Result reader is used to determine if the BIOS is running or not, it's used to check if the BIOS wants to continue the bootup or not, if true, it will continue the bootup, if false, it will go back a few hundred lines to pick up the next instruction. Then the code will move back to the beginning of the execution line and continue.
Null test is basically a test which tells the BIOS to validate an input parameter. Usually it's used in the video adapter initialization procedure. The code will first call a function which will check if the address is null, if it's null, the code will jump to a specific address which will lead to the BIOS startup. Anytime the video adapter something messed up, all you need to do is change the address with the NULL test opcode and reset the POST.
In order for the base board to get its power, there's a method to go through which is called extended BIOS function, also known as EAX (Extended Address) or E/AX (Extended Function). This function has a status or a result bit which indicate whether the base board can perform the operation or not. This way, the status of the bit is set at the beginning of the BIOS code (bit is set if running, and reset if not), and the code checks if the status is set OR if this function is the power-on function, the only condition where the power-on function is not set is in battery ROM, we will talk more about that stuff later). If the status is set, the base board calls the function, if not, the operation is skipped. If the status is reset, the code will go into normal execution. 7211a4ac4a