boot el capitan iso virtualbox no bootable medium found

So, I found that data starts at some address (in my case it is 0x00200000 from the start of the VDI file). So, the start of VDI represents a header and the the rest is actual data stored on virtual drive. I was researching how VDI works and I found that it is usually dinamically allocated and that only the beginning of data is stored. So, I decided to put the binary machine code of my OS to virtual hard drive ( VDI file) and then set it to the top of boot order and load it instead of loading from virtual optical drive ISO. I don't want to convert my binary file to ISO every time. So, that is a lot of work I want to avoid. I found that I must compile C code to binary format, extract text section and save it as a file, then convert it to ISO, then mount it to virtual optical dive of diskete and then load my OS in VirtualBox. I am creating an operating system in assembler and C.

I read all of them, but none of them answers my problem. There are a lot of question on stackoverflow with the similar title.

Boot el capitan iso virtualbox no bootable medium found