Scor­pio News

When faced with the choice of what 80-BUS disk controller or video card to buy, there are several choices on the market. The final choice must depend not only on the depth of pocket, but on the eventual planned requirements of the system. So, what are these for a disk-based system?

If we want to run CP/M, then we need some means of ‘booting’ from the system disk. This can either take the form of a program loaded from tape running under some older monitor, or as is more usual, to load the system boot sector direct from disk using an EPROM routine running upon RESET. This boot sector in turn pages out the EPROM, loads the main system, and eventually we get the CP/M prompt.

The most common Boot EPROM are Gemini’s SIMON and RP/M, and the Map-80 VSOFT. The Gemini versions sits on the CPU card. RP/M. is unsuitable for the Nascom-2. It can, however be used on its native card with no problem. There is a version of SIMON for the Nascom. Map-80’s VSOFT is the controlling software for the Map-80 VFC. Whichever boot system is chosen reflects on the choice of supplier of CP/M as they include checks on the boot sector to determine that it is in fact a system disk being read. It can also affect your choice of RAM card(s), as some CBIOSs will support some types and not others.

MAP-80 VFC – Video / Floppy Controller

This card, consists of a 25x80 video display, a parallel keyboard port, a floppy disk controller and the controlling software (VSOFT) EPROM. The card is available either ready built, or in kit form. The advantage of the kit approach is that you only need to pay for the options you require. (VSOFT is part of the base card – you can choose any or all of the Video, Video Switch, Floppy or Keyboard options.) It will work with the Nascom-2, GM811, GM813, and Map-CPU cards.

How does it compare ? The standard card uses 16 ports addressed ED – EF but alternate ports C0 – CF are available on request. (This is necessary if using the GM809 FDC or Map-80 MPI.)

The video section is controlled by a 6845, and all character handling is done by the main system CPU running routines in VSOFT, unlike the Gemini IVC and SVC. It should therefor be slightly slower, as the IVC and SVC contain their owe Z80s. Because the system CPU runs VSOFT, both VSOFT and the video RAM can be individually paged into the memory map under port control when required. (The overlaid system RAM is available at all other times.) The address at which VSOFT executes is also flexible – it can reside at a software controlled 4K boundary, and told where it is in an initialisation call. In addition, the auto-boot facility is link and software selectable. The card does not have a programmable character set, but does have a socket for a 2716/​2732 second character generator. This socket is under software control – you can select character generator 1 or 2, and normal 00H-FFH, or normal 00H-7FH + inverse video of 00H-7FH for 8OH-FFH with the selected set.

The final (optional) part of the video circuitry as a video switch which selects the output to be either the VFC screen, or pass through an external video signal – e.g. normal output from the Nascom-2 under software control. This means that although the VFC cannot generate the old Nascom format screen, (which would be at the wrong address anyway), the original Nascom screen can still be displayed when running NAS-SYS (as modified to MONITOR.COM to work with disks or loaded from disk and executed for standard NAS-SYS operation). This theoretically allows you to use your old software provided it doesn’t attempt to change the non-existent EPROM or corrupt VFC work areas. The parallel Keyboard input consists of a strobed latch which is read via VSOFT. The strobe polarity is link selectable.

The final section is the Floppy Disk Controller. This uses the WD2797, and is GM809 compatible. As with the GM809, however, the clock frequency of the FDC is link selectable, meaning that selection between 5.25″ and 8″ drives can only be done by link changing. If you want software selection, go for the Map-MPI card or Gemini GM829 or GM849 instead.

Control codes for the Map VFC are similar to the Gemini IVC, but since there is no onboard CPU, there is no facility to load or run user programs on the card. Almost all screen and character editing commands are identical, but differences creep it concerning character set selection, format and graphics. The VFC has a programmable (any) key string table, whereas the IVC can only reprogram its function keys. The VFC does not support a light pen. (If any of these are of concern, you should naturally obtain and compare the relevant manuals.)