Interworking enables you to mix ARM and Thumb code so that:

This means that, if you compile or assemble code for interworking, your code can call a routine in a different module without considering which instruction set it uses. The ARM compiler and ARM assembler both use the --apcs=/interwork command-line option to enable interworking.

You can freely mix code compiled or assembled for ARM and Thumb, provided that the code conforms to the AAPCS. See the specification, in install_directory\Documentation\Specifications\...\PDF\aapcs.pdf.

An error is generated if the linker detects:

The ARM linker detects when an interworking function is being called from a different state. Call and return instructions are changed, and small code segments called veneers, are inserted to change processor state where necessary. See Veneers in the Linker User Guide for more information.

The ARM architecture v5T and later provide methods to change processor state without using any extra instructions. There is almost no cost associated with interworking on ARMv5T and later processors.