Can Someone Solve the mistery of this function please

rename tha var result;

Resulr is already declared into the function as byte but the function result is a boolean value
That's why it says [DCC Error] PCI.PAS(86): E2010 Incompatible types: 'Boolean' and 'Byte'

so the correct one could be
function readPCIRegisterByte(RegisterNumber:word;BusNumber:Byte;FunctionNumber:Byte;var InternalResult:byte):boolean;
var okay:boolean;
    res:byte;
begin
  okay:=false;
  res:=0;
  asm
    db 66h; pusha
    mov AX,0B108h
    mov BH,busNumber
    mov BL,functionNumber
    mov DI,RegisterNumber
    int 1Ah
    jc @noaction
    mov res,cl
    mov okay,true
  @noaction:
    db 66h; popa
  end;
  InternalResult := res;
  readPCIRegisterByte:=okay;
end;

It solved my problem thanks
However before accept the solution just one more consideration.
What if someone calls the result from out side of the function.
You know, normally result refers the actual output of the function.
do you think it may cause confliction?

well, a function result is of course the result of the function itself, so for definition "Result" become reserved word for this case.
Using this word assigning it to a variable can of course cause a conflict, and that's why modern compilers give errors in this case.

so how can I solve this confliction without ruin the function of the function? This is the matter.

Yes I think this is the solution, compiler made no complain after I added {$X-}