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SCI Syntax Help / Re: Can someone explain Flags to me?
« on: June 13, 2019, 05:33:44 AM »
Why would you just say z directly in a conditional expression instead of (== z TRUE)? For those playing along at home and don't know yet, this is because of a few things. First, TRUE and FALSE are just constants representing 1 and 0. Second, any non-zero value is "truthy". You'd think TRUE is the most truthy of all but it's just a number so it's equally truthy, this isn't Animal Farm. The expression in an if statement and such has to evaluate not to true but to truthy to execute, and that can be any expression whatsoever.
By that same token, (if mover (mover dispose:)) only executes if mover is non-zero and assumes it's a valid reference to some instance.
To elaborate on Phil's post about procedure calls and byte counts: bytecode-wise, (if z ...) turns into the following:
Compare that with a procedure call:
And finally, checking your own properties as above is four bytes again:
Now, by "returns non-zero" in the first half of this post above, we actually mean "sets the accumulator register to a non-zero value before popping back to the caller". You should be able to figure out what that means by now if you didn't already know.
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(if (AskConsent "Are you sure you want to shoot that?") ...) ; will execute if AskConsent returns non-zero
(if (Btst fShotTheThing) ...) ; will execute if the "shot the thing" flag is set
(if (== z TRUE) ...) ; will execute if z is exactly 1
(if z ...) ; will execute if z is 1 or higherBy that same token, (if mover (mover dispose:)) only executes if mover is non-zero and assumes it's a valid reference to some instance.
To elaborate on Phil's post about procedure calls and byte counts: bytecode-wise, (if z ...) turns into the following:
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83 00 lal local0 //or 85 00 lat temp0, you get the point.
31 08 bnt justAfterThatLoad a local or temp into the accumulator, branch if the acc is not truthy. Four bytes, maybe one more if the code in the if block is large enough. The branch goes down eight bytes because that's how tiny the block itself is.Compare that with a procedure call:
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78 push1 // one parameter
39 2a pushi 2a // flag #42
45 01 02 callb Btest 2 // call with two bytes worth of param data
31 08 bnt justAfterThatThat's six bytes to execute (if (Btest 42) ...). So if you test the same flag a bunch of times, consider testing it once and storing it to a temp.And finally, checking your own properties as above is four bytes again:
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63 12 pToa mover
31 08 bnt moverWasNullNoting that mover is either null or something far greater than 1.Now, by "returns non-zero" in the first half of this post above, we actually mean "sets the accumulator register to a non-zero value before popping back to the caller". You should be able to figure out what that means by now if you didn't already know.
