struct BG3Cnt {
     u16 field_0:13;
     u16 field_d:1;
     u16 field_e:2;
};

void sub_80789D4(bool8 a1)
{
 // BUG: 8-bit access to an io reg is disallowed.
 #define BG3CNT (*(volatile struct BG3Cnt*)REG_ADDR_BG3CNT)
     if (!a1) {
         BG3CNT.field_e = 0;
         BG3CNT.field_d = 1;
     } else if (sub_8076BE0()) {
         BG3CNT.field_e = 0;
         BG3CNT.field_d = 1;
     } else {
         BG3CNT.field_e = 1;
         BG3CNT.field_d = 0;
     }
}