Print Page - Syntax for pointers to fields of struct pointers?

Title: Syntax for pointers to fields of struct pointers?
Post by: wossname on October 29, 2005, 04:19:23 PM

I'm having some problems with the syntax in this simple procedure.

I'm just trying to get the address of a byte field. See the highlighted line...

Code Select


;-----------------------------
WriteGameString PROC USES eax edx,
	gameMsg:PTR GAMESTRING
;
;	Draws a color and xy position prefixed string
;
;	Receives:	gameMsg = pointer to a GAMESTRING struct that we want to draw
;	Returns:	NOTHING
;-----------------------------

	mov dl, (GAMESTRING PTR gameMsg).xPos
	mov dh, (GAMESTRING PTR gameMsg).yPos
	CALL GotoXY
	
	movzx eax, (GAMESTRING PTR gameMsg).color
	CALL SetTextColor
	
[b][color=Red]	mov edx, OFFSET gameMsg.message          ;     WRONG :([/color][/b]
	CALL WriteString
	
	ret
WriteGameString ENDP

Here is my struct definition...

Code Select

GAMESTRING STRUCT
	color	BYTE	16						; defaults to white on black text
	xPos	BYTE	0						; defaults to column 0
	yPos	BYTE	0						; defaults to row 0
	message	BYTE	80 * 25 DUP(0), 0		      ; enough to fill a default screen (with the compulsory null terminator)
GAMESTRING ENDS

EDX needs to be set to the absolute offset address of that byte field. I can't seem to work out the right syntax for it. I keep getting "invalid operand for OFFSET" errors so i'm obviously missing some vital bit of syntax somewhere.

I did think of simply adding a number of bytes to the pointer itself but somehow that seems a bit of a dirty way to do it.

Can anyone help a struggling old C# coder? :)

Title: Re: Syntax for pointers to fields of struct pointers?
Post by: arafel on October 29, 2005, 04:36:02 PM

how about

Code Select

mov     eax, gameMsg
lea     edx, [eax.GAMESTRING.message]

Title: Re: Syntax for pointers to fields of struct pointers?
Post by: wossname on October 29, 2005, 06:27:49 PM

Thanks, but it doesn't seem to make any difference.

Title: Re: Syntax for pointers to fields of struct pointers?
Post by: tenkey on October 29, 2005, 10:40:08 PM

Your usage of gameMsg in the other instructions is in error.

According to your PROC, gameMsg is a pointer argument. That is stored in memory (stack is memory). The x86 processor does not support data pointer usage from memory. You need to copy the pointer value into a register.

In addition to what arafel posted, you need:

Code Select


	mov eax, gameMsg
	mov dl, [eax.GAMESTRING.xPos]
	mov dh, [eax.GAMESTRING.yPos]
	CALL GotoXY
	
	mov eax, gameMsg
	movzx eax, [eax.GAMESTRING.color]
	CALL SetTextColor

Title: Re: Syntax for pointers to fields of struct pointers?
Post by: Retsim_X on October 29, 2005, 11:16:19 PM

helo
wossname

have a look at this post and check out my code in the zip fille

it dosnt have any comments and i am having a different problem with my app but if you want to look at a way to use registers as ptrs to memory.
and accessing structures through it aswell :)

i might not be relevent but have a look any way it may be of some use

http://www.masmforum.com/simple/index.php?topic=3086.0

Title: Re: Syntax for pointers to fields of struct pointers?
Post by: wossname on October 30, 2005, 11:32:17 AM

Thanks guys I think i get it now.

Cheers :U

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