IBM had demonstrated use of a mainframe instruction set in their first desktop computer—the
IBM 5100, released in 1975. This product used microcode to execute many of the
System/370's processor instructions, so that it could run a slightly modified version of IBM's
APL mainframe program interpreter. In 1980 rumors spread of a new IBM personal computer, perhaps a miniaturized version of the 370. In 1981 the
IBM Personal Computer appeared, but it was not based on the System 370 architecture. However, IBM did use their new PC platform to create combinations with additional hardware that could execute S/370 instructions locally.
Personal Computer XT/370 In October 1983, IBM announced the IBM Personal Computer XT/370. This was essentially a three-in-one product. It could run
PC DOS locally, it could also act as
3270 terminal, and finally—its most important distinguishing feature relative to an
IBM 3270 PC—was that it could execute S/370 instructions locally. The XT/370 was an
IBM Personal Computer XT (System Unit 5160) with three custom 8-bit cards. The processor card (370PC-P), contained two modified
Motorola 68000 chips (which could emulate most S/370 fixed-point instructions and non-floating-point instructions), and an
Intel 8087 coprocessor modified to emulate the S/370 floating point instructions. The second card (370PC-M), which connected to the first with a unique card back connector contained 512
KiB of memory. The third card (PC3277-EM), was a 3270 terminal emulator required to download system software from the host mainframe. The XT/370 computer booted into DOS, then ran the
VM/PC Control Program. The card's memory space added additional system memory, so the first (
motherboard) memory could be used to move data to the expansion card. The expansion memory was dual ported, and provided an additional to the XT Machine bringing the total RAM on the XT side to . The memory arbitrator could bank switch the second 128 KiB bank on the card to other banks, allowing the XT
Intel 8088 processor to address all the RAM on the 370PC-M card. Besides the of usable RAM for S/370 applications, the XT/370 also supported up to of
virtual memory using the hard drive as its paging device. IBM claimed the XT/370 reached 0.1
MIPS (when the data fit in RAM). In 1984, the list price of an XT/370 in its typical configuration was approximately so compared favorably with IBM's own mainframes on a $/MIPS basis; for example, an
IBM 4341 delivered 1.2 MIPS for . While it theoretically reduced demand on customers' mainframes by offloading load onto the smaller computer, as customers purchased more XT/370s they likely increased the overall load on the mainframes, increasing IBM's mainframe sales. Similarly to the mainframe version of
VM/CMS, the VM/PC also created the illusion of
virtual disks, but on the PC version these were maintained as PC DOS files, either on floppy or hard disk. For example, the CMS virtual disk belonging to user FRED at device address 101 was stored as the DOS file FRED.101. The CMS IMPORT and EXPORT commands allowed extraction of files from these virtual drives as well as
ASCII/
EBCDIC conversion. The XT/370 came with an XT-style 83-key keyboard (10 function keys). Newer revisions of the XT/370 dropped the PC3277-EM in favor of the IBM 3278/79 boards. The XT/370 was among the XT systems that could use a second hard drive mounted in the 5161 expansion chassis.
BYTE in 1984 called the XT/370 "a qualified success". The magazine praised IBM for "fitting all of the 370's features into the XT", and hoped for technical improvements that "might result in an even better computer". The XT/370 was discontinued in April 1987.
Personal Computer AT/370 In 1984, IBM introduced the IBM Personal Computer AT/370 with similar cards as for the XT/370 and updated software, supporting both larger hard disks and DMA transfers from the 3277 card to the AT/370 Processor card. The system was almost 60% faster than the XT/370. VM/PC version 2, launched in November 1985, improved performance by up to 50%; it allowed add-on memory (in addition to the disk) to be used as a
page cache for VM. A November 1985
Computerworld article noted that the machine was "slow selling". Officially called the IBM 7437 VM/SP Technical Workstation (and later also known as the Personal System/370), it was a freestanding tower that connected to a
MCA card installed in a
PS/2 Model 60, 70, or 80. The 7437 tower contained the processor and a 16Mbytes main memory, and the PS/2 provided I/O and disk storage. The 7437 ran the IBM
VM/SP operating system, and one IBM representative described the 7437 "like a
9370 with a single terminal". It was intended for existing S/370 users and its November 1988 list price was $18,100 for a minimum 25-unit order. One of its intended roles was to provide a single-user S/370-compatible computer that could run
computer-aided design and
engineering applications that originated on IBM mainframes such as
CADAM and
CATIA. Graphics support was provided by an IBM 5080 graphics system, a floor-standing tower. The 5080 was connected to the 7437 through the PS/2 via a cable and MCA adapter.
Personal/370 Later, IBM introduced the Personal/370 (aka P/370), a single slot 32-bit
MCA card that can be added to a
PS/2 or
RS/6000 computer to run System/370 OSs (like
MUSIC/SP,
VM,
VSE) parallel to
OS/2 (in PS/2) or
AIX (in RS/6000) supporting multiple concurrent users. It is a complete implementation of the S/370 Processor including a
FPU co-processor and 16 MB memory. Management and standard I/O channels are provided via the host OS/hardware. An additional 370 channel card can be added to provide mainframe-specific I/O such as 3270 local control units, 3400/3480 tape drives or 7171 protocol converters. Although a single-card product, the P/370 ran three times faster than the 7437, attaining 3.5 MIPS, on par with a low-end
IBM 4381. A subsequent book (by the same author) claims 4.1 MIPS for the P/370. The Personal/370 was available as early as November 1989 although on a "special bid basis". ==System/390==