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ThatGuy22 wrote:I have made a program that makes a pointer to a variable, and then I tried to use a DOS function to print it out, I seem to be doing somthing wrong since it doesn't print out what my variable is. Here is my program:

Code: Select all

        mov bx, PointTo  ;Make bx point to the variable PointTo
	mov ah, 0x0e     ;DOS function to print character.
	mov al, [bx]       ;Print character that bx points to ('H')
	int 0x10            ;Print character
	
PointTo:
	db 'H'               ;char to print

What may I be doing wrong?
-I think that I may have a problem with moving a value from a pointer stored in a larger register, to the one I am transferring it to that is smaller. This part of my code is on line 3 of my code(mov al, [bx]).

ThatGuy22 wrote:O sorry, and why would I need to zero bx?

ThatGuy22 wrote:Ok, so I tried it using cx for the pointer, but got the error "invalid effective address". Note that I am using nasm to assemble this. My new code is as follows:

Code: Select all

   mov cx, PointTo
   mov ah, 0x0e
   mov al, [cx]
   int 0x10
   
PointTo:
   db 'H'

So what may be wrong with this?

   mov cl, PointTo
   mov ah, 0x0e
   mov al, cl
   int 0x10
   
PointTo:
   db 'H'

mikr00lz wrote:because you dont want [cx] you just want cx. but when you fix that you'll find that you can't move a 16-bit register to an 8-bit register.
the braces in [cx] mean that you're want to move the value of the LOCATION of cx into another register...... being that cx is a REGISTER, it doesnt HAVE a memory location.
and please keep such trivial issues out of "OS Development"?

Gigasoft wrote:

mikr00lz wrote:because you dont want [cx] you just want cx. but when you fix that you'll find that you can't move a 16-bit register to an 8-bit register.
the braces in [cx] mean that you're want to move the value of the LOCATION of cx into another register...... being that cx is a REGISTER, it doesnt HAVE a memory location.
and please keep such trivial issues out of "OS Development"?

Oh, please do share more great words of wisdom with us.

On a more serious note, in the x86 family, the possible 16-bit addressing modes are as follows:
[bx+si+displacement]
[bx+di+displacement]
[bp+si+displacement]
[bp+di+displacement]
[si+displacement]
[di+displacement]
[bp+displacement]
[bx+displacement]
[address]
where displacement is a word, signed byte or nothing.

The 32-bit addressing modes are as follows:
[base+displacement]
[base+index+displacement]
[base+index*scale+displacement]
[address+index*scale]
[address]
where base is any register, index is any register except esp, scale is 1, 2, 4 or 8, and displacement is a double word, signed byte or nothing.

[cx] is therefore not a valid addressing mode, but [bx] is valid, and so is [ecx].

By the way, the meaning of mov bx, PointTo is different between assemblers. In NASM, this indicates an immediate operand, which is what you want here. In MASM, it indicates a memory operand, and you would write mov bx, offset PointTo to use the address as an immediate operand.

If you have further questions, please don't hesitate to RTFM.

unsigned short getea(unsigned char rmval, unsigned char mod) {
         switch (mod) {
                case 0:
                     if (rmval==0) return((*segptr<<4)+bx+si);
                        else if (rmval==1) return((*segptr<<4)+bx+di);
                        else if (rmval==2) return((*segptr<<4)+bp+si);
                        else if (rmval==3) return((*segptr<<4)+bp+di);
                        else if (rmval==4) return((*segptr<<4)+si);
                        else if (rmval==5) return((*segptr<<4)+di);
                        else if (rmval==6) return((*segptr<<4)+disp);
                        else if (rmval==7) return((*segptr<<4)+bx);
                     break;
                case 1:
                     if (rmval==0) return((*segptr<<4)+bx+si+disp);
                        else if (rmval==1) return((*segptr<<4)+bx+di+disp);
                        else if (rmval==2) return((*segptr<<4)+bp+si+disp);
                        else if (rmval==3) return((*segptr<<4)+bp+di+disp);
                        else if (rmval==4) return((*segptr<<4)+si+disp);
                        else if (rmval==5) return((*segptr<<4)+di+disp);
                        else if (rmval==6) return((*segptr<<4)+bp+disp);
                        else if (rmval==7) return((*segptr<<4)+bx+disp);
                     break;
                case 2:
                     if (rmval==0) return((*segptr<<4)+bx+si+disp);
                        else if (rmval==1) return((*segptr<<4)+bx+di+disp);
                        else if (rmval==2) return((*segptr<<4)+bp+si+disp);
                        else if (rmval==3) return((*segptr<<4)+bp+di+disp);
                        else if (rmval==4) return((*segptr<<4)+si+disp);
                        else if (rmval==5) return((*segptr<<4)+di+disp);
                        else if (rmval==6) return((*segptr<<4)+bp+disp);
                        else if (rmval==7) return((*segptr<<4)+bx+disp);
                     break;
                case 3: return 0; //should never get to this
         }
}

ThatGuy22 wrote:First off, I thought you can't move pointers into 8 bit registers(cl), that was why I was using cx. Second I wan't the full address to be stored in cx not just the offset or base address. Also I put Breackets('[]') on cx because I do want the value that cx points to.

Hobbes wrote:ThatGuy22,
No offense intended, but this is not the forum to post such questions. You really should learn assembly language elsewhere.

A good tutorial in my opinion is the Art of Assembly Language Programming. It covers all your questions. It uses MASM syntax which is a little different from NASM syntax, but that shouldn't be a problem. The differences are explained quite well in the NASM manual.

Solar wrote:Explaining pointer semantics? On "The Place to Start for Operating System Developers"?

Seriously?

berkus wrote:

Solar wrote:Explaining pointer semantics? On "The Place to Start for Operating System Developers"?

Seriously?

YA RLY. And also explaining some assembler basics to miker00lz. Dude, find the mistake in your code below (NASM syntax):

miker00lz wrote:

Code: Select all

   mov cl, PointTo
   mov ah, 0x0e
   mov al, cl
   int 0x10
   
PointTo:
   db 'H'