3.6.8.2 Output Section LMA

Every section has a virtual address (VMA) and a load address (LMA); see Basic Script Concepts. The virtual address is specified by the see Output Section Address described earlier. The load address is specified by the AT or AT> keywords. Specifying a load address is optional.

The AT keyword takes an expression as an argument. This specifies the exact load address of the section. The AT> keyword takes the name of a memory region as an argument. See MEMORY. The load address of the section is set to the next free address in the region, aligned to the section's alignment requirements.

If neither AT nor AT> is specified for an allocatable section, the linker will use the following heuristic to determine the load address:

• If the section has a specific VMA address, then this is used as the LMA address as well.
• If the section is not allocatable then its LMA is set to its VMA.
• Otherwise if a memory region can be found that is compatible with the current section, and this region contains at least one section, then the LMA is set so the difference between the VMA and LMA is the same as the difference between the VMA and LMA of the last section in the located region.
• If no memory regions have been declared then a default region that covers the entire address space is used in the previous step.
• If no suitable region could be found, or there was no previous section then the LMA is set equal to the VMA.

This feature is designed to make it easy to build a ROM image. For example, the following linker script creates three output sections: one called ‘.text’, which starts at 0x1000, one called ‘.mdata’, which is loaded at the end of the ‘.text’ section even though its VMA is 0x2000, and one called ‘.bss’ to hold uninitialized data at address 0x3000. The symbol _data is defined with the value 0x2000, which shows that the location counter holds the VMA value, not the LMA value.

     SECTIONS
       {
       .text 0x1000 : { *(.text) _etext = . ; }
       .mdata 0x2000 :
         AT ( ADDR (.text) + SIZEOF (.text) )
         { _data = . ; *(.data); _edata = . ;  }
       .bss 0x3000 :
         { _bstart = . ;  *(.bss) *(COMMON) ; _bend = . ;}
     }

The run-time initialization code for use with a program generated with this linker script would include something like the following, to copy the initialized data from the ROM image to its runtime address. Notice how this code takes advantage of the symbols defined by the linker script.

     extern char _etext, _data, _edata, _bstart, _bend;
     char *src = &_etext;
     char *dst = &_data;
     
     /* ROM has data at end of text; copy it.  */
     while (dst < &_edata)
       *dst++ = *src++;
     
     /* Zero bss.  */
     for (dst = &_bstart; dst< &_bend; dst++)
       *dst = 0;