The kOS CPU hardware — kOS documentation (2024)

While it’s possible to write some software without knowing anythingabout the underlying computer hardware, and there are good designprinciples that state one should never make assumptions about thecomputer hardware when writing software, there are still some basicthings about how computers work in general that a good programmerneeds to be aware of to write good code. Along those lines, the KSPplayer writing a Kerboscript program needs to know a few basic thingsabout how the simulated kOS CPU operates in order to be able to writemore advanced scripts. This page contains that type of information.

  • Update Ticks and Physics Ticks

  • Electric Drain

  • Triggers

    • Triggers for Cooked Steering

    • Triggers for WHEN and ON statements

    • Triggers for GUI callbacks

    • Wait in a Trigger

    • Do Not Loop a Long Time in a Trigger Body!

    • But I Want a Loop!!

  • Trigger Interrupt Priority

    • Deliberately reducing your priority in long running triggers

  • Wait!!!

  • CPU Update Loop

    • How an interrupt works

  • The Frozen Universe

Update Ticks and Physics Ticks

Kerbal Space Program simulates the universe by running the universe insmall incremental time intervals that for the purpose of thisdocument, we will call “physics ticks”. The exact length of timefor a physics tick varies as the program runs. One physics tick mighttake 0.02 seconds while the next one might take 0.021 seconds and maybethe next one takes 0.019 seconds.

The game tries to simulate the universe using 50 physics ticks persecond (0.02 seconds per tick), but there is no guarantee it succeedsat this. There is a lot of variation depending on how fast yourcomputer is, and how heavily you are loading it with large rockets orcomplex mods.

If the KSP game is unable to execute physics ticks fast enough tokeep up the 50-per-second rate, that’s when you see the time displayin the upper-left of the Kerbal Space Program screen turn red as awarning that simulation is getting coarse-grain and might startgetting error-prone because of it.

Note that the game may also resort to slowing down the presentationof the simulated world in order to make the simulated time stillbe fine-grained at 0.02 seconds per physics tick even though thecomputer can’t keep up with it. In this state it is showingthe game in slow motion. This is what it means when the clock inthe upper-left corner of the screen is yellow.

The relevant take-away from that is this: When calculating physicsformulas, never assume elapsed time moves in constant amounts. Itis typically about 0.02 seconds per physics tick, but not reliablyso. You need to actually measure elapsed time in the TIME:SECONDSvariable in any formulas that depend on delta time.

The entire simulated universe is utterly frozen during the duration ofa physics tick. For example, if one physics tick occurs at timestamp10.50 seconds, and the next physics tick occurs 0.02 seconds later attimestamp 10.52 seconds, then during all the intervening times, suchas at timestamp 10.505 seconds, 10.51 seconds, and 10.515 secondsnothing has moved. TIME:SECONDS will claim the time is still 10.50seconds during that whole time, and the fuel isn’t being consumed, andthe vessel is at the same position. On the next physics tick at 10.52seconds, then all the numbers are updated. The full details of thephysics ticks system are more complex than that, but that quickdescription is enough to describe what you need to know about how kOS’sCPU works.

Physics ticks are NOT your FPS:There is another kind of time tick called an Update tick. It issimilar to, but different from, a physics tick. Update ticksoften occur a bit more often than physics ticks. Update ticks areexactly the same thing as your game’s Frame Rate. Each time your gamerenders another animation frame, it performs another Update tick.Essentially, physics ticks get the first dibs on execution time,while update ticks use up whatever time is leftover after that.If your computer is super fast so there’s a lot of leftover timeafter physics ticks are satisfied, it just uses that time to makemore update ticks, not to make more physics ticks. A fastcomputer might have 2 or 3 update ticks per physics tick. A slowcomputer might only be able to manage 1 update tick per physicstick, or in extreme cases, less than 1 so animation is in factpainting the picture at a slower frame rate than the frame rate thatthe physical world is actually being simulated under the hood.

It is important to note that versions of kOS prior to v0.17 executedprogram code during these update ticks so they were tied to youranimation FPS. But versions more recent than that started executingcode on physics ticks, as is more proper for the simulation, andto make script behvaior more consistent across different computers withdifferent frame rates.

