Use a single character for use in a comparison statement - Assembly (Motorola 68k)

Question

I have this C++ code I'm trying to replicate in Assembly (68K):

int main()
{
    int i=0;
    char *string = "This is a string"
    while(string[i]!=' ')
    {
        /.../
        i++;
    }
    return 0;
}

I'm stuck on the string[i]!=0, indexing part of assembly. I need to CMP.B with a letter string[i] with some ' ' in memory. I tried CMP.B [STRING, D3],D5 with STRING being the string stored as a variable, D3 being the current index as a number stored in a register and D5 being the empty space I'm comparing it with in a register,

Answer 1

CMP.B [STRING, D3],D5

This won't work: You need to use an address register and you cannot use a 32-bit offset when using a register.

Instead, load the address of STRING into an address register - for example A4:

LEA.L (STRING), A4

Then perform CMP.B (0,D3,A4),D5

EDIT

I don't know the assembler you are using. Using GNU assembler, the instruction CMP.B (0,D3,A4),D5 is writen as CMP.B (0,%D3,%A4),%D5.

I just looked up the 68000 programmer's manual and it seems that the official syntax is: "CMP.B (0,A4,D3.L),D5" (or "CMP.B (0,A4,D3.W),D5" if only the low 16 Bits of D3 shall be used).

The instruction accesses the byte at the address A4+D3, so if the register A4 contains the address of string[0] (this means: A4 contains the value of the pointer string!) and the register D3 contains the value i, the instruction accesses the value string[i].

That value will be compared to the low 8 bits of D5.

Answer 2

If string is at a PC-relative address, you can write something like this:

cmp.b   string(pc,d3.w),d5

But very often it is actually more efficient to load the address of the string into an address register and use post-increment addressing (it depends a bit on whether you finally need the address or the index of the first matching character.

    moveq  #' ',d5
    lea    string,a4     ; or string(pc)
loop:
    cmp.b  (a4)+,d5
    bne.s  loop
; now (a4,-1) has the address of the first space character