80-Bus News


September–October 1983, Volume 2, Issue 5

Page 43 of 67

software, they can all be used together from the one card!! A further extension to the disk system is the option of fitting Gemini GM833 512K RAM-DISK cards which are organised as a port driven memory plane (not page mode). This is intended to be used exactly like an additional 512K disk drive (or more with multiple cards), and the Gemini CP/M BIOS is designed to cope with this. The disadvantage of course is that data is lost on power down, so good discipline is required to ensure that the virtual disk is always adequately backed up. However, in use the virtual disk card is incredibly fast and is a boon when used in large data base programs where updating large disk based indices inevitably slows down work. Naturally the virtual disk may be used with the existing 5.25″ drives, and of course the 8″ and Winchester drives as well. A truly flexible configuration.

The Video Card

The video card, as already mentioned, handles all the video processing under the control of a 6845 video processor. The card also features the second Z80A mentioned in the advertising for internal control of the video processor and to provide the very flexible range of video card commands. As the video card is not memory mapped, I/O from the bus is via three ports, one for input and output, one for status and one for reset. The keyboard is also connected to the video card. The card carries a 4K EPROM which is the operating system for the Z80A handling all output to the video and all input from the keyboard and a further 2K EPROM is fitted for the 128 character fixed character set. A total of 6K of RAM is provided on this card, 2K for the video display, 2K for the programmable character set, 1/2K for the function keyboard tables, 64 bytes of keyboard buffer, 128 bytes of video input buffer and the remainder as workspace, leaving about 1K free. It is possible to further extend the range of commands to the video card by writing programs for use in this 1K of spare memory, however, because of the extreme difficulty of debugging programs in this area, this is best left to experts. On power up the fixed character set is complimented and then loaded to the programmable character generator providing 128 normal characters and 128 inverse video characters. Individual characters within the programmable set may be reprogrammed at will or complete character sets changed as required. The preset function key definitions are down loaded from the EPROM into the RAM table on power up, and may be changed by keyboard entry or by program from disk. The function key table space is allocated dynamically. Normally, each function key would be assigned the few bytes it needs in the table space, however, it is possible to program one key with almost all the 1/2K available at the expense of programming space for the other keys. The 64 byte keyboard buffer allows a useful type ahead facility.

Twin Z80s?

The Gemini advertising boasts that the Galaxy features two ZB0A processors, which whilst strictly true does not present the full picture. The second Z80A certainly speeds video operations and greatly enhances the flexibility of the video card, however, it would be difficult in the extreme and probably be a self defeating exercise to try and make the second processor share in some multi-processor role. In view of this, as each drive contains a dedicated processor, the 6845 and the WD1797 are also both dedicated processors and the keyboard contains a further derivative of the 8045 family, why not advertise that the machine contains no fewer than seven processors?

Page 43 of 67