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assembly68000easy68k

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

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,

Solution 
  • 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.