the previous generation. All KC-series computers from Mühlhausen were capable of displaying graphics at a resolution of 320×256 pixels. But the color possibilities were limited (each 4×8-pixel cell had a single foreground (out of 16) and background color (out of 8, slightly darker than foreground equivalents). This limitation was brought down to 1×8 on the KC 85/4, which also featured a video RAM addressing mode and a special 4-color (black, white, red, cyan) mode which could color every pixel independently. The colors were not paletted. There was no "text mode", everything had to be painted; this combined with the video RAM layout described above and ROM code made the KC 85/2-3 rather slow at printing and scrolling (improved very much on KC 85/4). There were no blitters, and the video subsystem was developed in-house and implemented entirely with a few dozen series 7400 ICs. With the KC 85/2 and KC 85/3 CPU access to video memory would interfere with screen redraw and cause visual distortions as pixel data could not be read from VRAM and the previously loaded pixel was simply drawn again. This issue was fixed in the KC 85/4 too - one video period of 8 horizontal pixels is divided evenly into 3 phases: (1) an 8 bit wide CPU access (r/w), (2) fetch 8 bits of pixel data and (3) fetch 8 bits of color data. The KC 85/4 was also the first capable of switching between 2 independent locations in video ram, allowing double buffering. Sound and tape output was implemented by a CTC IC driving flipflops to generate square waves. A zero was represented by one period of a 2400 Hz tone, a one by a 1200 Hz tone. There was also a sync tone of 600 Hz prefixing each byte. The signal from the tape was read back, passed through a band pass filter and an interrupt would be generated each time the audio signal crossed the 0 V base line. Data was stored on tape in blocks of 128 byte. Each block was prefixed by brief silence and a series of 1 bits. Approximately 8000 (7 seconds) for the first block and 160 (133 msec) for each subsequent block. Following that was the 1 byte block number, 128 data bytes and 1 byte checksum. The tape could be rewound in case a block was not read correctly and loading would continue at that