Editing IBM PC XT/370 (section)

== The Three 370PC Cards ==

The XT/370 is defined by three full-length 8-bit ISA cards that occupy three of the XT's eight expansion slots:

=== 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.

=== 370PC-M (Memory Card) ===

* '''512 KB of dual-ported RAM''', shared between the 370PC-P card and the XT host through a back-edge connector unique to the 370PC card pair.
* Of the 512 KB, '''416 KB is usable for S/370 applications'''; the remainder is reserved for the emulator runtime.
* The card also '''contributes 384 KB to the XT host''', bringing the host's RAM from 256 KB to the full 640 KB.<ref>Mueller, S. ''Upgrading & Repairing PCs'', 2nd edition, 1992, pp. 73–75, 94.</ref>

=== PC3277-EM (3277 Emulation Adapter) ===

* '''3270 terminal emulation adapter''' — required for the XT/370 to download VM/PC system software from a host mainframe, and for connecting CMS to mainframe-resident files.
* '''Late-production XT/370s replaced this with the standard 3278/79 Emulation Adapter''' (same card used by the [[IBM 3270 PC]]).<ref>IBM SA38-0037-00 ''Personal Computer Family Service Information Manual'', July 1989, §6-17.</ref>