Amstrad PC1640 NVR Configuration Utility — Comprehensive Edition

POSIX-style NVR (Non-Volatile RAM) configuration and system diagnostic utility for the Amstrad PC1640, targeting ELKS (Embeddable Linux Kernel Subset) running on real hardware.

This replaces the original NVR.EXE that shipped on the Amstrad User Disk, and goes far beyond it — covering every hardware feature the BIOS exposes.

Hardware Reference

All register definitions and I/O protocols were reverse-engineered from the original Amstrad PC1640 BIOS ROMs (40043.v3 / 40044.v3, interleaved hi/lo byte pair, 8KB each = 16KB combined), and verified against the PCem emulator source code.

I/O Port Map (PC1640-Specific)

Port	Dir	Function
0x60	Read	Keyboard data / system status 1 (PB.7 sel)
0x61	R/W	PB register (speaker, keyboard, nibble sel)
0x62	Read	System status 2 (PB.2 selects nibble)
0x64	Write	System status 1 latch
0x65	Write	System status 2 latch / NVR address
0x66	Write	Soft reset trigger
0x70	Write	MC146818 RTC/CMOS address register
0x71	R/W	MC146818 RTC/CMOS data register
0x78	R/W	Amstrad mouse X counter
0x7A	R/W	Amstrad mouse Y counter
0x0201	Read	Game/joystick port
0x0378	R/W	LPT1 data (OR'd with language bits on read)
0x0379	Read	LPT1 status: lang (0-2), DIP (5), display (6-7)
0x03DB	Write	Video CGA/EGA switch (bit 6)
0x03DE	Read	IDA status (0x20 = internal display disabled)
0xDEAD	Read	Dead-man diagnostic port (POST progress)

NVR Protocol

The PC1640's "NVR" is a standard MC146818 RTC/CMOS chip with 64 bytes of battery-backed RAM (addresses 0x00-0x3F).

Read: OUT 0x70, address → IN 0x71 returns data Write: OUT 0x70, address → OUT 0x71, data

Amstrad-Specific Differences from Standard AT

• CMOS address mask is 0x3F (64 bytes only, not 128)
• Bit 7 of port 0x70 does NOT control NMI mask
• RTC alarm interrupt routes to IRQ 1 (not IRQ 8)
• Single 8259A PIC only (no secondary — 8086 system)
• Port 0x62 reads system configuration with nibble select via PB bit 2
• Port 0x66 triggers a soft reset — never write to it accidentally
• Amstrad mouse uses ports 0x78/0x7A with keyboard scancodes for buttons
• LPT1 status register is overloaded with language and display type info

CMOS Register Layout

Addr	Register	Format
0x00	Seconds	BCD
0x01	Alarm seconds	BCD/WC
0x02	Minutes	BCD
0x03	Alarm minutes	BCD/WC
0x04	Hours	BCD
0x05	Alarm hours	BCD/WC
0x06	Day of week	BCD
0x07	Day of month	BCD
0x08	Month	BCD
0x09	Year (2-digit)	BCD
0x0A	Status Register A	Binary
0x0B	Status Register B	Binary
0x0C	Status Register C	R/O
0x0D	Status Register D	R/O
0x0E	Diagnostic status	Binary
0x0F	Shutdown status	Binary
0x10	Floppy drive types	Binary
0x12	Hard disk types	Binary
0x14	Equipment byte	Binary
0x15	Base memory (low)	Binary
0x16	Base memory (high)	Binary
0x17	Extended mem (low)	Binary
0x18	Extended mem (high)	Binary
0x19	HD 0 extended type	Binary
0x1A	HD 1 extended type	Binary
0x2E	Checksum (high)	Binary
0x2F	Checksum (low)	Binary
0x32	Century	BCD

WC = Wildcard (0xC0-0xFF means "don't care")

Building

Prerequisites

Install the ia16-elf cross-compiler:

# Debian/Ubuntu
sudo add-apt-repository ppa:tkchia/build-ia16
sudo apt-get update
sudo apt-get install gcc-ia16-elf libia16-elf-dev

# Or from source: https://github.com/tkchia/gcc-ia16

Build for ELKS

make                # Build ELKS binary
make strip          # Build and strip for smaller size
make disasm         # Generate disassembly listing

Build native (for testing UI only)

make native         # Linux x86 native binary

Install to ELKS root filesystem

make install ELKS_ROOT=/path/to/elks/rootfs

Usage

nvr [options] <command> [args...]

