pad data structure

typedef union {
    struct { // meta commands
        union {
            struct { // set up data pointer
                u8* ptr;
                u8  pad[6];
            } c00_data;
            struct { // write <= 9 bytes
                u8 count;
                u8 bytes[ 9 ];
            } c01_data;
            struct { // write N zeroes
                u32 number_of_zeroes;
                u8  padding[ 6 ];
            } c02_data;
            struct { // checksum/ack/nak
                // ?????
            } c03_data;
            struct { // jump to given function pointer
                void (*func_pointer)(void);
                u8 padding[ 6 ];
            } c04_data;
            u8 bytes[ 10 ];
        };
        u8 meta_command;
    } meta;
    struct { // DMA
        u32 dest_ram;
        u32 src_vrom;
        u32 size:24;
    } lowlevel;
    u8 bytes[ 12 ]; // last byte is padding
} pad_data_t;
// 12 bytes long