Two methods of accessing the absolute address of the MCU in C language programming of 51 MCU

When programming an 8051 microcontroller application system, it is often necessary to directly operate the various memory address spaces of the system. The target code generated by the C51 program after compilation has a floating address, and its absolute address must be determined after being linked and located by BL51. In order to be able to directly operate on any specified memory address in the C51 program, the extended keyword "at", pointer, pre-defined and link positioning control commands can be used.

Among these methods, I think the simplest and most effective method is to use the "_at_" keyword to specify the absolute address of the specified variable memory space. The general format is as follows:

                             [Memory type]            Data type             identifier_at_address             constant

            in:

                     The memory types                 include idata, data, xdata and other types that can be recognized by C51. It is best not to omit them.

                     The data type                    can be basic types such as int, long, float, and of course, complex data types such as arrays and structures can also be used              .                 I think that unsigned int can generally solve many problems.

                    The identifier                       is the name of the variable to be defined, and the programmer decides it himself

                   The address constant                     is the absolute address of the memory to be directly operated and must be located within the valid memory space.

Note: Variables cannot be initialized, only global variables can be used. Generally, do not use them lightly to avoid errors.

Example:

                     xdata            unsigned int             addr1            _at_ 0x8300;

Another method is to use a header file absacc.h, which is not difficult. Let's take an example to explain it:

               #include

                XBYTE[0x8300]=0;               /*Write 0 to the external memory (XDATA) address 0x8300

I strongly recommend using at, which may be better. Another point is that it should not be used indiscriminately, because some memory space cannot be occupied casually, the C51 compiler has already made other uses, and some space is very important for the microcontroller itself. If it is used indiscriminately, it is easy to make mistakes.