Electric Drain

Real world CPUs often have low power modes, and sleep modes, and these arevital to long distance probes. In these modes the computer deliberatelyruns slowly in order to use less power, and then the program can tell it tospeed up to normal speed again when it needs to wake up and do something.

Older versions of kOS implemented this concept with a constant electric drain regardless of CPU load. As of version 0.19.0, this concept is simplified by just draining electric charge by “micropayments” of charge per instruction executed.

To change this setting if you want to re-balance the system, see thepage about kOSProcessor part config values.

The shorthand version is this: The more instructions per updateactually get executed, the more power is drained. This can be reducedby either lowering CONFIG:IPU or by making sure your main loophas a WAIT statement in it. (When encountering a WAIT statement,the remainder of the instructions for that update are not used and endup not counting against electric charge).

The system always costs at least 1 instruction of electric charge perupdate no matter what the CPU is doing, unless it’s powered down entirely,because there’s always at least 1 instruction just to check if it’s timeto resume yet in a WAIT. The electric cost is never entirely zeroas long as it’s turned on, but it can be very close to zero while it isstuck on a wait.

If your program spins in a busy loop, never waiting, it can consumequite a bit more power than it would if you explicitly throw in aWAIT 0.001. in the loop. Even if the wait is very small, themere fact that it yields the remaining instructions still allowedthat update can make a big difference.


There are multiple things within kerboscript that run “in the background”always updating, while the main script continues on. The way these work isa bit like a real computer’s interrupt handling system, but not quite.

Collectively all of these things are called “triggers”.

Triggers come in these varieties:

  • Recurring triggers: Triggers that once they are started keep gettingcalled again and again on a regular basis, until they are made to stop.

    • LOCKS which are attached to flight controls (THROTTLE, STEERING,etc), but not other LOCKS.

    • User Delegates assigned to recurrently updating suffixes such asVecDraw:VECUPDATER.

    • WHEN and ON triggers:

      • WHEN condition THEN { some commands }

      • ON condition { some commands }

  • CallbackOnce triggers: Triggers that only happen once per event. Tomake the trigger happen again, the event has to happen again:

    • Callback delegates you tell the system to call when the userperforms GUI events (for example a button’s ONCLICK).

These two types of trigger don’t have the same priority level.It is possible for a recurring trigger to interrupt a callback-oncetrigger, but not the other way around. Further information aboutthis is described in the interrupt prioritydocumentation below.

All triggers work essentially like this:

The kOS CPU decides it’s time to cause a call to the trigger. (How itdoes this is explained below ininterrupt priority.) Once it decides itstime to call the trigger, it does so by inserting a subroutine callat the current moment that interrupts the normal program flow andjumps to the trigger’s subroutine as if the program itself had chosento call the subroutine. It manipulates the call-stack in such a waythat the normal work of the Return instruction at the end of thetrigger routine will pop back to the current location of the programflow. This system works because all variables in kOS are on thestack without any registers, and so popping back to where theinterruption happened puts everything back in the state it was inbefore the interruption so the program can continue as if nothinghad happened.

Prior to kOS 0.19.3, this section was quite different but large changes to how triggers work required a re-write of this whole page. Any old kOS scripts you find that were written prior to kOS 0.19.3 that used triggers might have different behaviour because of this.

Triggers for Cooked Steering

This is a kind of recurring trigger.

The lock expressions associated with Cooked Control,meaning STEERING, THROTTLE, WHEELSTEERING, andWHEELTHROTTLE, have triggers associated with them.kOS will keep calling these expressions repeatedly as frequentlyas it can (once per physics tick if it can). That is whythey are a kind of recurring_trigger.

Note, the LOCK command does not normally result in a triggerthat runs every physics tick. It just does this when dealing withone of these specific values, of STEERING, THROTTLE,WHEELSTEERING, and WHEELTHROTTLE. The normal behaviour ofa lock expression is to only execute the expression when it’s usedinside another expression. It’s just that in the case of thesespecial locks, the kOS system itself is repeatedly doing that.To do this kOS needs to interrupt whatever your code was doing at thetime to perform this expression and it uses the trigger interruptsystem to do so.

