Glitch with NeoSD on AES console

[juicelink77]

Hi !

Thanks for this wonderfull SDK !

I work on a fighting game and I have good results under emulator but many glith on real hardware with NeoSD cartdrige.

Here a screen shot on emulator :
https://flashkof.fr/temp/TPF_NG/01.jpg

And here a picture of my neogeo aes with neosd cartdrige :
https://flashkof.fr/temp/TPF_NG/02.jpg

Does this resonate with you?

Regards,
Jul

[dciabrin]

Hello Jul,

I know that the timing for writing to the Video RAM is fairly sensible, and sometimes the emulator is more lenient than the real hardware. This has been reported at least once by the creator of Shadow Gangs Zero.

Based on https://wiki.neogeodev.org/index.php/VRAM, you might want to wait some cycles between writes to the VRAM to make sure the operations are correctly read by the graphics chip.

Hope that helps

[juicelink77]

Hi !
Thank you, I will check that :)

Regards,
Jul

[dogruis]

Use the shadow-RAM commit pattern: build all your SCB / VRAM-bound writes into a RAM shadow during active display, then burst the dirty ranges to VRAM inside vblank only (after the vblank IRQ fires, watchdog kicked).

Max words per vblank: ~1,100.

Why: the Neo Geo's vblank window is ~40 scanlines ≈ 28k usable m68k cycles. Each VRAM-write word costs ~25 cycles at -O2 (REG_VRAMADDR/REG_VRAMRW poke + loop overhead), so the safe ceiling is about 1,100 words per frame. Spill past that and the burst leaks into active display — the LSPC starts reading VRAM mid-write and you get tearing.

If you need more than 1,100 words of changes in one frame, either (a) split the commit across multiple frames, (b) compress to dirty-range bursts so unchanged data isn't re-written, or (c) use auto-increment regions so you only pay the address-set cost once per run.

[dogruis]

Honestly, I only figured this out once I built an on-screen profiler. Before that I was guessing — the tearing was intermittent, hard to reproduce, and didn't correlate with anything obvious from reading the code. The moment I had a fix-layer WMAX readout showing peak words written per vblank (and turning red when it overran), the budget became obvious and the regressions became catchable. Strongly recommend wiring up a profiler before you trust any word-count estimate — measured cycles beat reasoned ones every time on this hardware.

How to know you're in vblank:

1. Land at the top of vblank using the VBI callback. ngdevkit's linker script declares rom_callback_VBlank as an optional weak symbol; if you define it, the BIOS routes the vblank IRQ to it. Define a volatile u16 counter, bump it from the callback, and spin-wait on it changing before you do any VRAM writes:

static volatile u16 neogeo_vblank_count = 0;

void rom_callback_VBlank(void) {
    neogeo_vblank_count++;
}

void wait_for_vblank(void) {
    const u16 seen = neogeo_vblank_count;
    while (neogeo_vblank_count == seen) { }   /* blocks until next VBI fires */
}

Important: don't latch a "vblank happened" flag and consume it — by the time your frame work finishes, the flag may already be set from earlier in active display, and your commit will start mid-frame. Always sample-then-spin so you block for the next vblank, which lands you at the top of the window with the full ~40 scanlines ahead of you.

2. Verify with REG_LSPCMODE (raster line readout). The high 9 bits of *REG_LSPCMODE (bits 7..15) hold the current scanline:

u16 raster = (*REG_LSPCMODE >> 7) & 0x01FF;
Vblank runs roughly lines 248..287 (NTSC, 264 visible + ~40 blank). Read this after your commit finishes and confirm it's still inside the vblank range — if it's wrapped back to a low number (1..247) you've already spilled into active display and the bottom of the screen will tear. The profiler's RASM (worst end-of-vblank line) and RAS (last-frame end line) readouts are exactly that check, sampled at commit-end so the figure includes the profiler's own cost.

3. Don't forget the watchdog. On MVS, write *REG_WATCHDOGW = 0 once per frame inside vblank — skip it and the hardware resets the cart after ~3 frames. Harmless on AES but free insurance.