memory, the Super-80 used discrete TTL logic to implement the video display and 512 bytes of system RAM was shared between the video display and the CPU. Fifty times per second, the CPU was turned off for around 10 ms by asserting the Z80 BUSREQ (DMA) pin. The video display circuitry would then read from the shared RAM while it refreshed the image on the screen. In addition to a 50% degradation in processor performance, this meant that it was not possible to perform any accurate timing in software, since the programmer had no control over when the next video display refresh cycle would occur. The video display could be switched off under software control for greater processing speed, or when accurate software timing was required. The most common situation in which that occurred was when the built-in cassette interface was being used. The location of the 512 bytes of video memory was normally at the top of the available RAM, but could be changed by writing to an I/O port. The keyboard was part of the main computer PCB, but before assembly, the constructor could opt to cut the keyboard section of the printed circuit board off and connect it to the main board with ribbon cable. The keyboard was wired as an 8 × 8 matrix and connected to the computer via the two 8-bit ports of a Z80 PIO chip. Pressing the keys <CTRL>, <C> and <4> at the same time generated an interrupt that would perform a "warm start" of the monitor program. The keyboard 'read' routine supplied in ROM was "negative edge triggered" and would block while a key was down. As a result, most action games incorporated their own keyboard driver. The standard computer had no serial or parallel I/O as such, relying on the optional S-100 bus interface for I/O and expansion. A 10-pin connector at the back of the board was labeled "PORT" and had power, as well as a pair of digital outputs and two available digital input lines. The connector was for a future RS-232 / 20 mA current loop serial interface, but that was never implemented. Mass storage was available in the form of a cassette tape interface running at 300 baud. Accessing the cassette interface required the video display to be switched off, so an LED was provided to show activity during a tape load or save operation.