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denkid
Joined: 17 Jan 2005
Posts: 10
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| Why did not supports? |
 Posted: Fri Jan 21, 2005 7:32 am |
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Hello,
Why did not supports
This is not standard ANSI C?..
My Version : CCS-C 3.190 |
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Mark
Joined: 07 Sep 2003
Posts: 2838
Location: Atlanta, GA
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| Posted: Fri Jan 21, 2005 7:49 am |
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I think you are trying to do a pointer to a constant which is not allowed.
you can do this:
| Code: |
char buf[]="ABCDEFG";
char *p;
p=buf;
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SherpaDoug
Joined: 07 Sep 2003
Posts: 1640
Location: Cape Cod Mass USA
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| Posted: Fri Jan 21, 2005 8:28 am |
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1) CCS C is not ANSI C. It is closer to Kernighan & Ritchie first edition C.
2) The main reason pointers to constants are not allowed is due to the Harvard architecture of the PIC processor, which keeps ROM space and RAM space seperate. Constants are stored in ROM space, which has a wider data width (12, 14, or 16 bits) than RAM space (8 bits) where pointers operate.
_________________
The search for better is endless. Instead simply find very good and get the job done. |
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Mark
Joined: 07 Sep 2003
Posts: 2838
Location: Atlanta, GA
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| Posted: Fri Jan 21, 2005 8:58 am |
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| SherpaDoug wrote: | | 2) The main reason pointers to constants are not allowed is due to the Harvard architecture of the PIC processor, which keeps ROM space and RAM space seperate. Constants are stored in ROM space, which has a wider data width (12, 14, or 16 bits) than RAM space (8 bits) where pointers operate. |
No offense, but that is a load of crap. The reason that I say that is the some of the other PIC C compilers support pointers to constants. Fact of the matter is that CCS just didn't do it. They could if they wanted to and I believe it will be in there soon enough. They didn't have pointers to functions, but now they do. It is only a matter of time. |
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Ttelmah
Guest
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| Posted: Fri Jan 21, 2005 9:45 am |
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| Mark wrote: | | SherpaDoug wrote: | | 2) The main reason pointers to constants are not allowed is due to the Harvard architecture of the PIC processor, which keeps ROM space and RAM space seperate. Constants are stored in ROM space, which has a wider data width (12, 14, or 16 bits) than RAM space (8 bits) where pointers operate. |
No offense, but that is a load of crap. The reason that I say that is the some of the other PIC C compilers support pointers to constants. Fact of the matter is that CCS just didn't do it. They could if they wanted to and I believe it will be in there soon enough. They didn't have pointers to functions, but now they do. It is only a matter of time. |
In part, but with pointers to ROM, there is a cost. If you think of a typical large PIC, the RAM space is perhaps 1.5KB, while many now have ROM spaces of 64KB, or those supporting external memory, even more. If you allow pointers to ROM, then you force 'pointers' fo be a much larger value (an int16, will address every possible RAM address on ony current PIC, but with the ROM address spaces allowed on some chips, a pointer would need to grow to at least an int24). With this there would also be an overhead for pointer operations, of working out which memory area to access. Ideally, implementation would be in the form of an 'option', so that the extra overheads involved, do not have to apply where this feature is not needed.
Historically, the 'reason', was that it is basically impossible to implement ROM pointers on the early chips, it became simply possible, on the chips that implement the ability to read a byte from the memory (most of the flash chips), and except for the C syntax involved, the read_program_memory function, implements the required access operation. I must admit, it is suprising that this has not been offered as an option for these chips, and it may well apear in the future.
Best Wishes |
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Mark
Joined: 07 Sep 2003
Posts: 2838
Location: Atlanta, GA
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| Posted: Fri Jan 21, 2005 9:51 am |
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| Quote: | | but with the ROM address spaces allowed on some chips, a pointer would need to grow to at least an int24 |
You can specify the memory model which would dictate the size of the pointer. Also, this is only true for the constant pointer. RAM based pointers can still be 8 or 16 bit regardless of the size of the constant pointer. |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Fri Jan 21, 2005 10:16 am |
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Mark, have you examined the ASM code in other compilers ?
