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Instruction width selection - forward/external reference

Hi,

I am learning the Thumb 2 instruction set and at the moment I am trying to understand when to use the instruction width specifier.

e.g.:

ldmia.w 	sp!,{r3,lr}

I understand sometimes it is necessary to make some instructions 32 bit wide because the content/addresses is/are not fitting into the 16 bit encoding.

But reading the ARM Keil explanation about it I am wondering what exactly is meant by internal/external reference?

And more precisely when I have to worry about adding .w and when not?

Top replies

42Bastian Schick over 6 years ago +1 suggested

In general, the assembler will take care. You only need to worry about it if you want to force a special instruction. I suggest to read the relevant reference manual as it lists all opcodes and the syntax...
Joseph Yiu over 6 years ago +1 suggested

In the case of using Branch instructions in assembly programming, if the branch target is in a different object file (i.e. extern), then you might need to specify 32-bit version of branch instructions...