Usage of absolute address in C51

ena

Usage of absolute address in C51 [Copy link]

 

Is it possible to place a variable in a fixed area and initialize it at the same time?

damiaa

If you look up "absolute address in C51" on the Internet or in a book, someone will tell you.

ena

damiaa posted on 2018-3-16 11:46 Look up "absolute address in C51" on the Internet or in a book and someone will tell you.

But they are just definitions, not assignments.

yang_alex

ena posted on 2018-3-16 12:00 But they are just definitions, not assignments

If you define it, isn't it just assigning a value to the variable address? Dead brain!

ena

yang_alex posted on 2018-3-16 12:52 When you define it, isn't it just assigning a value to this variable address? Dead brain!

, it just defines this variable to be stored at a certain address, but the specific value is unknown?

yang_alex

ena posted on 2018-3-16 13:02 , just define this variable to be stored at a certain address, but don’t know the specific value?

Do you mean you don’t know the address of the variable? Or do you mean you don’t know the value of the variable? Didn’t you specify the address of the variable when defining it? Like this: C51 provides a way to access absolute addresses: 1. Absolute macros: In the program, use “＃include"You can use the macros defined therein to access absolute addresses, including: CBYTE, XBYTE, PWORD, DBYTE, CWORD, XWORD, PBYTE, DWORD For specific usage, please refer to absacc.h. For example: rval=CBYTE[0x0002]; points to the address 0002h of the program memory rval=XWORD [0x0002]; points to the address 0004h of the external RAM 2. The _at_ keyword can be added directly after the data definition with _at_ const, but please note: (1) Absolute variables cannot be initialized; (2) Bit-type functions and variables cannot be specified with _at_. For example: idata struct link list _at_ 0x40; specifies that the list structure starts at 40h. xdata char text[25b] _at_0xE000; specifies that the text array starts from 0E000H. Tip: If the external absolute variable is an I/O port or other data that can change on its own, it needs to be described using the volatile keyword. Please refer to absacc.h.

ena

yang_alex posted on 2018-3-16 13:09 Do you mean you don't know the address of the variable? Or do you mean you don't know the value of the variable? Didn't you specify the address of the variable when defining it? Like this...

I mean how to assign a value to this variable, because what I saw in the document is "absolute variables cannot be initialized". But in MDK, using __attribute__ is OK: const int ID __attribute__((at(0xf200)))=0x10; I don't know if there is such a method in C51.

damiaa

Any of the text[25b] array can be assigned a value. rval = XWORD [0x0002]; points to the 0004h address of external RAM. In this way, rval can be assigned a value. As long as it is not CCODE CBYTE, etc. (restricted to code space) or const-restricted (RAM XDATA IDATA).

yang_alex

ena posted on 2018-3-16 13:35 I mean how to assign a value to this variable, because what I saw in the document is "absolute variables cannot be initialized". But in MDK, use __a ...

You need to understand what an absolute variable is. I understand that it is defined to an address in the FLASH space. You definitely cannot initialize it in the program! If it is defined in the RAM space, it can be initialized.

ena

yang_alex posted on 2018-3-16 14:09 You need to understand what an absolute variable is. I understand that it is defined to an address and defined to the FLASH space. You certainly cannot initialize it in the program! For example...

Well, I guess you can't put

xiaoshishuai

Come to learn, popular teaching

wsdymg

const int ID __attribute__((at(0xf200)))=0x10; In the example given by the host, ID is actually a constant. It is stored at the address 0XF200 and can be accessed by accessing ID in the program.

ena

wsdymg posted on 2018-3-16 20:53 const int ID __attribute__((at(0xf200)))=0x10; In the example given by the host, ID is actually a constant. It is stored at 0XF200...

But it fails to compile in KEILC51

huo_hu

51 uses the keyword code code unsigned char Arry[256] _at_ (0x1000) = { 0,1,2,3,4,5 };

ena

huo_hu posted on 2018-3-18 17:12 51 uses the keyword code code unsigned char Arry[256] _at_ (0x1000) = { 0,1,2,3,4,5 };

This will prompt an error;

error C274: "icode": absolute specifier illegal

huo_hu

ena posted on 2020-6-13 11:51 This will prompt an error; error C274: "icode": absolute specifier illegal

You have to check that the value specified later is indeed in the flash area

hailidianzi

Same question, after specifying an absolute address in C51 code, is there no way to assign an initial value?

If it is mixed programming, there used to be a way:

Specify the initialization data in the A51 assembly code:
CSEG AT 0x1000
DB 12H,34H,56H,78H
Then define the reference in the C51 code:
unsigned char code *pinfo =0x1000;

or code unsinged char pinfo[4] _at_ 0x1000;