68k: Adding 0x100 to 9-bit register and doing BEQ.s - how does it make sense?

Question

During my current project I found such code:

move.w  (0xFFFC0C).l, d0 | read SCSR
andi.w  #0x100, d0       | add 256 to d0
beq.s   location         | branch if LSB = 0x00

I cannot get the idea of this fragment. I guess author tried to check the status of the UART, but how does andi.w and beq.s serves the purpose, when SCSR may contain maximum of 9 bits?

Answer 1

It's not adding. It is binary and operation, and in translation to C the code looks as follows:

d0 = readSCSR(); // move.w  (0xFFFC0C).l, d0 | read SCSR
if (d0 & 0x100) 
//andi.w  #0x100, d0       | not add, but AND 256 to d0
//beq.s   location         | branch if LSB = 0x00
{
    // do whatever needed
}
location: // else branch to location