Nascom

Did You Know?

• The Nascom keyboards used Licon solid-state (induction transformer) key switches in a software-scanned matrix, mounted in a metal frame riveted to a single-sided fibreglass PCB.
• An annotated disassembly listing of the Nascom 2 Microsoft ROM BASIC was published, and the code was later re-purposed in retrocomputing projects such as Grant Searle's Multicomp and Spencer Owen's RC2014.

History

John A. Marshall, the "& Son" in the family electronics component business A Marshall & Son (London) Ltd, had been advertising in hobby magazines since 1967. He also served as a director of Nasco Sales Ltd, a UK distributor of American semiconductors, and had ties to Lynx Electronics (London) Ltd, another regular advertiser in the hobby press from 1976. In the autumn of 1976, while on a business trip to California, Marshall dropped in on an amateur computer club meeting at Stanford University.

On the flight back to the UK, he began to wonder if there was a market for a kit computer, using the roughly £200 cost of an SLR camera as a benchmark for what a hobbyist might spend. Later that year, at a microprocessor seminar at Imperial College, he met Phil Pitman, marketing manager for Mostek, which had just become a second source for Zilog's Z80 processor. Pitman introduced Marshall to design consultant Chris Shelton, and in the spring of 1977, Marshall commissioned Shelton Instruments to design the Nascom 1.

Most of the design details later appeared in a series of articles by Pitman in Wireless World from November 1977 to January 1979. By July 1977, Lynx Electronics' monthly adverts were hinting at an upcoming microprocessor seminar and a new computer product. On Saturday, 26 November 1977, Lynx Electronics launched the Nascom 1 at its Home Microcomputer Symposium at Wembley Conference Centre in London. Tickets were £3.50, and holding the event on a Saturday aimed it squarely at amateurs rather than professionals.

Around 550 people attended, and more than 300 kits were sold in the two weeks after the launch. The symposium was covered in detail in the first issue of PCW magazine, which featured the Nascom 1 on its cover.

Hardware

The Nascom 1 and Nascom 2 shared several hardware features: a 16 MHz crystal biased into oscillation and divided down to create clocks for the CPU, serial communications, and video interface; a Z80/Z80A CPU; a Z80/Z80A PIO; a Harris 6402 UART (or equivalent) for serial communication or cassette tape storage; a memory-mapped video display with a UHF modulator for a domestic TV; circuitry on I/O port 0 for a software-scanned keyboard, a 'DRIVE' LED, and a timed non-maskable interrupt for hardware single-stepping; and a 'HALT' LED on the Z80 CPU /HALT output.

The I/O address map was common between the two machines, and the Nascom 2 memory map was a superset of the Nascom 1 map, ensuring software compatibility. The Nascom 1 used only off-the-shelf integrated circuits; the Nascom 2 used four 16-pin bipolar PROMs as glue logic for decode functions. The Nascom 2 added power-on reset with timing control, gating to reset the Z80-PIO, a reset-jump circuit to start execution from any 4-Kbyte boundary, Microsoft BASIC in an 8 Kbyte ROM, two groups of four uncommitted 24-pin DIL sockets for ROM or RAM, a socket for a second character-generator ROM, and full buffering of the CPU bus to generate the NAS-BUS expansion bus.

Video Display

The display of the Nascom 1 and 2 was memory-mapped, consisting of 16 rows of 48 characters. Each row used 64 consecutive memory locations, with 16 characters per line hidden by video blanking. Scrolling was software-controlled; due to an idiosyncrasy of video memory decoding on the Nascom 1 (retained on the Nascom 2), lines were decoded discontiguously, with the top line 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 displaying 128 characters (bit 7 ignored); the Nascom 2 used an identical character set in a ROM footprint-compatible with a 2716 2 Kbyte device. The Nascom 2 allowed a second 2 Kbyte character generator ROM (the NAS-GRA ROM) to display codes 0x80–0xFF, enabling a crude 96×48 graphics display via Microsoft BASIC. Each character was 8 pixels wide and 16 pixels high, allowing true descenders, with 16 bytes per character in ROM.

Characters were abutted vertically and horizontally, with inter-character spacing built into the design. On the Nascom 1, all 16 rows of the character were displayed, giving 256 rows total. On the Nascom 2, the top 12 or 14 rows were displayed, controlled by a switch or jumper, with 12 rows for 525-line 60 Hz displays and 14 rows for 625-line 50 Hz displays.

Software

Initially, users wrote their own software, often assembling Z80 code by hand and entering it in hexadecimal via the monitor. The Nascom 1 provided two 24-pin DIL sockets for ROM, each accepting a 2708 1 KB device. The first monitor was NAS-BUG (1 KB EPROM), superseded by NAS-BUG T2. Later 2 KB monitors included BBUG (a 1 KB extension co-existing with T2), T4, NAS-SYS 1, and NAS-SYS 3.

The Nascom 2 provided one 24-pin DIL socket for a 2716 2 KB device; kits initially came with NAS-SYS 1 in masked ROM, the only monitor supplied as masked ROM. All monitors allowed examining and modifying memory, starting execution, inserting breakpoints (in RAM only), single-stepping through ROM or RAM with register display (using hardware NMI support), and saving or loading memory to cassette tape. As the user base grew, user-group magazines published type-in programs in assembly, hexadecimal dumps, or BASIC.

Commercial software appeared on cassette or EPROM. When disk drives became available, disk operating systems included PolyDos (developed by Anders Hejlsberg, inspired by PolyMorphic Systems Poly-88 software), NAS-DOS, and CP/M. The predecessor of Borland's Turbo Pascal compiler and IDE for CP/M and DOS was developed by Anders Hejlsberg of Blue Label Software for the Nascom 2, as Blue Label Software Pascal (BLS Pascal). The Nascom 2 came with an onboard ROM containing an early Microsoft Basic 8 KB interpreter. Commercial games were also produced for Nascom.