Editing IBM PC XT/370 (section)

=== 370PC-P (Processor Card) ===

The processor card carries:

* '''Two modified Motorola 68000s''' — IBM had Motorola rewrite the 68000 microcode to '''directly execute most System/370 fixed-point and non-floating-point instructions'''. When the 68000 encounters an opcode it does not recognise as native S/370, the instruction traps and is handled by a software interpreter running on the unmodified 68000. The custom 68000 is sometimes called the '''Micro/370'''.<ref>https://www.cpushack.com/2013/03/22/cpu-of-the-day-ibm-micro-370/</ref><ref>https://thechipletter.substack.com/p/motorola-intel-ibm-make-a-mainframe</ref>
* '''A modified Intel 8087''' — IBM had Intel modify the 8087 floating-point unit to implement '''IBM System/370 floating-point arithmetic''' (excess-64 hexadecimal floating-point, not the IEEE 754 binary floating-point the standard 8087 implements).
* Glue logic to bridge the 370PC-P to the host XT's ISA bus and to the 370PC-M memory card via a dedicated back-edge connector.