#define	TXTBASE		0x0400
#define	TXTWIDTH	32
#define	TXTLINES	16
#define	TXTSIZE		TXTWIDTH*TXTLINES
#define	TXTEND		TXTBASE+TXTSIZE

static unsigned char *_getpos(unsigned char x, unsigned char y)
{
	unsigned char *pos;

#if 0 /* Ummm...why does this act like TXTBASE == 0? */
	pos = TXTBASE + ((y / 2) * TXTWIDTH) + (x / 2);
#else /* But this works? */
	pos = TXTBASE;
	pos += ((y / 2) * TXTWIDTH);
	pos += (x / 2);
#endif

	if (pos > TXTEND)
		return NULL;

	return pos;
}

--- non-working assembly (#if clause) ---

?0_getpos LEAS -2,S
 LDB 5,S
 CLRA
 TFR D,X
 LDB #2
 CLRA
 JSR ?div
 TFR D,X
 LDB #32
 CLRA
 JSR ?mul
 ADDB #1024
 ADCA #0
 PSHS A,B
 LDB 9,S
 CLRA
 TFR D,X
 LDB #2
 CLRA
 JSR ?div
 ADDD ,S++
 STD 0,S
 LDD 0,S
 TFR D,X
 LDD #1536
 JSR ?ugt
 LBEQ ?8
 LDD #0
 JMP ?9
?8 EQU *
 LDD 0,S
?9 EQU *
 LEAS 2,S
 RTS

--- working assembly (#else clause) ---

?0_getpos LEAS -2,S
 LDD #1024
 STD 0,S
 LDB 5,S
 CLRA
 TFR D,X
 LDB #2
 CLRA
 JSR ?div
 TFR D,X
 LDB #32
 CLRA
 JSR ?mul
 ADDD 0,S
 STD 0,S
 LDB 7,S
 CLRA
 TFR D,X
 LDB #2
 CLRA
 JSR ?div
 ADDD 0,S
 STD 0,S
 LDD 0,S
 TFR D,X
 LDD #1536
 JSR ?ugt
 LBEQ ?8
 LDD #0
 JMP ?9
?8 EQU *
 LDD 0,S
?9 EQU *
 LEAS 2,S
 RTS