Options

Option	Description
-d	Enable debug output
-dd	More verbose debug
-ddd	Maximum debug (port-level traces)
-h, --help	Show help

Commands — Configuration Display

Command	Alias	Description
show		Show all configuration (default)
time		Show current date and time
alarm		Show alarm settings
floppy		Show floppy drive configuration
harddisk	hd	Show hard disk configuration
equipment	equip	Show equipment byte breakdown
memory	mem	Show memory configuration
status		Show RTC status registers (detailed)
diag		Show diagnostic & shutdown status
battery	bat	Show battery health report

Commands — Amstrad-Specific

Command	Alias	Description
amstrad		Show all Amstrad system status
language	lang	Show language selection (DIP switches)
display	video	Show display type detection
mouse		Show mouse port counters
mouse-test		Interactive mouse test (5 seconds)
mouse-reset		Reset mouse counters to 0

Commands — Hardware Diagnostics

Command	Alias	Description
ports		Detect serial/parallel ports
gameport	joystick	Show game/joystick port status
pic		Show 8259A PIC status
dma		Show 8237A DMA status
pit	timer	Show 8253 PIT timer status
deadman	dead	Read dead-man diagnostic port
speaker-test		Play test tones through speaker
beep FREQ		Play tone at FREQ Hz (20-20000)

Commands — Time/Date Setting

Command	Description
set-time HH:MM:SS	Set the RTC time
set-date DD/MM/YYYY	Set the RTC date
set-dow N	Set day of week (1=Sunday, 7=Saturday)
set-alarm HH:MM:SS	Set alarm (-1 for wildcard)
alarm-enable	Enable alarm interrupt
alarm-disable	Disable alarm interrupt
watch	Continuously display time (Ctrl+C stop)

Commands — Drive Configuration

Command	Description
set-floppy A|B TYPE	Set floppy type (0-4)
set-harddisk 0|1 TYPE	Set hard disk type (0-15)

Commands — Equipment Configuration

Command	Description
set-equip FIELD VAL	Set equipment: fpu, video, floppy-count
set-basemem KB	Set base memory (64-640 KB)

Commands — RTC Mode Configuration

Command	Description
set-rtc 24h 0|1	Toggle 24-hour / 12-hour mode
set-rtc bcd 0|1	Toggle BCD / binary data mode
set-rtc sqw 0|1	Toggle square wave output
set-rtc dse 0|1	Toggle daylight savings
set-rtc pie 0|1	Toggle periodic interrupt
set-rtc uie 0|1	Toggle update-ended interrupt
set-rtc rate N	Set periodic rate (0-15)

Commands — CMOS Operations

Command	Alias	Description
dump		Hex dump all 64 CMOS bytes
read ADDR		Read single CMOS byte
write ADDR VAL		Write single CMOS byte
fill START END VAL		Fill CMOS range with value
checksum		Verify/repair CMOS checksum
save FILE		Save CMOS image to file
load FILE		Load CMOS image from file
compare FILE	diff	Compare live CMOS vs saved file
factory-reset		Reset to PC1640 factory defaults
clear-diag		Clear diagnostic status byte

Commands — Debug

Command	Alias	Description
probe		Full hardware port probe
trace		NVR port protocol trace
inb PORT		Read I/O port (hex)
outb PORT VAL		Write I/O port (hex)
soft-reset	reboot	Trigger soft reset via port 0x66

