And it explains why IBM did such a good job in documenting the PC architecture:
*[https://userpages.umbc.edu/~squire/intel_book.pdf Intel Microprocessors Practical Reference]
*[https://bitsavers.org/pdf/ibm/pc/GG24-3057-00_IBM_PC_Internals_Fundamentals_Course_Notes_198603.pdf IBM PC Internals Fundamentals Course]
*[https://bitsavers.org/pdf/ibm/pc/pc/6025008_PC_Technical_Reference_Aug81.pdf IBM PC Hardware Technical Reference]