Triggers for WHEN and ON statements

This is a kind of recurring trigger.

Each of the ON and WHEN triggers also behavemuch like a function, with a body like this:

if (not conditional_expression) return true. // premature quit. preserve and try again next time.do_rest_of_trigger_body_here.

WHEN and ON Triggers always interrupt to check the condition even whenthe body doesn’t happen yet.

Even a trigger who’s condition isn’t true yet still needs to executethe few instructions at the start of the trigger that discover thatit* condition isn’t true yet. The trigger causes a subroutine callonce per physics tick (or less often if the system has toomuch trigger work to accomplish all the triggers in one tick).This call gets at least far enough into the routine toreach the conditional expression check and discover that it’s nottime to run the rest of the body yet, so it returns. An expensiveto calculate conditional expression can really starve the system ofinstructions because the system is attempting to run it everyphysics tick if it can.

It’s good practice to try to keep your trigger’s conditional checkshort and fast to execute. If it consists of multiple clauses, tryto take advantage of short circuit booleanlogic by putting the fastest part of the check first.

Triggers for GUI callbacks

Another type of trigger is the callback delegates that you canwrite for the GUI system when using theCallback technique. (For example,using Button:ONCLICK, Slider:ONCHANGE, and so on.)

When you give a GUI a callback hook to call, the CPU will implementthat as a trigger as well. When you click the button or move theslider, etc, then kOS will interrupt your program at the next availableopportunity (usually the start of the next IPU’s worth of instructions),to call your callback delegate.

Wait in a Trigger

While WAIT is possible from inside a trigger and it won’t crashthe script to use it, it’s probably not a good design choice to useWAIT inside a trigger. Triggers should be designed to executeall the way through to the end in one fast pass, if possible.

Exception: If you are careful, there is a built-in function youcan call that will have your trigger willingly relinquish its priorityincrease, reducing it back down to whatever the priority was beforeit rudely interrupted things. Doing that can allow other triggers ofequal priority to itself to interrupt it again. To see how this works,look at DROPPRIORITY(), explained below on this page. In general,however, it’s a better idea not to use this unless you fully understandhow the prioriy system here works.

Do Not Loop a Long Time in a Trigger Body!

For similar reasons to the explanation above about the WAIT commandused inside triggers, it’s not really a good idea for a trigger tohave a long loop inside it that just keeps going and going.

The system does allow a trigger to take more than one physics tickto finish. There are cases where it is entirely legitimate to do soif the trigger’s body has too much work to do to get it all done in oneupdate. However, all triggers should be designed to finish their tasksin finite time and return. What you should not do is design a trigger’sbody to go into an infinite loop, or a long-lasting loop that you thoughtwould run in the background while the rest of the program continues on.

This is because while you are in a trigger, main-line code isn’t beingexecuted, and other triggers of equal or lesser priority aren’t beingexecuted. A trigger that performs a long-running loop will starve therest of the code in your kerboscript program from being allowed to run.

Exception: If you are careful, there is a built-in function youcan call that will have your trigger willingly relinquish its priorityincrease, reducing it back down to whatever the priority was beforeit rudely interrupted things. Doing that can allow other triggers ofequal priority to itself to interrupt it again. To see how this works,look at DROPPRIORITY(), explained below on this page. In general,however, it’s a better idea not to use this unless you fully understandhow the prioriy system here works.

But I Want a Loop!!

If you want a trigger body that is meant to loop a long time, the onlyworkable way to do it is to design it to execute just once, butthen make it return true (or use the preserve keyword, which isbasically the same thing) to keep the trigger around for the nextphysics tick. Thus your trigger becomes a sort of “loop” thatexecutes one iteration per physics tick.

