Acorn Archimedes A4000

Acorn Archimedes A4000
	Acorn Archimedes A4000
Specifications
Manufacturer	Acorn Computers Ltd
Type	Personal Computer
Released	June 1992
Discontinued	1993
Intro price	£899 (1992)
CPU	ARM250 @ 12 MHz
Memory	2 MB RAM (expandable to 4 MB)
Storage	3.5" floppy drive (1.6 MB), 80 MB IDE hard drive
Display	640×512 (16 colours), 640×480 (256 colours), 800×600 (16 colours)
Sound	8-channel stereo, 8-bit logarithmic DAC
Dimensions	430 mm × 340 mm × 95 mm
Weight	6 kg
OS / Firmware	RISC OS 3.10
Predecessor	Acorn Archimedes A3000
Successor	Acorn A4

The Acorn Archimedes A4000 was a mid-range desktop computer released in June 1992 as part of Acorn's second generation of Archimedes machines. Positioned between the entry-level A3010/A3020 and the professional A5000, the A4000 combined the cost-effective ARM250 processor with a built-in IDE hard drive, making it suitable for small businesses and advanced educational use.

Overview

The A4000 represented Acorn's response to market demands for an affordable machine with built-in mass storage. Unlike the A3010 and A3020 which targeted home and education markets respectively, the A4000 aimed at users requiring a complete system without the expense of the ARM3-based A5000. The inclusion of an 80 MB IDE hard drive as standard distinguished it from its contemporaries, providing sufficient storage for RISC OS 3, applications, and user data without external additions.

Acorn designed the A4000 using the same fundamental architecture as the A3010 and A3020, sharing the ARM250 system-on-chip that integrated the ARM2 core, MEMC memory controller, VIDC video controller, and IOC input/output controller into a single package. This integration reduced manufacturing costs while improving reliability through fewer interconnections.

Architecture

The ARM250 processor formed the heart of the A4000, manufactured using a 1.5µm CMOS process that reduced power consumption to approximately 3 watts. The integration of previously discrete components into a single chip eliminated many timing issues present in earlier designs while enabling the clock speed increase from 8 MHz to 12 MHz.

Memory System

The A4000's memory architecture differed from both its predecessors and contemporaries through its use of standard 30-pin SIMMs rather than proprietary modules:

Configuration	SIMM Type	Banks Used	Total RAM
Standard	2× 1MB 30-pin	2 of 4	2 MB
Expanded	4× 1MB 30-pin	4 of 4	4 MB
Alternative	2× 2MB 30-pin	2 of 4	4 MB

The use of industry-standard SIMMs reduced upgrade costs significantly compared to earlier machines. The memory controller within the ARM250 supported fast page mode access, achieving 50ns access times with 70ns rated SIMMs through careful timing optimization.

Hardware Design

The A4000 motherboard represented a significant redesign from the A3000, optimizing for manufacturing efficiency and the integrated ARM250 processor. The two-layer PCB reduced costs compared to the four-layer boards used in earlier Archimedes models, though this required careful trace routing to minimize signal interference.

Integrated Components

The ARM250's integration provided several advantages:

Reduced chip count - Single chip replaced four major components
Lower power consumption - 3W versus 8W for discrete implementation
Improved reliability - Fewer solder joints and interconnects
Simplified cooling - Passive heatsink sufficient for all conditions
Cost reduction - Approximately 40% lower component cost

The integration also introduced limitations. The fixed relationships between processor, memory controller, and video controller prevented independent upgrades, and the 12 MHz clock speed could not be increased without replacing the entire chip.

IDE Interface

Specification	Details	Performance
Controller	82C711 Multi-I/O chip	PIO Mode 0
Interface	40-pin IDE connector	16-bit data
Transfer rate	3.3 MB/s maximum	1.8 MB/s typical
Drive supplied	80 MB Conner CP3084	28ms average seek
File system	ADFS with RISC OS 3 extensions	Long filenames

The IDE interface represented a departure from the SCSI interfaces used in higher-end models. While offering lower performance, IDE drives cost significantly less than equivalent SCSI units, contributing to the A4000's attractive price point. The 82C711 controller also provided the floppy drive interface, serial port, and parallel port, further reducing chip count.

Video Capabilities

The VIDC1a video controller within the ARM250 supported enhanced display modes compared to the original VIDC, with improved timing flexibility for VGA monitors becoming standard in the early 1990s.

Display Modes

Mode	Resolution	Colors	Refresh	Memory Used
12	640×256	16	50 Hz	80 KB
15	640×256	256	50 Hz	160 KB
20	640×480	16	60 Hz	150 KB
21	640×480	256	60 Hz	300 KB
24	800×600	16	56 Hz	234 KB
27	640×480	16	72 Hz	150 KB
28	640×480	256	60 Hz	300 KB
31	800×600	16	60 Hz	234 KB

The addition of proper VGA timing modes (20, 21, 27, 28) eliminated the need for expensive multisync monitors, allowing the use of standard PC displays. This compatibility improvement significantly reduced total system cost for business users migrating from PC platforms.

