the background level have distinct sets of registers. Therefore, entry into most interrupt levels does not require saving the registers of the interrupted program. The infrequently used level which processed program and hardware errors shares registers with the next highest level and thus has to save and restore registers. The five program levels are: Error processing Communication line events Channel adapter events Service requests from other levels Background level The 3705 has a flashing light style control panel which is superior to most other IBM panels of that style. The two unusual (for IBM) features are: Storage (and some external registers) can be displayed while the program is executing. The hex input switches and the two register-sized displays can be used by the program while the machine is running. This capability derived from the objective of emulating the IBM 2703 which has a fairly rich control panel. The 3705 has a maximum memory capacity of 512 KB, but this requires four physical frames (the base frame and three expansion frames). An Amdahl 4705 has a maximum memory capacity of 1024 KB, all of which is contained within the base frame. Peripherals Three types of peripherals were available: communications scanners, channel adapters and 8" floppy disk (only available in models without a channel adapter). The first Communication Scanner and the first Channel Adapter occupied the same frame as the CCU. Extra peripherals required extra frames. All Amdahl 4705s were provided with an 8" floppy drive, which was used for diagnostics as well as for remote program loading. This floppy drive and all Channel Adapters were contained in the Base Frame, although additional S/370 channel I/O interfaces could require an Expansion Frame. That is, more than four I/O interfaces required an Expansion Frame, which accommodated four additional I/O interfaces. Communication scanners Three different communication scanners were offered: The Type I Communication Scanner was an entry level device which presented an interrupt on every received bit. Transmission also required an interrupt for every bit. In theory this would have allowed for rather imaginative uses such as Morse Code and connection to devices with unusual framing methods. A maximum of 64 half-duplex lines could be attached. The aggregate bandwidth was restricted due to the heavy processing requirements. The Type II Communication Scanner performed functions similar to a USART. There was an interrupt for every transmitted or received character. Six different asynchronous character formats, two