Hello, Can someone explain what it means??
#define IOPIN0 (*(volatile unsigned long *)(GPIO_BASE_ADDR + 0x00))
The controller using is LPC23xx/24xx.
BR Chethan
Standard C pointer cast, telling the address of a volatile 32-bit register at a specific address that should be used with the name IOPIN0.
So the address - as an integer - is said to be a pointer to volatile 32-bit unsigned, and the macro then does a pointer indirection to "follow" the pointer to allow you to either read or write that register.
So bring out a decent book about the C language - or even better the language standard - and have a look at type declarations and type casts.
Hi Westermark,
Thanks for the reply.
Based on your explanation what i understood is : that address is trying to making as a type of pointer (in this case 'volatile 32-bit integer pointer' and it can be dereferenced) is it correct???
Chethan
Not only can it be dereferenced. It already is.
The middle * is for the type cast from integer to pointer. The left-most * is the dereference being done.
So you can use IOPIN0 as if it was a normal 32-bit volatile unsigned integer. You can read from it and you can write to it.
So the construct is kind of placing a variable at an absolute address, but without involving any link-time symbol - the generated code will instead get the hard-coded address of that processor special function register.
Ya got it now!!
In this case why we using volatile.....is their any special purpose??
Haven't you looked at the description of the volatile keyword?
What is the difference between a normal memory variable and a register mapped to real hardware? When may a memory variable change content? When may a I/O register associated with processor pins change value? Would that matter to the compiler, when it generates the processor instructions?
Actually i know the concept.
Volatile is used to modify the memory content is existing in different file.
Is it correct ???
For remaining questions i don't have much deep knowledge. Kindly explain me those things also it looks to be interesting :) :)
No. You really should read up about volatile.
You seem to think about external declarations, which is something completely different.
Didn't you consider the hints I left about the need for volatile because of the difference between a normal RAM variable and a special function register? Volatile is even part of non-technical English. Nitroglycerin is volatile, as is the situation in Ukraine.
(making allowances for English as a foreign language)
Well, one common use of the 'volatile' qualifier is to indicate that memory content may bechanged by "another file".
So think how a hardware SFR (special function register) might have similar issues...
Is this essentially the same question as http://www.keil.com/forum/58484 ?