Input/Output Systems

The A4000 provided comprehensive I/O capabilities through a combination of integrated and discrete controllers:

Standard Ports

Port	Type	Controller	Features
Serial	9-pin D-sub	82C711	115200 bps maximum
Parallel	25-pin D-sub	82C711	Bidirectional, EPP compatible
Mouse	9-pin mini-DIN	IOC quadrature	PS/2 compatible with adapter
Keyboard	5-pin DIN	8051 MCU	PC/AT protocol
Video	15-pin D-sub	VIDC1a	Analog RGB, separate syncs
Audio	3.5mm jack	VIDC1a	Stereo line output
Network	Optional	Econet module	250 kbps half-duplex

The serial port implementation in the 82C711 provided full 16550-compatible buffering, improving reliability at high data rates compared to the unbuffered 6551 used in earlier models. The parallel port's EPP compatibility enabled connection to newer peripherals designed for PC systems.

Expansion Options

Unlike the A3000 which provided only a single internal expansion slot, the A4000 included:

Single podule slot - Standard 96-pin Archimedes expansion
Network slot - Internal Econet interface option
RAM expansion - Two free SIMM slots
VRAM upgrade - Optional 1 MB VRAM for improved video

The single podule slot limitation proved restrictive for users requiring multiple expansions. Common expansion cards included SCSI interfaces for external devices, Ethernet adapters for TCP/IP networking, and MIDI interfaces for music applications.

Operating System

The A4000 shipped with RISC OS 3.10 in 2 MB of ROM, providing immediate access to the desktop environment and core applications without loading from disk. This version introduced several improvements over RISC OS 3.00 used in the A3010/A3020:

RISC OS 3.10 Features

System enhancements included:

Improved ADFS - Better IDE support and faster disk operations
Enhanced printing - Bidirectional parallel communication
Bug fixes - Resolution of numerous RISC OS 3.00 issues
Application updates - Newer versions of Edit, Paint, and Draw
Driver improvements - Better third-party hardware support

The 2 MB ROM contained the complete operating system, desktop environment, and bundled applications. This approach provided instant startup, immunity from disk corruption, and consistent system behavior across all machines.

Memory Management

With 2 MB RAM as standard, RISC OS 3.10 memory allocation typically appeared as:

Area	Size	Purpose
Screen memory	160-300 KB	Depending on mode
System heap	128 KB	OS workspace
Module area	256 KB	Loaded modules
Font cache	64-128 KB	Outline font rendering
RAM disc	64 KB	Default size
Free memory	1.0-1.2 MB	Applications

The inclusion of a hard drive reduced RAM disc importance, allowing more memory allocation to applications. The system could dynamically adjust memory areas based on requirements, though this occasionally led to fragmentation requiring a restart.

Software Compatibility

The A4000's 2 MB minimum RAM and included hard drive ensured compatibility with virtually all contemporary RISC OS software. The ARM250's instruction set compatibility with ARM2 and ARM3 processors meant existing software ran without modification, though without the performance benefits of the ARM3's cache.

Popular software categories included:

Office applications - Impression Publisher, Pipedream, DataPower
Graphics software - Artworks, ProArtisan, Photodesk
Programming tools - Acorn C/C++, BBC BASIC V, ARM assembler
Educational software - Granada Learning titles, Sherston Software
Games - Zarch, Lemmings, SimCity

The hard drive transformed software installation and use, eliminating disk swapping and providing space for multiple applications. Users could maintain extensive font collections, clip art libraries, and document archives previously impractical on floppy-only systems.

Performance Analysis

The A4000's performance fell between the A3020 and A5000, offering adequate speed for most applications while lacking the raw processing power of ARM3-based systems:

Benchmark	A3020 (12 MHz ARM250)	A4000 (12 MHz ARM250)	A5000 (25 MHz ARM3)
Dhrystone 2.1	5,400	5,400	21,000
Screen redraw	100%	100%	180%
BASIC loop	1.2 sec	1.2 sec	0.3 sec
File copy (HD)	N/A	850 KB/s	1.4 MB/s

The identical processor performance to the A3020 meant the A4000's advantages lay primarily in its storage capabilities and expansion options rather than raw computing power. The IDE interface's lower performance compared to the A5000's SCSI system was rarely noticeable in typical use.

Manufacturing and Variants

Acorn produced the A4000 for approximately 18 months, with minor revisions addressing component availability and cost reduction:

Production Timeline

Period	Changes	Notes
Jun-Dec 1992	Initial production	Conner CP3084 drives
Jan-Jun 1993	Drive change	Western Digital drives
Jul-Oct 1993	Component updates	Cost-reduced PSU
Nov 1993	End of production	Replaced by Risc PC

Production volumes remained modest, with approximately 15,000 units manufactured. The education market absorbed roughly 40%, with the remainder split between small businesses and advanced home users. The relatively short production run reflected Acorn's transition to the Risc PC architecture announced in 1994.

