Hi,
I'm somewhat confused with the Thumb mode code size. My understanding is compiling with ARM mode will generate 32-bit instructions and compiling with Thumb mode will generate 16-bit instructions. When I compile my Cortex-M0+ project (which should use Thumb mode instructions only), the output binary file always increments in multiple of 4 bytes. Do I miss something here?
Here are the instructions I use:
armcc -c -O3 -Ospace --cpu Cortex-M0plus --thumb --apcs=interwork -I ./ -g -c aaa.c -o aaa.o
armasm --cpu Cortex-M0plus --apcs=interwork -I ./ -g startup.s -o startup.o
armlink --cpu Cortex-M0plus --ro_base 0x00000000 --rw-base 0x00100000 --entry Reset_Handler --first __Vectors aaa.o startup.o -o aaa.axf
Thanks,
First of all, there are at least 3 "modes" of ARM instructions - ARM, Thumb and Thumb 2. ARM indeed uses 32-bits for each instruction. Thumb indeed uses 16-bits for each instruction. However Thumb 2 uses 16-bits of 32-bits for instructions, depending on the instruction used and its variant (for example branch can use 16-bits of 32-bits). ARM Cortex-M0+ chip that you have supports most of Thumb instructions and some Thumb 2 instructions, so you cannot expect each instruction to take exactly 2 bytes. The next thing is alignment, which is usually set to 4 bytes. The last thing would be that usually instructions that load something to/from RAM store the address as a separate word in flash (because you cannot fit a full 32-bit address into a 16-bit or 32-bit instruction). Such additional words are 32-bit long (because these are addresses) and must be 32-bit aligned.
Regards,
FCh