间接寻址如何在汇编语言中使用Motorola 68k

Question

I have this code:

loop2
        move.b  (a4)+,d3      * moving the morse code array input to d3
        muls    #5,d3
        add.b   $d3(a6),d3    * moving the character in morse code array to d4
        move.b  d3,d4
        cmp.b   #dot,d4

Here, I am accessing an array starting at a4 . I am taking an element of the array and multiplying it with 5 to move to the memory location where I have the desired element.

a6 represents the starting point of an array which contains certain character.

the statement $d3(a6),d3 wokrs but the code gets faulty as i know the code d3(a6),d3 should be correct but it show me an error. how should i do it ?

Answer 1

What's happening is since $ is for specifying hex constants, and d3 is technically a valid hex constant, it's using a6 + 0xd3 as the memory address. What you're trying to accomplish uses different syntax, namely both registers are in the parentheses: (a6,d3)

So the correct statement is:

add.b   (a6,d3),d3

Answer 2

In add.b (a6,d3),d3 you probably should declare size of d3 used for address generation: either (a6,d3.w) or (a6,d3.l) , depending on what you need. In general case, after a multiplication you have full 32bit result and should use d3.l . Only if you are sure that the result of multiplication fits 16bit you can use d3.w .
If you write code specifically for 68020+ (020,030,040,060), also consider using scaled addressing mode like (a6,d3.w*n) , where n is one of 1, 2, 4, 8.