Each of the 2K characters was represented by two bytes: a personality byte, and an attribute byte. When the "plot" bit (bit 7) of the attribute byte was excessive, it indicated that instead of generating one of many 128 glyphs, the 8b of the character byte must be displayed as an alternative as a 2x4 grid of fats pixels. The 8b parallel port was multiplexed with a little bit of exterior logic as a way to scan the keyboard and to entry the internal floppy disk drive. The Compucolor design relied on LSI chips as much as possible in order to suit into the small cabinet at a reasonable value. With the intention to see a coherent character, the identical character and attribute bytes needed to be stored in corresponding positions of an even/odd row pair. The attribute byte acts as a sentinel to point the primary unused entry. When the "plot" bit (bit 7) of the attribute byte was low, it indicated that the character byte was to be interpreted in line with the desk below. There isn't a reason why a few the I/O port ranges appear twice; it's just incomplete decoding of the port deal with bus, saving a bit of logic.

One obvious product was to put 8 KB of RAM there, and one other product put an socketed EPROM that filled that address range, permitting storing varied preloaded packages, akin to an assembler and debugger. The empty 8 KB vary at 0x4000 to 0x5FFF was put to use in varied after-market merchandise. It means the info stream might need long-time period DC content, yet the recording medium is incapable of monitoring it; it also implies that there are no unlawful bit encodings, such that there are not any straightforward to differentiate encodings to use to sync to; it additionally means that the read logic can't use AGC (automatic acquire management) and will need to have an absolute threshold for distinguishing a one bit from a zero bit. In a single mode, the chip operated as supposed, and drove the RS-232 serial port, permit the same old vary of link speeds (one hundred ten bps to 9600 bps) and cease bit options. The opposite mode, described later, used an undocumented function of the chip to drive the floppy interface. 3 would fireplace, concern an RST three instruction to the 8080. That routine would then drive one row of the keyboard, then read that parallel port to sense which keys related to that row were depressed.

Most other system have a tiny ROM routine to learn a boot sector from the disk, which then hundreds in a 2nd level disk operating system, allowing for bug fixes and enhancements after the very fact. To handle the primary problem, the software blindly steps the r/w head out forty two times when it wants to residence the top to track 0. The supply code for FCS literally calls this the POUND routine. In the traditional handle vary, the place the user is often simply poking a byte at a time whereas typing or printing text, stalls the 8080 CPU till the horizontal blanking interval, at which level the video technology would not must access RAM. The opposite handle vary, 0x6000 to 0x6FFF, gives precedence to the CPU over video technology. One product even allowed stacking eight eight KB banks into that one vary, with software choice between which financial institution was lively at any given time. If the CPU accesses the display RAM at the identical time the video era logic does, the CPU wins and there is a visible "tear" within the video stream.

Because the show RAM is multiplexed between the video generation circuitry and the need for CPU accesses, one thing have to be achieved when both want to entry video reminiscence at the same time. Aside from the scroll offset and the cursor addressing registers, the timing was set shortly after the CPU came out of reset, and the expectation was that it wouldn't be written again. The chip additionally accepted an input derived from the vertical sync timing of the video timing generator. Rather than having a self enough keyboard which used an somewhat expensive LSI chip to map key presses to ASCII codes, the Compucolor as an alternative would scan the keyboard itself. One advantage of this type of scanning is that for game play, it was doable to tell when a given key was depressed, held down, or released, not simply that it had been pressed. Not solely did this save pricey electronics, the coloration bandwidth was higher than NTSC would permit, resulting in crisper graphics than would have been potential with modulation. Although it was fundamentally a personality oriented show (not bitmapped), it was possible to combine text of two different sizes with 128x128 graphics in eight different colors. There was no need to module and demodulate the video sign: the graphics generator drove the CRT red, green, and blue guns instantly.

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