/*
* Looks up a vertical scroll value and sets some related variables.
*/
#undef LOOKUP_YSCROLL_REC
#define LOOKUP_YSCROLL_REC(rec_no) \
do { \
yscroll_amount = get_word(vsram + rec_no * 2) & 0x7ff; \
\
/* interlace ? */ \
if (reg[12] & 2) \
yscroll_amount >>= 1; \
\
/* Offset for the line */ \
yscroll_amount += line; \
\
yoff = ((yscroll_amount >> 3) & (ysize - 1)); \
tile_line = (tiles + ((xsize * yoff) & 0x1fff)); \
scan = (yscroll_amount & 7); \
} \
while (0)
{
int xsize, ysize;
int x, scan = 0, w, xstart;
static int sizes[4] = { 32, 64, 64, 128 };
unsigned which;
unsigned char *where, *hscroll_rec_ptr, *tiles, *tile_line = NULL;
int xoff, yoff, xoff_mask;
int hscroll_amount, yscroll_amount = 0;
uint8_t two_cell_vscroll = 0;
/*
* when VSCR bit is set in register 11, this is 'per 2-cell'
* vertical scrolling as opposed to full screen vscrolling.
*/
two_cell_vscroll = ((reg[11] >> 2) & 0x1);
#if PLANE == 0
// Plane 0 is only where the window isn't
// This should make Herzog Zwei split screen work perfectly, and clean
// up those little glitches on Sonic 3's level select.
if (reg[18] & 0x80) {
// Window goes down, plane 0 goes up! :)
if ((line >> 3) >= (reg[18] & 0x1f))
return;
}
else {
// Window goes up, plane 0 goes down
if ((line >> 3) < (reg[18] & 0x1f))
return;
}
#endif
/*
* Get the vertical/horizontal scroll plane sizes
*
* 0b00: 32 cell
* 0b01: 64 cell
* 0b10: prohibited, but unlicensed games use this
* turns out to be 64.
* 0b11: 128 cell
*/
xsize = (sizes[(reg[16] & 3)] << 1);
ysize = sizes[((reg[16] >> 4) & 3)];
/*
* Here we compute pointer to the beginning of the hscroll table.
* The base address of the table is stored in reg[13] << 10.
*/
#if PLANE == 0
hscroll_rec_ptr = (vram + ((reg[13] << 10) & 0xfc00));
tiles = (vram + (reg[2] << 10));
#else // PLANE == 1
hscroll_rec_ptr = (vram + ((reg[13] << 10) & 0xfc00) + 2);
tiles = (vram + (reg[4] << 13));
#endif
// Wide or narrow?
if (reg[12] & 1) {
w = 40;
xstart = -8;
}
else {
w = 32;
xstart = 24;
}
/*
* Lookup the horizontal offset.
* See Charles MacDonald's genvdp.txt for explanation.
*/
switch (reg[11] & 3) {
case 0:
// full screen
// NOP - pointer in the right place
break;
case 1:
// invalid, but populous uses it
hscroll_rec_ptr += ((line & 7) << 2);
break;
case 2:
// per tile
hscroll_rec_ptr += ((line & ~7) << 2);
break;
case 3:
// per line
hscroll_rec_ptr += (line << 2);
break;
}
hscroll_amount = get_word(hscroll_rec_ptr);
xoff_mask = xsize - 1;
xoff = ((-(hscroll_amount>>3) - 1)<<1) & xoff_mask;
where = dest + (xstart + (hscroll_amount & 7)) * (int) Bpp;
/*
* If this is not column vscroll mode, we look up the
* whole screen vertical scroll value once and once only.
*/
if (two_cell_vscroll == 0)
LOOKUP_YSCROLL_REC(PLANE);
/*
* Loop cells, we draw 2 more cells than expected (-1 and w) because
* previously off-screen cells can be horizontally scrolled on-screen.
*/
for (x = -1; (x <= w); x++) {
/*
* If we are in 2-cell vscroll mode then lookup the amount by
* which we should scroll this tile.
*
* If we are not in 2-cell vscroll then we looked up the value
* for the whole screen vscroll earlier.
*
* We lookup vscroll values on even x values and this is the
* vscroll value for the next two cells. Note that cell -1 is
* a special case as we never looked up the vscroll value for
* cell -2.
*/
if ((two_cell_vscroll) && ((x % 2 == 0) || (x == -1))) {
/*
* Note that the underflow and overflow of the table
* for cell -1 and cell w is intentional.
*
* http://gendev.spritesmind.net/forum/viewtopic.php?t=737&postdays=0&postorder=asc&start=30
*/
uint8_t cell_index = (uint8_t) x % w;
int vscroll_rec_no = 2 * (cell_index / 2);
/*
* The records alternate, PLANE A, PLANE B, PLANE A,
* ...
*/
#if PLANE == 1
vscroll_rec_no++;
#endif
LOOKUP_YSCROLL_REC(vscroll_rec_no);
}
#if PLANE == 0
if (reg[17] & 0x80) {
// Don't draw where the window will be
if (x >= ((reg[17] & 0x1f) << 1))
goto skip;
}
else {
// + 1 so scroll layers in Sonic look right
if ((x + 1) < ((reg[17] & 0x1f) << 1))
goto skip;
}
#endif
which = get_word(tile_line + xoff);
#if (FRONT == 0) && (PLANE == 1)
draw_tile_solid(which, scan, where);
#elif FRONT == 1
if (which >> 15)
draw_tile(which, scan, where);
#else
if (!(which >> 15))
draw_tile(which, scan, where);
#endif
#if PLANE == 0
skip:
#endif
where += Bpp_times8;
xoff = ((xoff + 2) & xoff_mask);
}
}