Trigger Interrupt Priority

New in version The multiple priorities of interruption described below (GUI callbacksbeing lower priority than recurring callbacks) were introduced inkOS v1.1.6.0

When the CPU wants to interrupt the normal program flow and redirect itinto a trigger, there are some priority rules for which kind of triggeris allowed to interrupt the program flow depending on what the programis doing right now. This is accomplished by having a few prioritylevels, shown in this list:

  • Priority 30: Cooked control Interrupts (i.e. LOCK STEERING)

  • Priority 20: Recurring Interrupts (i.e. WHEN or ON)

  • Priority 10: Callback-Once Interrupts (i.e. GUI callbacks)

  • Priority 0: Normal (non-interrupting) code.

A Trigger will only interrupt something of lower priority than itself.

If the CPU is currently running normal non-interrupting) code, then anytrigger is allowed to interrupt it. But if it is currently already inthe middle of running a trigger, and another trigger of equal prioritywants to interrupt it, the second trigger will wait until the firsttrigger is over and the CPU has dropped back down to normal codebefore the second trigger will be allowed to happen.

The reason the priorities are laid out the way they are is thatthe assumption is that recurring interrupts need to be thehighest priority because they’re often time sensitive and needto happen again and again with speed, while the callback-onceinterrupts are probably not as time-sensitive since they respondto one-shot events like user clicks.

most triggers cannot interrupt *themselves* if they’re still running.

When you have recurring triggers that keep re-running themselvesagain and again, the way they work is that they wait till the previousinstance of themselves has finished running before a new instance willhappen. Thus a recurring trigger will not run every single physicstick if the trigger takes longer than 1 tick to finish. Instead itwill wait for the start of the next physics tick after the currentexecution of the trigger is over. (This is to prevent it from queuingup calls faster than they get dispatched, which would make a backlog.)

These priorities are subject to change in later future versions ofkOS. Right now they’re pretty coarse-grain, which is why they countby 10’s - so there is room to split them up and make them morefine-grained if that becomes necessary later. Never write code thatis too dependant on the priorities being exactly this way. (This iswhy these numbers aren’t even exposed to the script at the moment,to avoid that design pattern.)

Deliberately reducing your priority in long running triggers

Normally if you did something like this:

local done is false.set Gwin to GUI(200).set b1 to Gwin:addbutton("beep").set b1:onclick to { getvoice(0):play(note(300,0.2)). }.set b2 to GWin:addbutton("count").set b2:onclick to count@.set b3 to Gwin:addbutton("quit").set b3:onclick to { set done to true. }.GWin:show().wait until done.GWin:Dispose().function count { local i is 5. until i = 0 { print "Counting.. " + i. set i to i - 1. wait 1. }}

It would mean that while you press the “count” button, and it prints thecountdown from 5 to 1, the other buttons, including “beep” and “quit”would have no effect until the countdown is done. Because count()is the callback for a GUI button, it runs at a higher than normal priority,which means it won’t let itself get interrupted by other GUI callbacks.Instead those other GUI callbacks will be delayed until count() is done.

If you wish, you can cause your trigger, or callback, to deliberatelyrelinquish its hold on other interrupts, allowing them to interrupt itdespite the fact that it is itself in the middle of an interrupt.You do this by deliberately reducing your current priority levelback down a step to whatever it was prior to being incresed by theinterrrupt, which is what this special built-in function does:


After this built-in function is executed by a trigger’s body,the current interrupt priority is dropped back down to whatever thepriority of the code you interrupted was. This is your trigger’sway of saying “I don’t actually want to block interrupts anymore.Please let me be interrupted just as much as whatever Iinterrupted was allowed to be interrupted.”

SO, for example, if Priority 0 code (normal code) got interruptedby priority 10 code (GUI callback code), and the GUI callbackcode executed DROPPRIORITY, then it would now be running atpriority 0 instead of 10, because priority 0 is what got interrupted,and thus allow other GUI code to interrupt it again.

On the other hand, if GUI callback code (priority 10) gotinterrupted by WHEN-THEN code (priority 20), and the WHEN-THENcode had called DROPPRIORITY(), then the priority level ofthat pass through the WHEN-THEN would only be dropped down to10, NOT all the way to 0, because it was interrupting priority 10code.

