in the Expansion Interface). The OS ROMs, I/O area, video memory and OS work space occupy the first 16 KB of memory space on the Model I. The remaining 48 KB of the 64 KB memory map space is available for program use, subject to the amount of physical RAM installed. Although the Z80 CPU can use port-based I/O, the Model I's I/O is memory-mapped aside from the cassette tape and RS-232 serial ports. Keyboard The TRS-80 Model I keyboard uses mechanical switches that suffer from "keyboard bounce", resulting in multiple letters being typed per keystroke. The problem was described in Wayne Green's editorial in the first issue of 80 Micro. Dirt, cigarette smoke, or other contamination enters the unsealed key switches, causing electrical noise that the computer detects as multiple presses. The key switches can be cleaned, but the bounce recurs when the keyboard is reexposed to the contaminating environment. Keyboard bounce only occurs in Model I computers with Level II BASIC firmware; Level I BASIC has a "debounce" delay to the keyboard driver to avoid the noisy switch contacts. Tandy's utility, the Model III, the last Model I firmware, and most third-party operating systems also implement the software fix, and Tandy changed the keyboard during the Model III's lifetime to an Alps Electric design with sealed switches. The Alps keyboard was available as an upgrade for the Model I for $79. The keyboard is memory-mapped so that certain locations in the processor's memory space correspond to the status of a group of keys. Video and audio The color of the 12" KCS 172 RCA monitor's text is faintly blue (the standard P4 phosphor used in black-and-white televisions). Green and amber filters, or replacement tubes to reduce eye fatigue were popular aftermarket items. Later models came with a green-on-black display. Complaints about the video display quality were common. As Green wrote, "hells bells, [the monitor] is a cheap black and white television set with a bit of conversion for computer use". (The computer could be purchased without the Radio Shack monitor.) CPU access to the screen memory causes visible flicker. The bus arbitration logic blocks video display refresh (video-RAM reads) during CPU writes to the VRAM, causing a short black line. This has little effect on normal BASIC programs, but fast programs made with assembly language can be affected. Software authors worked to minimize the effect, and many arcade-style