2.3. "Map" Files
avr-objdump is very useful, but sometimes it's necessary to see information about the link that can only be generated by the linker. A map file contains this information. A map file is useful for monitoring the sizes of your code and data. It also shows where modules are loaded and which modules were loaded from libraries. It is yet another view of your application. To get a map file, I usually add -Wl,-Map,demo.map to my link command. Relink the application using the following command to generate demo.map (a portion of which is shown in Example 2-3.
% avr-gcc -g -mmcu=at90s2313 -Wl,-Map,demo.map -o demo.out demo.o
Some points of interest in the map file are:
- The .text segment (where program instructions are stored) starts at location 0x0.
- The next available address in the .text segment is location 0xec, so the instructions use up 234 bytes of FLASH.
- The .data segment (where initialized static variables are stored) starts at location 0x60, which is the first address after the register bank on a 2313 processor.
- The next available address in the .data segment is also location 0x60, so the application has no initialized data.
- The .bss segment (where uninitialized data is stored) starts at location 0x60.
- The next available address in the .bss segment is location 0x64, so the application uses 4 bytes of uninitialized data.
- The .eeprom segment (where EEPROM variables are stored) starts at location 0x0.
- The next available address in the .eeprom segment is also location 0x0, so there aren't any EEPROM variables.
Example 2-3. Portion of demo map file
.text 0x00000000 0xec