Someone

BG3_CR = ROTBG_SIZE_512x512 | BG_TILE_BASE(4) | BG_MAP_BASE(0) | BG_COLOR_256;

uint16 val;
for(val = 0; val < 256; val++) {
        *(BG_PALETTE+val) = RGB15(val%31,val%31,val%31);
}
for(val = 0; val < (64*64); val++) {
        *((uint16*)(BG_MAP_RAM(0))+val) = 0x0000;
}
for(val = 0; val < 64; val++) {
        *((uint16*)(BG_TILE_RAM(4))+val) = 0x1e00;
}