Assembly language

Assembler for PCs
http://home.snafu.de/nkomin/html/assembe.htm

how to program a boot-manager
https://museo.freaknet.org/gallery/software/xenix/bootman.html

Writing a boot loader in Assembly and C
https://www.codeproject.com/Articles/664165/Writing-a-boot-loader-in-Assembly-and-C-Part