What causes the L121 Improper Fixup error in Keil C51? How can I fix it?
The most common cause is that, when the code was compiled, the compiler assumed functions were going to be close enough together to use the ACALL instruction, which can only jump 2k from its current destination. However, if the linker places functions farther apart than this, this fixup error occurs.
You can resolve this issue by changing the Memory Model setting in the Compiler to Large (program size 64k).