Can you tell us how they make pointers to ROM for the PIC16 ? |
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Darren Rook
Joined: 06 Sep 2003
Posts: 287
Location: Milwaukee, WI
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| Posted: Fri Jan 21, 2005 11:44 am |
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| Mark wrote: | | RAM based pointers can still be 8 or 16 bit regardless of the size of the constant pointer. |
No.
Example:
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char *ptr;
const char rom[]="CONSTANT IN ROM";
char ram[]="CONSTANT IN RAM";
ptr=rom;
ptr=ram;
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ptr will have to hold extra overhead to know if it's point to RAM or ROM. Also it will have to be an int32 value to hold the full 32 bit address for ROM. Probably a 32bit value with bit31 set if it's ROM, clear if RAM.
BTW - There are some undocumented typemod features you can use to get functionality very very close to pointers to ROM. So I don't think CCS is far away. If I have time later I will try to show some examples.
Last edited by Darren Rook on Fri Jan 21, 2005 12:38 pm; edited 1 time in total |
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Mark
Joined: 07 Sep 2003
Posts: 2838
Location: Atlanta, GA
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| Posted: Fri Jan 21, 2005 12:08 pm |
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| Darren Rook wrote: | | Mark wrote: | | RAM based pointers can still be 8 or 16 bit regardless of the size of the constant pointer. |
No.
Example:
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char *ptr;
const char rom[]="CONSTANT IN ROM";
const char ram[]="CONSTANT IN RAM";
ptr=rom;
ptr=ram;
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ptr will have to hold extra overhead to know if it's point to RAM or ROM. Also it will have to be an int32 value to hold the full 32 bit address for ROM. Probably a 32bit value with bit31 set if it's ROM, clear if RAM.
BTW - There are some undocumented typemod features you can use to get functionality very very close to pointers to ROM. So I don't think CCS is far away. If I have time later I will try to show some examples. |
[/quote]
As you put it, you would be right. But you are wrong. The compilers that support (at least the ones I use) wouldn't allow that. It has to be a pointer to a const. It is a differenct data type thus a different kind of pointer.
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const char *pointer;
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For example with the HI-TECH you would get this warning:
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Warning[000] C:\HT-PIC\samples\picdem2\picdem2.c 30 : illegal conversion between pointer types |
Last edited by Mark on Fri Jan 21, 2005 12:15 pm; edited 1 time in total |
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Mark
Joined: 07 Sep 2003
Posts: 2838
Location: Atlanta, GA
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| Posted: Fri Jan 21, 2005 12:13 pm |
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| PCM programmer wrote: | Mark, have you examined the ASM code in other compilers ?
Can you tell us how they make pointers to ROM for the PIC16 ? |
Hi-Tech uses the retlw approach. They simply have a routine they call. The high byte of the const pointer is loaded into what is referred to internally as the high byte of "code_ptr". The low byte is put into the W register and the internal routine is called. The W reg is then put into the code_ptr low byte. Tests are made on the high byte. The next action depends on how the upper 2 bits are set but basically the code_ptr is loaded into the PCL and the const is returned in the W reg and then acted upon. |
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Darren Rook
Joined: 06 Sep 2003
Posts: 287
Location: Milwaukee, WI
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| Posted: Fri Jan 21, 2005 12:43 pm |
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| Mark wrote: |
As you put it, you would be right. But you are wrong. The compilers that support (at least the ones I use) wouldn't allow that. It has to be a pointer to a const. It is a differenct data type thus a different kind of pointer.
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const char *pointer;
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I see what you mean - you are right I was wrong. |
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Mark
Joined: 07 Sep 2003
Posts: 2838
Location: Atlanta, GA
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| Posted: Fri Jan 21, 2005 2:26 pm |
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| Darren Rook wrote: | | Mark wrote: |
As you put it, you would be right. But you are wrong. The compilers that support (at least the ones I use) wouldn't allow that. It has to be a pointer to a const. It is a differenct data type thus a different kind of pointer.
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const char *pointer;
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I see what you mean - you are right I was wrong. |
The C18 compiler requires you to use 'rom' in front of the declaration. It will also allow you to declare a pointer to a const and the pointer to the const can actually be stored in rom to conserve RAM. This comes in handy when doing menus and command parsers. |
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