The Acorn System 4 was a microcomputer introduced around 1979 by the British company Acorn Computers as a more powerful and expandable successor to the earlier System-series machines. Unlike many home computers of the time, the System 4 was designed primarily for laboratories, industrial control, and educational environments. It used the MOS Technology 6502 microprocessor, typically running at about 1 MHz, and was packaged in a Eurocard rack format rather than a small desktop case. The system supported larger amounts of RAM than earlier Acorn systems and was commonly paired with floppy disk storage, which was a significant step forward from cassette-based systems.

The System 4 ran the Acorn Disk Operating System, allowing users to manage files on floppy disks and run larger programs than were practical on tape-based systems. It was often supplied with development tools and high-level languages such as BASIC and Assembler, making it suitable for programming, data acquisition, and control tasks. Because it was built as a modular rack system using plug-in cards, the machine could be configured with additional memory, I/O cards, and storage controllers depending on the application.

Compared with the System 3, System 4 had more memory expansion capability and a less limited I/O architecture. The System 3 was compact and inexpensive relative to the System 4, but it lacked the infrastructure required for larger multi-user or disk-based applications.

The biggest difference between System 4 and its predecessor was their intended role. The System 3 was essentially a development or hobby system built around a single board and tape storage, whereas the System 4 was a professional rack-mounted computer designed for disk-based operation and larger installations. The System 4’s floppy disk subsystem, modular Eurocard architecture, and greater RAM capacity made it suitable for more demanding workloads such as laboratory automation, software development, and industrial control—tasks that were difficult to perform efficiently on the more limited System 3.

The Acorn Visual Display Unit Controller Board connects to the Standard Acorn Computer Bus and contains a memory mapped character storage R.A.M. which is transparently written to or read from by the C.P.U.

An MC6845 programmable controller I.C. provides all the synchronisation signals to drive a 625 line 50 fields per second V.D.U. together with read addresses for the character R.A.M. Characters are then fed to an SAA5050 character generator IC which produces the necessary dot patterns to create the characters to refresh the V.D.U.

The SAA5050 produces Teletext standard characters and has Red, Green and Blue drive outputs giving coloured characters or graphics.

The R.G.B. and sync outputs may be used to drive a colour encoder and modulator for a U.H.F. Television; There is also a 1 volt/75 ohm composite sync and video output which can directly drive a Monochrome Monitor on which the different colours will appear as different scales of grey.

The VDU board came with listings for programs which set up the MC6845, display 25 instructions in hex on the V.D.U. (with double or treble byte instructions on a single line) and allow the drawing of graphics or characters on the V.D.U. These programs may be loaded and run using the Acorn System 1 Monitor. Later new monitor ROM was able to link the VDU and an ASC 11 keyboard to Acorns' 4K Fast BASIC.

The versatility of the programmable MC6845 and the SAA5050 combination may be employed to give other screen formats e. g. 80 characters x 16 lines and double height characters. Thus the Acorn VDU was of great value to experimenters and producers of specialist display systems. The VDU controller PCB was supplied in kit form with a full set of I.C. sockets. It was easily assembled using a small soldering iron and useful hints on assembly may be found in the Acorn Micro-computer System 1 Technical Manual. The board operates from a single +5v supply from which it draws not more than 500 mA.

The Motorola MC6845 commonly referred to as a CRTC (Cathode Ray Tube Controller) served as the architectural backbone for the video subsystems of many iconic 8-bit computers, including the IBM PC (CGA/MDA) and the BBC Micro. Unlike modern GPUs that handle complex rendering, the MC6845 was a programmable timing generator. Its primary function was to manage the synchronization signals (HSYNC and VSYNC) and generate the memory addresses required to fetch pixel or character data from video RAM. By offloading these repetitive timing tasks from the CPU, it allowed for flexible display formats through a set of 18 internal 8-bit registers.

At its core, the chip operates using a series of internal counters that track the horizontal and vertical position of the electron beam. The programmer defines the display geometry by writing to registers that control the total number of horizontal characters, the horizontal displayed characters, and the sync pulse widths. This programmability allowed developers to implement various "tricks," such as hardware scrolling or split-screen effects, by manipulating the Start Address Register (R12 and R13) mid-frame. This changed which part of the video memory the controller pointed to at the beginning of a raster scan.

The interface between the MC6845 and the system memory is fundamentally passive regarding data content. The controller outputs a Memory Address (MA0–MA13) and Row Address (RA0–RA4) but it does not actually "see" the data being fetched. In a typical character-mapped configuration, the MA lines point to the character code in RAM, while the RA lines determine which specific scanline of that character is being pulled from a Font ROM. This separation of concerns made it highly versatile, supporting both alphanumeric modes and "all-points-addressable" (APA) graphics modes depending on how the external logic interpreted the address lines.

Timing and synchronization are governed by the Character Clock (CLK) input, which increments the internal horizontal counter. To prevent "snow" or visual artifacts, the system must carefully manage bus contention between the CPU and the MC6845, as both require access to the video RAM. This was often solved using "interleaved" access, where the CPU and CRTC traded cycles, or by restricting CPU writes to the vertical blanking interval. The chip's ability to provide a Light Pen strobe input further expanded its utility, allowing the hardware to latch the current memory address when a light pen hit was detected, providing a primitive but effective form of user interaction.

To program the MC6845 for a specific resolution, you must calculate the values for its internal registers based on your system's Character Clock (CLK) and the target monitor's refresh rates. The chip operates by counting character units rather than individual pixels, so your first step is determining how many pixels wide a "character" is (typically 8) to find your horizontal frequency.

The 6502 is an 8-bit MicroProcessor designed by MOS Technology. The team was led by Chuck Peddle and had also worked on the Motorola 6800. The 6502 is a simplified, but faster and cheaper design than the 6800.

The 6502 was introduced in 1975 and was the cheapest microprocessor on the market. Together with the Zilog Z80, the 6502 helped start the home computer revolution of the 1980s. The 6502 was used in a wide range of devices: the Atari 2600, the 8-bit Atari home computers, the Apple II, the Nintendo Entertainment System, the Commodore 64, the BBC Micro and many others. All used the 6502 or a variation of it.

The 6502 is a 1MHz design, while the 6502A is designed for 2MHz. The 6502A is 100% compatible with the original 6502.

Commodore soon bought MOS Technology, but conitnued to sell the microprocessor to competitors and licensed the design to other manufacturers.