The reason it works this way (instead of just dropping it all theway down to normal (0) priority directly) is that, effectively,it means a trigger only has the authority to undo its ownpriority increase that it caused itself. It can’t force thepriority down to something less than the code that got interruptedhad to begin with. Had it been allowed to do that, it could havebeen a back-door to circumventing the priority of the thingthat it interrupted.

Be aware that once you DROPPRIORITY(), you also are making itso that the SAME trigger you are currently inside of could fire offa*gain too. It may be a good idea to protect yourself against that,if it is not desired, by setting a flag variable to record the factthat you are inside the trigger at the time and should not re-run it,and then test this flag variable at the top of your trigger code,skipping the body if it’s set.

So in the above GUI example, if you added DROPPRIORITY as shownin the edited version of the example, below, then the other buttonslike the “beep” button, would work while the count() is happening:

local done is false.set Gwin to GUI(200).set b1 to Gwin:addbutton("beep").set b1:onclick to { getvoice(0):play(note(300,0.2)). }.set b2 to GWin:addbutton("count").set b2:onclick to count@.set b3 to Gwin:addbutton("quit").set b3:onclick to { set done to true. }.GWin:show().wait until done.GWin:Dispose().function count { DROPPRIORITY(). // <--- NEW LINE ADDED HERE local i is 5. until i = 0 { print "Counting.. " + i. set i to i - 1. wait 1. }}

Once you call DROPPRIORITY(), then from then on, you are effectively nolonger a trigger, as far as the interruption system is concerned.

BE CAREFUL - if you do this then it is possible for the same trigger orcallback to interrupt itself again. In the above example whereDROPPRIORITY() was added, you could press the “count” button twice inquick succession and one press would interrupt the other. It’s up to you,if you use DROPPRIORITY() to deal with this problem and stop it fromhappening if it’s a bad thing for your program. You can do this bysetting a flag that checks if your trigger is already running and if so,skips it, like so:

local count_is_running is false.function count { if not(count_is_running) { set count_is_running to true. DROPPRIORITY(). local i is 5. until i = 0 { print "Counting.. " + i. set i to i - 1. wait 1. } set count_is_running to false. }}

Again, using DROPPRIORITY() is an advanced topic that should be avoideduntil after you understand what you’ve read here. Even then, it’s usuallysimpler and better to just avoid using it and instead design your script insuch a way that it’s unnecessary to use it. (It’s only necessary to use itif you have interrupt triggers that run a long time instead of finishingquickly like they should.)


Any WAIT statement causes the kerboscript program to immediately stop executing the main program where it is, even if far fewer than Config:IPU instructions have been executed in this physics tick. It will not continue the execution until at least the next physics tick, when it will check to see if the WAIT condition is satisfied and it’s time to wake up and continue.

Therefore ANY WAIT of any kind will guarantee that your program will allow at least one physics tick to have happened before continuing. If you attempt to:

WAIT 0.001.

But the duration of the next physics tick is actually 0.09 seconds, then you will actually end up waiting at least 0.09 seconds. It is impossible to wait a unit of time smaller than one physics tick. Using a very small unit of time in a WAIT statement is an effective way to force the CPU to allow a physics tick to occur before continuing to the next line of code.In fact, you can just tell it to wait “zero” seconds and it will stillreally wait the full length of a physics tick. For example:


Ends up being effectively the same thing as WAIT 0.01.or WAIT 0.001. or WAIT 0.000001.. Since they all contain atime less than a physics tick, they all “round up” to waiting afull physics tick.

Similarly, if you just say:


Then even though the condition is immediately true, it will still wait one physics tick to discover this fact and continue.

CPU Update Loop

New in version As of version, the entire layout of the CPU update loopwas re-written to handle the new trigger priority system.

The guts behind the kOS emulated CPU is the main loop explained belowthat runs once per physics tick. (A “FixedUpdate” in Unity3d terms).

    1. instructionsExecuted = 0

  • 2. how_many_instructions_this_time = config:IPU plus or minus one. (Itwavers slightly because doing so can help prevent edge cases wherethe interrupt triggers syhnc up perfectly with the end of an updateand thus starve main code.)TODO: THIS +/- 1 thing ISN’T TRUE IN THE CODE YET. I’m WRITING THISDOCUMENT BEFORE I’M IMPLEMENTING THIS. COME BACK AND REMOVe THISTODO WHEN I ACTUALLY IMPLEMENT THIS.

