Why can't symbols defined with _at_ be initialised!?

Next Thread | Thread List | Previous Thread Start a Thread | Settings

Details	Message
Read-Only Author Andrew Neil Posted 14-Jan-2002 10:10 GMT Toolset C51	Why can't symbols defined with _at_ be initialised!? Andrew Neil Well, we've just had another question about initialising constant data defined with _at http://www.keil.com/forum/docs/thread1162.asp This is a very frequently-asked question, which begs the further question, "why* can't symbols defined with _at_ be initialised??"* After all, there's obviously a lot of people out there wanting to do it. I gather that the Raisonance compiler does allow this. http://www.8052.com/forum/read.phtml?id=13182&top=

Read-Only Author Mikhail Podgouzov Posted 14-Jan-2002 12:03 GMT Toolset C51	RE: Why can't symbols defined with _at_ be initialised!? Mikhail Podgouzov I think your question is of no importance. Is it really that difficult to initialize whatever you want at the start of your program? Michael.

Read-Only Author Andrew Neil Posted 14-Jan-2002 15:39 GMT Toolset C51	RE: Why can't symbols defined with _at_ be initialised!? Andrew Neil "I think your question is of no importance." Take a look back through the last year's posts - you'll see that it's been asked at least half a dozen times. I think it was important to them, and to all the others who worked it out for themselves without ever asking on this forum. "Is it really that difficult to initialize whatever you want at the start of your program?" It is if you want your constants in CODE!! But why should you have to do this, when the Compiler handles all other initialisations automatically!?

Read-Only Author Michael Podgouzov Posted 14-Jan-2002 17:33 GMT Toolset C51	RE: Why can't symbols defined with _at_ be initialised!? Michael Podgouzov Excuse me, I couldn't imagine it was so hard for you to have an initializing block in your code. Probably, a little practice will help. M.

Read-Only Author Mike Kleshov Posted 14-Jan-2002 17:47 GMT Toolset C51	RE: Why can't symbols defined with _at_ be initialised!? Mike Kleshov An unusual way of intruducing yourself :-)) That's why I try to think twice before posting messages.

Read-Only Author Andrew Neil Posted 14-Jan-2002 20:33 GMT Toolset C51	RE: Why can't symbols defined with _at_ be initialised!? Andrew Neil I obviously wasn't clear when I said, "...if you want your constants in CODE!!" What I meant was that your suggestion cannot be used if the initialisations are required in the 8051's CODE memory space - because there is no instruction to write to CODE space! (it's normally ROM)

Read-Only Author tom mazowiesky Posted 14-Jan-2002 14:06 GMT Toolset C51	RE: Why can't symbols defined with _at_ be initialised!? tom mazowiesky But when a lot of people ask for a certain feature, doesn't that imply that it is important? This is one of those odd limitations that everyone has in their products. While it is not critically important, it would be a nice addition to have.

Read-Only Author Jon Young Posted 14-Jan-2002 18:24 GMT Toolset C51	RE: Why can't symbols defined with _at_ be initialised!? Jon Young I was trying to think up reason why you want to declare constants at absolute locations. I can understand the need for declared constants residing in a particular segment. Can someone give me an example?

Read-Only Author Andrew Neil Posted 14-Jan-2002 20:37 GMT Toolset C51	RE: Why can't symbols defined with _at_ be initialised!? Andrew Neil I think one of the previous posts which asked this question did actually give a reason. I can't remember for sure, but it might've been that they wanted a "signature" in the ROM at a known location?

Read-Only Author David Lively Posted 20-Jan-2002 21:23 GMT Toolset C51	RE: Why can't symbols defined with _at_ be initialised!? David Lively Typically I'd use a small A51 file that declared an absolutely located segment, and db the required data in place. Of course, this _should_ be possible in C; I'm not sure of the reason for this particular limitation.

Read-Only Author Andy Neil Posted 29-Jan-2004 23:38 GMT Toolset C51	RE: Why can't symbols defined with _at_ be initialised!? Andy Neil "Of course, this _should_ be possible in C" No, it should not! The allocation of absolute addresses is not the job of a Compiler - it is the Linker's job (or, strictly, the Locator's job - but the functions are commonly combined these days).

Read-Only Author Stefan Duncanson Posted 30-Jan-2004 09:54 GMT Toolset C51	RE: Why can't symbols defined with _at_ be initialised!? Stefan Duncanson "The allocation of absolute addresses is not the job of a Compiler - it is the Linker's job (or, strictly, the Locator's job - but the functions are commonly combined these days)." I couldn't care less if my cat has to do it as long as it can be done. This _at_ with initialisers issue rumbles on and on. It seems like the most frequently asked question on the forum and is therefore presumably the most desired missing feature in Keil. Come on Keil, when are you going to implement it? Stefan

Read-Only Author Drew Davis Posted 30-Jan-2004 19:32 GMT Toolset C51	RE: Why can't symbols defined with _at_ be initialised!? Drew Davis I was trying to think up reason why you want to declare constants at absolute locations This feature would be convenient for any shared constant created by an external program. For example, you might have a CRC over your code. You might also some sort of table of calibration constants or control parameters, which have default values (built into the code), but known locations so it's easy to update them with a flash programmer without having to rebuild the code. You might have some sort of version / product identification string read by another device. All such problems have solutions (thoroughly discussed on the forum). But while you can object "you can do it some other way", that overlooks the fact that _at_ itself is a mere convenience extension. You might not expire without being able to have _at_ and an initializer, but you wouldn't expire without _at_ at all. Given that _at_ exists, though, it's annoying to have a half-implemented feature. I'd rather have "long long" support, though (along with some other C99 features). Or better control over object placement within segments. Or a better interface for managing projects with multiple results (libraries and executables) in uVision. Or a "medium" memory model where auto variables and temporaries go into overlaid data, and file scope and statics go into xdata, by default.

Read-Only Author Stefan Duncanson Posted 2-Feb-2004 09:53 GMT Toolset C51	RE: Why can't symbols defined with _at_ be initialised!? Stefan Duncanson "But while you can object "you can do it some other way", that overlooks the fact that _at_ itself is a mere convenience extension." Indeed, but given the fact that Keil is a tool for programming in an embedded environment it is a very appropriate extension. Just the sort of thing one would expect to find in such an implementation of 'C'. There have to be non-portable and/or hardware specific features to make the embedded 'C' compiler useable - otherwise we'd have to code the majority of projects in assembler. We need to be able to do in 'C' pretty much anything that can be achieved in assembler. "Given that _at_ exists, though, it's annoying to have a half-implemented feature." Oh yes. Oh yes indeed. Stefan

Next Thread | Thread List | Previous Thread Start a Thread | Settings