contained the processor and up to 128 KB of RAM as well as most of the external interfaces: Two serial interfaces (SIO-4 and SIO-2) – providing full and cut-down RS-232 interfaces, respectively. Parallel I/O – used either as a high-speed link for the external disk drives or ROM Packs, or as a Centronics printer port. Cassette interface. Accessory input – two channel analogue joystick interface. Composite video and audio. RF TV output. Network connection (optional). The high resolution graphics and memory option board allowed an extra 128 KB of RAM to be added and included a TTL RGB monitor output. The option board could also be fitted with an IEEE-488 interface and an AMD 9511 or 9512 floating point coprocessor. Firmware The 480Z was fitted with up to 32 KB of firmware: ROM monitor (ROS) – 8 KB. Z-Net network firmware – 4 KB. BASIC in ROM – 20 KB. ROM monitor ROS (standing for Resident Operating System) provided a monitor program and a set of basic system services. The monitor could be used to start BASIC from ROM, load application programs from cassette, or boot the operating system. ROS also provided a software front panel allowing a display of registers and memory, and supporting breakpoints and single-stepping of machine code. ROS services ROS provided a number of basic hardware control functions, such as keyboard input, writing text to the video memory and disk input/output. ROS functions were called by means of the Emulator Trap (EMT) pseudo-opcode, which used the Z80 RST 30H instruction to call the EMT handler function. The EMT handler read the first byte following the RST 30H instruction to determine which EMT function was being requested; all parameters were passed in registers. A call-relative pseudo-opcode was also implemented using RST 28H. ROS services were largely compatible with the COS services provided on the earlier Research Machines 380Z. Z-Net The Z-Net firmware was used to allow the system to network boot from a file server over the local area network. It was switched out of the main address space once the system had booted. BASIC in ROM Research Machines provided a full version of their BASIC interpreter as part of the standard firmware. This was mainly for use on cassette-based systems and was switched out of the address space if the system was booted from disk or the network. Main ROS versions ROS versions were: ROS 1.0