    1. while instructionsExecuted < how_many_instructions_this_time do this:

    • 3.1 Execute one instruction. It will move the instruction pointer +1to the next opcode in the program, or in the case of a jump opcode, bysome other number than +1.

    • 3.2 Break out early from this loop if instruction was a WAIT or if programis over or errored out.

    • 3.3 Check if there’s enabled triggers with priority allowing an interrupt.

      • 3.3.1 - If so then insert a “faked” subroutine call right now that jumpsto trigger’s code, with the stack arranged so it will return back tothe current instruction pointer when it’s done.

    • 3.4 increment instructionsExecuted.

  • 4. Any trigger that wanted to interrupt but was waiting for the nextphysics tick boundary before it did so (recurring triggers areusually like this), gets moved from the “pending” trigger queue tothe “active” queue so it will get executed next time on step 3.3 above).

How an interrupt works

Whenever the CPU decides to cause an interrupt in step 3.3 above, it doesso by simulating how a subroutine call normally works in the system. Itdoes the following:

  • Create a subroutine context record which has its “came from” instructionpointer set to the current instruction pointer, and its “came from”priority level set to the current priority level.

  • Push that subroutine context record on the callstack just like a normalsubroutine call would do.

  • Set the instruction pointer to the first instruction of the trigger’scode.

  • Change the CPU priority to match the new priority of the interrupt.

Now if it just lets the CPU loop run as normal after that, it will beinside the trigger code, and when it reaches the Return instruction atthe end of the trigger code, it will pop the context record off the callstack and end up back where it was now before the interruption happened.Not only does Return go back to the instruction the call came from,but it also drops back down to the priority level the call came from.

Because the kOS CPU is a pure stack machine, with all variables andscopes stored on the stack, this ensures everything will be just likeit was before the interruption, and the main code can continue on,unaware that it was even interrupted.

Interrupts that happen at the same time

When more than one trigger of the same priority are in the queue and bothtry to interrupt at the same time before either one has started running,then what happens is this: The first trigger gets its interrupt to occur,but the second trigger, because the first trigger raised the prioritylevel of the CPU, will refuse to interrupt the first one… UNTILthe first one gets to the bottom and does its Return. Then beforeexecuting the next normal priority instruction, the CPU hits point 3.3 inthe loop above again with the priority level now reduced back to normalbecause the first trigger has returned, and right away it notices thesecond trigger still in the queue, and inserts a call to it before themain code can continue.

Thus the two interrupts happen back to back before normal code continues.

Note that the number of instructions being executed (CONFIG:IPU) are NOT lines of code or kerboscript statements, but rather the smaller instruction opcodes that they are compiled into behind the scenes. A single kerboscript statement might become anywhere from one to ten or so instructions when compiled.

The Frozen Universe

Each physics tick, the kOS mod wakes up and runs through all the currently loaded CPU parts that are in “physics range” (i.e. 2.5 km), and executes a batch of instructions from your script code that’s on them. It is important to note that during the running of this batch of instructions, because no physics ticks are happening during it, none of the values that you might query from the KSP system will change. The clock time returned from the TIME variable will keep the same value throughout. The amount of fuel left will remain fixed throughout. The position and velocity of the vessel will remaining fixed throughout. It’s not until the next physics tick occurs that those values will change to new numbers. It’s typical that several lines of your kerboscript code will run during a single physics tick.

Effectively, as far as the simulated universe can tell, it’s as if your script runs several instructions in literally zero amount of time, and then pauses for a fraction of a second, and then runs more instructions in literally zero amount of time, then pauses for a fraction of a second, and so on, rather than running the program in a smoothed out continuous way.

This is a vital difference between how a kOS CPU behaves versus how a real world computer behaves. In a real world computer, you would know for certain that time will pass, even if it’s just a few picoseconds, between the execution of one statement and the next.

The kOS CPU hardware — kOS documentation (2024)
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