Editing Nascom (section)

==Video Display==
[[File:Nascom vdu charset 0 127.jpg|thumb|alt=Photograph of Nascom character set, characters 0-127|Nascom character set, characters 0–127, displayed on a Nascom 2 and photographed on a monochrome non-interlaced CRT monitor]]

[[File:Nascom vdu character cell.jpg|thumb|alt=Photograph of individual character cell on Nascom 2|Character cell on Nascom 2: 8 pixels by 14 rows. Photographed on a monochrome non-interlaced CRT monitor]]

The display of the Nascom 1 and 2 was memory-mapped and consisted of 16 rows of 48 characters. Each row of characters used 64 consecutive memory locations; the extra 16 characters in each line were "hidden" by the video blanking circuitry.

Scrolling was implemented under software control. Due to an idiosyncrasy of the video memory decoding on the Nascom 1 (which was then retained on the Nascom 2), the lines were decoded discontiguously, with the top line of the display being the 16th region of memory. The top line was not scrolled, except by the Nascom [[CP/M]] implementation.

The Nascom 1 used a MCM6576P character generator to display 128 characters (bit 7 of the memory was ignored). The Nascom 2 used an identical character set but implemented it in a ROM that was footprint compatible with a 2716 2Kbyte device. The Nascom 2 allowed a second 2Kbyte character generator ROM (or EPROM) to be fitted (approximate price £20 in 1980) . The so-called NAS-GRA ROM was used to display characters with the byte codes 0x80–0xFF. The built-in [[Microsoft BASIC]] (8K ROM) interpreter could use these graphics to create a crude, blocky 96×48 graphics display.

Each character was 8 pixels wide and 16 pixels high, allowing display of true [[descender]]s. Therefore, a character occupied 16 bytes in the ROM (so that 256 characters required a total of 256*16=4Kbytes of character generator storage). Characters were abutted vertically and horizontally on the display and so the design of the characters within the character generator included vertical and horizontal inter-character spacing. On the Nascom 1, all 16 rows of the character were displayed, so that the whole image occupied 16*16=256 rows. On the Nascom 2, the top 12 or 14 rows of the character were displayed (controlled by the setting of a switch/jumper on the main board). The 12-row setting was intended for 525 line displays in 60&nbsp;Hz geographies and the 14-row setting was intended for 625-line displays in 50&nbsp;Hz geographies.

The design of the video display required that the CPU and the video circuitry shared access to the video RAM (the CPU had read/write access and the video circuitry had read-only access). If the CPU and the video circuitry accesses the video RAM simultaneously, the CPU was given priority and the video circuitry would read incorrect data. On the Nascom 1 this gave rise to white flicker on the screen that was termed "snow". The International Nascom Microcomputer Club (INMC) published a "snow plough" design that reduced the effect by blanking the video when simultaneous access occurred.<ref>{{cite web|url=https://80bus.co.uk.mirror.jloh.de/publications/magazines/INMC_news_2.pdf#page=4 |archive-url=https://web.archive.org/web/20170506182658/http://80bus.co.uk.mirror.jloh.de/publications/magazines/INMC_news_2.pdf |archive-date=2017-05-06 |url-status=live|title=INMC News issue 2|last=Hunt|first=Dave|date=1979|website=Nascom Magazines}}</ref> The Nascom 2 used a slightly different design but still allowed contention to occur, this time giving rise to black flicker (blanking) on the screen.