Common Issues and Reliability

The A4000 exhibited generally good reliability, benefiting from the integrated ARM250 design and improved manufacturing processes. However, several issues emerged over time:

Hard Drive Failures

The Conner CP3084 drives used in early production proved problematic:

Stiction problems - Drives failed to spin up after periods of inactivity
Head crashes - Particularly common after 5-7 years of use
Bearing wear - Caused increasing noise before complete failure

Replacement with modern IDE drives requires attention to geometry limitations in RISC OS 3.10, which cannot address drives larger than 512 MB without third-party software.

Power Supply Issues

The internal switch-mode power supply, while more efficient than earlier linear designs, developed characteristic faults:

Capacitor degradation - Particularly in the primary section
Thermal stress** - Inadequate ventilation caused component failure
Voltage drift - Aging components caused regulation problems

Battery Leakage

Like all Archimedes machines, the CMOS backup battery posed long-term risks:

Battery type - 3.6V lithium (improved from NiCd)
Typical lifetime - 7-10 years before replacement needed
Leakage damage - Less severe than NiCd but still problematic
Symptoms - Settings loss, clock failure, boot problems

Upgrades and Modifications

A4000 owners frequently pursued various upgrades to enhance system capabilities:

Memory Expansion

Upgrading to 4 MB required adding two 1 MB SIMMs:

Cost in 1993 - Approximately £80
Installation** - Simple insertion into empty slots
Benefits - Improved multitasking, larger RAM disc
Compatibility - 70ns or faster SIMMs required

Processor Upgrades

Several third-party processor upgrades became available:

ARM3 upgrade** - 25-33 MHz ARM3 with cache
FPA coprocessor - Hardware floating-point acceleration
Clock speedup - Replaced crystal for 16 MHz operation

The ARM3 upgrade provided the most significant performance improvement, offering 3-4 times the processing power of the standard ARM250.

Storage Expansion

Storage options included:

Larger IDE drives - Up to 512 MB with RISC OS 3.10
External SCSI - Via podule slot interface
Network storage*** - Using Econet or Ethernet

1. Network Options ##

Network connectivity options:

Econet - Internal interface card, 250 kbps
Ethernet - 10Base2/10BaseT via podule slot
Serial networking - SLIP/PPP via serial port

Ethernet gradually replaced Econet in educational settings, offering standard TCP/IP compatibility and higher speeds.

Educational Use

The A4000 found particular success in secondary schools and colleges where the built-in hard drive enabled new teaching approaches. Programming courses benefited from local storage for development tools and student projects. Art and design classes utilized the hard drive for storing large graphics files and font collections.

Network boot capabilities allowed the A4000 to serve as a classroom file server, with diskless A3010 or A3020 stations loading software from the A4000's hard drive via Econet. This configuration reduced total system cost while providing centralized software management.

Market Position

The A4000 occupied a challenging market position, competing against increasingly powerful and affordable PC compatibles. While offering superior ease of use and educational software, the platform's limited business software selection restricted commercial appeal.

Pricing remained competitive with similarly specified PC systems, but the PC platform's economies of scale and broader software availability gradually eroded Acorn's market share. The A4000's discontinuation in late 1993 marked the end of the traditional Archimedes line, with the revolutionary Risc PC representing Acorn's next generation strategy.

General Maintenance

Regular maintenance focuses on thermal management and mechanical components. The internal hard drive requires adequate cooling airflow, making case ventilation cleaning essential. The power supply's switch-mode design generates less heat than linear types but still requires periodic inspection. Battery replacement before leakage prevents motherboard damage. The integrated ARM250 has no socketed components, simplifying maintenance but preventing chip-level repairs.

Troubleshooting

Common faults include hard drive failures manifesting as boot errors or data corruption, power supply problems causing random resets or failure to start, and video output issues often traced to the VGA output circuitry. The integrated ARM250 design means processor, memory controller, or video controller failures require complete chip replacement. IDE interface problems may prevent hard drive detection, usually resolved by cable replacement or controller chip reseating.

Capacitor Replacement Guide

Critical capacitors requiring replacement after 30 years include power supply primary capacitors (47µF 400V, 100µF 200V), secondary filtering (1000µF 16V ×2, 2200µF 10V), and motherboard electrolytics (100µF 16V ×4, 47µF 16V ×6). The switch-mode power supply requires careful handling of high-voltage components. Unlike earlier models, the A4000's two-layer PCB simplifies trace repair if capacitor leakage causes damage.

Legacy

The A4000 represented the final evolution of the original Archimedes concept before the radical Risc PC redesign. Its combination of integrated processor technology, standard memory modules, and IDE storage influenced subsequent Acorn designs. While commercial success remained limited, the A4000 demonstrated the viability of highly integrated ARM processors, presaging the system-on-chip designs that would eventually dominate mobile computing.

The machine's reliability and expandability ensured many units remained in service well into the 2000s, particularly in educational environments. Modern retro computing enthusiasts appreciate the A4000's balance of capability and affordability, making it a popular entry point for exploring RISC OS and ARM architecture history.