Tiny C Compiler: "error: unknown opcode 'jmp'"

Question

Given this code:

int main(void)
{
    __asm volatile ("jmp %eax");

    return 0;
}

32-bit TCC will complain with:

test.c:3: error: unknown opcode 'jmp'

but the 64-bit version will compile just fine.

What's the problem with the 32 bit code?

Answer 1

The solution is to simply add a star (*) before the register, like this:

__asm volatile ("jmp *%eax");

I'm not exactly sure what the star means. According to this SO post:

The star is some syntactical sugar indicating that control is to be passed indirectly, by reference/pointer.

As for why it works with 64-bit TCC, I assume that it's a bug; 64-bit GCC complains with Error: operand type mismatch for 'jmp', as it should.