process". This can be done with run-now-in-process, by deactivating another process, or using go on another process. This changes pp, but not the kernel context. The process in pp is called the "current pp process".

The value of the pp register determines the current process.

Some notes on process-deactivate

To stop a process, you can do call the deactivate method of that process. The process-deactivate macro just does this for the current process.

This does the following:

  • Set state to dead-state.
  • Calls entity-deactivate-handler, if you have an entity
  • Calls exit of states
  • Cleans up any pending protect-frame (calling them with pp set for the process)
  • Disconnects it from the connection system
  • Deactivates all children process
  • Returns itself to the pool
  • If you deactivated the process that the kernel-dispatcher started running, immediately bail out of the thread
  • If you deactivated during a run-now-in-process, immediately bail out of the initialization and return to caller of run-now-in-process.

Some notes on go.

The go macro is used to change the state of the current process.

If you use go when in run-now-in-process, it will immediately return to the caller of run-now-in-process, and the actual state change will happen on the next execution of the main thread of that process.

If you use go-process on another process, the go-process will return immediately and the state transition will happen on the next run of that process.

If you use go in the main thread, it will immediately transition states, run exits, enter, trans, and begin running the new state code.

If you use go in trans it will set up the next run of the main thread, then abandon the current trans. If you use go in post, it will set up the next run of the main thread to transition, but not abandon the current post.