Permits slow access to a very large memory space and is perhaps the easiest model to use. I’m resigning as a moderator from all Stack Exchange sites, effective today. How do we handle problem users? When the cpu intends to access D: By using our site, you acknowledge that you have read and understand our Cookie Policy , Privacy Policy , and our Terms of Service.

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It is also a good place to locate the stack, as this is by definition indirectly addressed. I have some code like this: Stack Overflow works best with JavaScript enabled. There’s more to it than that; see Clifford’s answer. In summary, there are five memory spaces available for data storage, each of which has particular pros and cons.

Keil C51 Compiler Basics

I’m familiar with C, not with Keil C The documentation states that an sbit cannot be declared inside a function, so I am also assuming that this code fragment is elided? Permits slow access to a very large memory space and is perhaps the easiest model to use. As these data types require indirect addressing, they are ideally placed in the indirectly addressable area. Filippo Lauria Filippo Lauria 1, 8 8 c15 badges 16 16 bronze badges.


The compact model is rarely used on its own but more usually in combination with the SMALL switch reserved for interrupt routines. I think that c51 is an extension of standard C. It gives me 1 on that line when it is blinking, 0 when it is sleeping. The solution is to stick to one global memory model and then use the SMALL function attribute, covered in the previous section, to set the memory model locally.

Will support code sizes up to about 4K but a constant check must be kept on stack usage. Active 6 years, 1 month ago. So you are not declaring a variable X and reading it once before the loop, rather you are defining a reference to P3.

As I said, check the documentation for your MCU. While it is possible to change the global memory model half way through a project, it is not recommended! Obviously, being ROM, only constants can be stored here.

So the question is, what is the meaning of the operator! A fourth memory area is also off-chip, starting at X: Presumably it means the same thing it means in standard C. Failure to do this properly can result in very dangerous results, as data keill is at the whim of PORT2!

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It is only accessible via indirect addressing MOV A, Ri and effectively overlays the directly addressable sfr area. It is directly addressable, so that instructions like ‘MOV A,x’ are usable. It isn ormal to indicate that with an elipsis I believe that ona port is configured as an input by writing zero to it, which allows it to be asserted externally.


I have linked to the documentation of sbit in my answer. It is useful in that it can be both accessed byte-wise by the normal MOV instructions and addressed by special bit-orientated intructions, as shown below:. This starts at D: Slow or infrequently-used background variables. The ability to apparently change the value of X when it is externally pulled high, I am guessing is in the nature of the GPIO hardware rather than any strange behaviour of the!

Maybe is like you said, and for some reason unknown to me, the behaviour is, in my opinion, strange. Perhaps the most initially confusing thing about the is that there are three different memory spaces, all of which start at the same address.