Examples

nvr                         # Show full configuration
nvr time                    # Show date/time
nvr set-time 14:30:00       # Set time to 2:30 PM
nvr set-date 25/12/2026     # Set date
nvr set-dow 4               # Set day to Wednesday
nvr set-floppy A 4          # Set drive A to 1.44MB 3.5"
nvr set-harddisk 0 2        # Set drive C: to type 2 (20MB)
nvr set-equip video 0       # Set initial video to EGA
nvr set-equip fpu 1         # Mark 8087 as installed
nvr set-basemem 640         # Set base memory to 640KB
nvr set-alarm -1:-1:00      # Alarm every minute at :00
nvr alarm-enable            # Arm the alarm
nvr set-rtc 24h 1           # Switch to 24-hour mode
nvr battery                 # Check battery health
nvr mouse-test              # Test Amstrad mouse (5 sec)
nvr speaker-test            # Test PC speaker
nvr beep 440                # Play 440 Hz tone
nvr dump                    # Hex dump CMOS
nvr save backup.nvr         # Backup CMOS to file
nvr load backup.nvr         # Restore CMOS from file
nvr compare backup.nvr      # Show what changed vs backup
nvr factory-reset           # Reset to factory defaults
nvr -ddd probe              # Maximum-verbosity hardware probe
nvr inb 0xDEAD              # Read dead-man port
nvr pic                     # Show interrupt controller status
nvr checksum                # Verify and fix checksum
nvr watch                   # Continuously display time

Floppy Drive Types

Type	Description
0	Not installed
1	360KB 5.25"
2	1.2MB 5.25"
3	720KB 3.5"
4	1.44MB 3.5"

Hard Disk Types

Type	Cylinders	Heads	Sectors	Approx Size
0	—	—	—	Not installed
1	306	4	17	10 MB
2	615	4	17	20 MB
3	615	6	17	30 MB
4	940	8	17	62 MB
5	940	6	17	46 MB
6	615	4	17	20 MB
7	462	8	17	30 MB
8	733	5	17	30 MB
9	900	15	17	112 MB
10	820	3	17	20 MB
11	855	5	17	35 MB
12	855	7	17	49 MB
13	306	8	17	20 MB
14	733	7	17	42 MB
15	Extended (uses CMOS 0x19/0x1A)

Language Codes (DIP Switches)

Code	Language
0	Diagnostic
1	Italian
2	Swedish
3	Danish
4	Spanish
5	French
6	German
7	English

Display Types

Code	Display
0	EGA (built-in Paradise PEGA)
2	CGA
3	MDA/Hercules

PC1640 IRQ Assignments

IRQ	Function
0	Timer (8253 channel 0, 18.2 Hz)
1	Keyboard + RTC alarm (Amstrad!)
2	Reserved
3	COM2
4	COM1
5	LPT2
6	Floppy disk controller
7	LPT1

Notes

• Must run as root for direct port I/O access
• The PC1640 has an 8086 CPU at 8MHz — the binary is compiled for 8086
• Battery is in the monitor base (4× AA) — check with nvr battery
• If the battery is dead, all CMOS settings are lost on power-off
• The save/load commands can backup and restore the full CMOS state
• The compare command is useful for diagnosing unexpected changes
• The probe command reads but does not modify hardware state
• factory-reset restores: 720KB floppy A, no HD, EGA video, 640KB base, 24h BCD mode
• Never write to port 0x66 — it triggers a soft reset (use soft-reset command intentionally)
• Alarm wildcards: value ≥ 0xC0 means "don't care" (e.g. -1:-1:00 fires every minute)
• The Amstrad mouse is proprietary — NOT PS/2 or serial

BIOS ROM Analysis

The NVR protocol was determined by disassembling the interleaved BIOS ROMs:

• 40044.v3 — high byte ROM (8192 bytes)
• 40043.v3 — low byte ROM (8192 bytes)
• 40100 — Paradise PEGA video BIOS (32768 bytes)

Key BIOS routines identified:

ROM Offset	Function
0x00C9	POST entry (CLI, clear ports)
0x00E0	OUT 0x66 — soft reset during warm boot
0x03F3	System config verify via port 0x64/0x60
0x0465	NVR byte read via port 0x65/0x62
0x0793	NVR address select via port 0x65
0x089E	INT 19h — bootstrap loader

The BIOS contains POST messages in 7 languages: Danish, French, Italian, Swedish, Spanish, German, and English.

License

Public domain. Written for hardware preservation.