Summary: This application note describes a small problem in the SEGA C
compiler by Paul Lee, and shows an easy "work around" for it.

Probably the easiest way to explain the problem is with an example. Here's
a very small example program, made by stripping demo6.c.

- - - - - begin showbug.c - - - - -

#include "genesis.h"

ulong tiledata[] = {
    0x00000008,    /* tile #0 = slash character */
    0x00000080,
    0x00000800,
    0x00008000,
    0x00080000,
    0x00800000,
    0x08000000,
    0x80000000,
};

/*
 *  Variables...
 */
ulong vtimer;   /* VBL timer, incremented by ISR in sega.s */

/*
 *  Main program
 */
void main()
{
}

- - - - - end showbug.c - - - - -

If you compile this with the SEGA C compiler, you'll get the following:

- - - - - begin showbug.s - - - - -

	.data
	.globl	_tiledata
_tiledata:
	.dc.l	8
	.dc.l	128
	.dc.l	2048
	.dc.l	-32768
	.dc.l	524288
	.dc.l	8388608
	.dc.l	134217728
	.dc.l	-2147483648
	.bss
	.globl	_vtimer
	.comm	_vtimer,4
	.text
	.globl	_main
_main:
	bra	L1
L0:
L2:
	unlk	a6
	rts
L1:
	link	a6,#-0
	bra	L0
	.data

- - - - - end showbug.s - - - - -

If you look at the generated "tiledata" values, you'll notice that the fourth
and eight values are incorrect (negative). If you tried to use values 0x8000
through 0xffff in demo6.c (demo6.c comes with the compiler) you will get an
unwanted "stripe" in your tile. Instead of

.......O
......O.
.....O..
....O...
...O....
..O.....
.O......
O.......

you'll see

.......O
......O.
.....O..
OOOOO...
...O....
..O.....
.O......
O.......

(I discovered this while working on tiles for my game "moonbase".)

Basically, the compiler is incorrectly computing the TYPE of the constants,
based upon their VALUE.

While reading through the source code to the compiler (in hopes of finding
a quick fix) I came up with a "work around" for the problem. The compiler
isn't smart enough to use the variable type specified in the array
declaration, BUT you can explicitly tell it the correct type using suffix
characters.

- - - - - begin showfix.c - - - - -

#include "genesis.h"

ulong tiledata[] = {
    0x00000008UL,    /* tile #0 = slash character */
    0x00000080UL,
    0x00000800UL,
    0x00008000UL,
    0x00080000UL,
    0x00800000UL,
    0x08000000UL,
    0x80000000UL,
};

/*
 *  Variables...
 */
ulong vtimer;   /* VBL timer, incremented by ISR in sega.s */

/*
 *  Main program
 */
void main()
{
}

- - - - - end showfix.c - - - - -

The change from the previous version is the "UL" suffix characters.

- - - - - begin showfix.s - - - - -

	.data
	.globl	_tiledata
_tiledata:
	.dc.l	8
	.dc.l	128
	.dc.l	2048
	.dc.l	32768
	.dc.l	524288
	.dc.l	8388608
	.dc.l	134217728
	.dc.l	-2147483648
	.bss
	.globl	_vtimer
	.comm	_vtimer,4
	.text
	.globl	_main
_main:
	bra	L1
L0:
L2:
	unlk	a6
	rts
L1:
	link	a6,#-0
	bra	L0
	.data

- - - - - end showfix.s - - - - -

As you can see, the generated code no longer has the bad value for the fourth
array element. The eighth element is still negative, but that's harmless.

Acknowledgements
----------------
I would like to thank Paul Lee (for releasing the SEGA C compiler in the
first place), and Steven Fuller, for loaning me the source code so I could
research this problem.

Kevin Banks (kbanks@ro.com)