I want to define some values in code memory with specific location. For example
code unsigned char MyArray[] _at_ 0xF000 = {0x01, 0x02};
After that, I can use "MyArray[0]" and "MyArray[1]" to access the memory at 0xF000 and 0xF001. How can I achieve this?
I know this kind of memory allocation. But I want to achieve this in C file. Is it possible? For example, I want to define an array in 2 dimension. MyArray[][2] = { {0xABCD, 0x01}, {0x1234, 0x02}, }; In assembly, I have to do : DB 0xCD DB 0xAB DB 0x01 DB 0x34 DB 0x12 DB 0x02
The '0xABCD' is an index, if divided into 2 bytes, it will be very difficult to read for programmers.
I've never looked into doing this in C (for the C51), so I don't know if it is possible there.
But, again, I have done a similar thing with structures in assembler.
You could, for example, do something like:
;Within assembly file Public MyArray Cseg At 0F000h MyArray: Dw 0ABCDh,00001h Dw 01234h,00002h
;Within C file extern int code MyArray[][2]; ... Val = MyArray[0][1]; ...
I think you have to be a bit careful in your description, because it now looks like what you are wanting is an array of structures; i.e., element 0 is an int, element 1 is a char etc.
You can order the source in the assembler to make it more 'programmer friendly' and, so long as you have the C external declaration correct, you can access that data in any way you want.
In my case, the first element is 2-byte data and the second element is 1-byte data. So in C file, I have to use "unsigned char" to read the data and re-merge them to 2-byte data. Thanks for your fast reply. I've got a better way to program.
"I've got a better way to program."
Please let us know what the way is - Most of us here are always ready to learn.
"Better way" means better than current programming style. i.e. {0xABCD, 0x01}. My current programming CSEG AT 0xF000 ADDR_LBYTE: DB 0xCD ADDR_HBYTE: DB 0xAB DATA_BYTE: DB 0x01
But now, I can re-write to : CSEG AT 0xF000 ADDR_: DW 0xABCD DATA_BYTE: DB 0x01
It's much friendly to programmers. I just need to change the parsing algorithm to reverse high-low byte order.
I found some guys said KeilC does not support initial values in fixed allocation. If this is true, I think I've got the best way to program my firmware.
It can be done in 'C', at least in the sense that no assembly language is required to achieve it. The defining declaration and initialisation needs to be placed in a separate source file and included in the project. The linker project options can then be modified to locate the resulting segment at the required address.
If multiple objects are required at sequential addresses (as though they were contained in a structure) it is necessary to check the 'keep variables in order' box in the options for the source file or the entire project.
I understand.
DW stores data of a word with the high byte first - I thought there was an equivalent to store the low byte first. Can't remember it though. Might even have been another assembler???
Maybe someone else knows it - Or can correct me.
"The linker project options can then be modified to locate the resulting segment at the required address."
Yes, I'd forgotten that option. I've done it that way a number of times before, but have nearly always ended up going back to my assembler 'comfort zone'!
If you want the data to be 'easy to read' in the source code, consider using macros.
"I found some guys said KeilC does not support initial values in fixed allocation."
You don't need "some guys" to tell you that - Keil themselves clearly state it in their documentation:
http://www.keil.com/support/man/docs/c51/c51_le_absvarloc.htm
"If this is true"
Yes, it does exactly what it says on the tin!
"I think I've got the best way to program my firmware."
Maybe not "best", but perfectly acceptable!