IT Block Assembly

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Hello. This is Dr Miller and this episode 12.12 of assembly

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Today we're gonna again talk about conditional execution, but we're going to talk about I t blocks.

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So some i t conditional execution.

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So thumb to does not have the bits for standalone, conditional instructions,

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but in order to compensate for that a little bit, it does have I t blocks.

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And so these are gonna be available for both thumb to and our moon.

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And then when you get into arm version eight, it actually deprecate this functionality and removes it.

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So what are I t blocks?

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So I t blocks our if then blocks and they allow up to four different instructions to be conditionally executed

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when they run an arm thumb to mood.

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So we have this set of instructions I t t e Q. And then we have three instructions that follow this particular example.

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So an I t block. So it's I t. And then it's followed the then by X, y and Z,

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and then some flag that we care about.

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And so X, y and Z can be either tea or E. So they're one of the two letters to your E

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and then the number of them is optional. So we can have 012 or three,

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and then the conditions must be ordered correctly based on what you're doing with them.

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So the tea is then s O that must match our condition. The e is an else, and it must be the opposite of our condition. And then the flag is one of those conditional suffixes that we have seen.

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So again, we have seen the suffixes like equal, not equal, greater than or equal to less than greater than less than equal are typically the ones that you're going to see.

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So here are some example I T blocks.

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And so one of the things that you're going to see is that if I have to tease, then those will have to match this condition. So e que e que e que

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um and then our else is something that is not a Q or the opposite of U Q is not equal.

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So this I t block has I t t e e. And so we got I then then and then and else, Or

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if then else So I gotta then so we got a negative M I is

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negative and pl is positive. And so these conditions have to match what's in our I t block?

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Are we gonna have an I t t t

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So we got three then, right? So if negative here, I got three different negative instructions that I can use for that

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or i t and then e So I got one then and to else's so one then,

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um g t. And then we got less than able to lessen equal two in that one.

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So here's an example using the if then blocks so we can see that we have a function here and it says dot be aligned to So we're relying on to buy boundaries and then this is a thumb function.

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And then our function bar is going to move our zero into our one and then compare are zero to the number 10 and then it's got it. If then

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that if then then else So we've got to then is two equals and then are not equal.

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So we can movinto are zero the number one and then subtract the number one from r zero

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or we do move. Are 0 100 And so this one. If you type

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a 10 it will give you the number zero. And if you,

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um cyp something that's not equal to 10 it will give you the value. 100 right? And so this was around. What about way of doing it? I could have just done the move zero for it. But I wanted to show the if then block inside of there.

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Oh, and again, you're doing a branch of Lincoln Exchange, right? So I'm setting the link register, and then I'm also exchanging instruction sets to jump into bar because this is a regular armed function, and then this is a film function,

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but we can see that this is also useful for not doing some mood. So it works both ways. So all I did in this is I just did a branch in a link and I didn't do an exchange. And then I removed the header from my thumb code, and I can use the exact same code if this is an arm block or if this is a thumb block.

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So today we talked about some conditional execution using I t blocks

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looking forward, we will given I t block example.

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And then here's our quiz. So for an I t block, what does the T in the east and for

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so t stands for, then an e stands for else.