CP/M Plus (CP/M Version 3.1)

Now that memory is very cheaply and easily available for microcomputers it is noticeable that, as each year goes by, the size of memory commonly installed increases by a factor of two. At present 64K of RAM is the norm for the 8-bit micro and the step up to 128K has produced a problem. An 8-bit processor cannot address, directly at least, more than 64K. This is because the largest integer that can be stored in a Z80 processor address register of 16 bits is 65535 in decimal notation, or FFFF in hexadecimal. 65535 is in fact 64 times 1024 minus 1; 1K being, of course, 2 to the power 10, not 1000.