The core recreates a complete Nascom 2 workstation on the MEGA65. At its heart are a T80s Z80, the classic memory layout, NAS-SYS 3 and Microsoft BASIC. Extensions are integrated in a way that keeps the original system directly accessible.
Base system
- selectable CPU speed of 1, 2 or 4 MHz
- 48 × 16 characters in classic Nascom mode
- amber or green display with optional scanlines
- NAS-SYS 3, Microsoft BASIC and configurable expansion ROMs
- manual NMI function and system reset
The following components were not part of the standard Nascom 2 configuration. They were optional expansions, usually connected through the NAS-BUS, and are also recreated by the core:
- UART through the MEGA65 JTAG adapter and PMOD RS-232 with optional RTS/CTS
- AY-3-8910-compatible sound generator with three channels, noise and envelopes
- two digital joystick ports through the parallel AY I/O registers
Overlays and controls
The function keys open help pages without requiring software to run on the emulated Nascom. The CPU pauses while an overlay is visible.
| Key | Function |
|---|---|
F1 |
MEGA65 → Nascom 2 keyboard mapping |
F3 |
NAS-SYS 3 command overview |
F5 |
Hardware monitor, RAM editor and single-step mode |
F7 |
CP/M and FDD diagnostics |
F9 |
Save a memory range as a NAS file |
F11 |
File browser for loading and mounting files |
F13 |
Configuration and reset |
SD card and mass storage
The file browser reads files from the /NASCOM2 directory on the SD card. NAS, CAS and 640 KiB DSK files are supported. Disk images can be mounted and written by NAS-DOS.
The WD1793-compatible floppy controller supports reading, writing and formatting. When formatting, the existing image is first renamed to a BAK file; an existing backup prevents it from being overwritten.
The browser can also rename and delete unmounted FAT 8.3 files. Read-only files, long-filename entries and mounted DSK images remain protected.
CP/M 2.2 and AVC
CP/M mode starts through a boot ROM and uses an adapted BIOS. In addition to the usual CP/M tools, a native 640 KiB system disk contains programs for formatting, disk copying, RTC access and AVC graphics.
The emulated Lucas Logic AVC Model B provides colour planes, two resolution modes and a 96 × 24 character terminal mode for CP/M. When AVC is disabled, the classic Nascom display remains active.
Sound and joysticks
The AY-compatible device runs at 2 MHz independently of the Z80 speed. Its three channels are mixed to mono and output as 48 kHz/16-bit PCM on both HDMI channels and both channels of the 3.5 mm jack. Joysticks A and B are available through registers 14 and 15; analogue paddles are not emulated.
Communication and real-time clock
On R3/R3A and R6 machines, the core uses either a valid external DS3231 connected to the Grove port or the internal RTC. Optional Ethernet provides static IPv4 configuration or DHCP, ARP, ICMP ping, DNS and outgoing TCP connections. Programs for ping, Telnet, passive FTP and remote control are supplied. Network functions are disabled by default and can be enabled on the third configuration page.