PC-based IBM-compatible mainframes  Since the rise of the personal computer in the 1980s, IBM and other vendors have created PC-based IBM-compatible mainframes which are compatible with the larger IBM mainframe computers. For a period of time PC-based mainframe-compatible systems had a lower price and did not require as much electricity or floor space. However, they sacrificed performance and were not as dependable as mainframe-class hardware. These products have been popular with mainframe developers, in education and training settings, for very small companies with non-critical processing, and in certain disaster relief roles (such as field insurance adjustment systems for hurricane relief). Up until the mid-1990s, mainframes were very large machines that often occupied entire rooms. The rooms were often air conditioned and had special power arrangements to accommodate the three-phase electric power required by the machines. Modern mainframes are now physically comparatively small and require little or no special building arrangements. 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 some exotic combinations with additional hardware that could execute S/370 instructions locally. 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.