The TMS9918A VDP was rendering background tiles even when the display blank bit (register 1, bit 6) was set. This caused games like Bomb Jack on SG-1000 to show a blue screen with garbled tiles instead of just the backdrop color during initialization.

Changes

crates/core/src/tms9918a.rs

• Moved display blank check from sprite-only to encompass both tile and sprite rendering
• When blank bit is set, only backdrop color renders (no tiles, no sprites)

Before:

fn render_scanline(&mut self, line: u8) {
    // Clear to backdrop
    // Render tiles (always)
    if !display_blanked {
        self.render_sprites(line, line_offset);  // Only sprites checked blank bit
    }
}

After:

fn render_scanline(&mut self, line: u8) {
    let display_blanked = (self.registers[1] & 0x40) != 0;
    // Clear to backdrop
    if !display_blanked {
        // Render tiles
        // Render sprites
    }
}

Impact

Affects SG-1000 and ColecoVision (both use TMS9918A). Games using display blanking during screen transitions or initialization will now render correctly.

Documentation:

• Updated SG-1000 and ColecoVision READMEs with fix details

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