LINK 480Z
Encyclopedia
The LINK 480Z was an 8-bit
microcomputer
produced by Research Machines Limited in Oxford
, England
, during the early 1980s.
The 480Z used a Z80
microprocessor
with up to 256 KB
of bank-switched
RAM
. The system could be used as a stand-alone unit with cassette-based storage and the BASIC
programming language run from ROM
, or it could boot CP/NOS (a network version of CP/M
) over a local area network
from a file server
. When fitted with an optional external floppy disk
drive the system could boot the CP/M
operating system
directly.
The 480Z was sold mainly to the UK educational market as cassette-based system or as a diskless node which could be connected, via the proprietary CHAIN Network, to a Research Machines 380Z
acting as a file server
.
drive unit was external.
The only microprocessor offered was a 4 MHz
Z80A
.
(32 × 1024 byte
s) of RAM
, most 480Zs were fitted with at least 64 KB of memory. Bank switching
allowed memory to be extended to 128 KB on the main board, and up to 256 KB by using the option board (which also included the high resolution graphics hardware). As the Z80 processor could only directly address 64 KB without software support the additional memory was typically used as a RAM disk
, specifically the Silicon Disk System
. The silicon disk could be automatically loaded with software and data when the 480Z booted to the network. This saved considerable time in a classroom setting, where software could be loaded in advance of the children arriving. The silicon disc retained its contents if the system underwent a soft reset.
The 480Z was also fitted with up to 32 KB of firmware
that could also be bank-switched out of the normal address space when not in use, leaving a total of 58 KB of RAM directly available to the user, with 2 KB of RAM reserved for system use and 4 KB inaccessible because of the firmware ROM
.
monochrome video hardware
. Composite video
output was provided for an external monitor, and an internal RF modulator
provided a separate output to drive a television set. The text-mode display had its own dedicated memory.
In addition to the text-mode video interface the system could be enhanced with an option board providing a high-resolution
graphics capability and an additional TTL RGB interface for a colour monitor. The board was fitted with a dedicated bank of 16 KB of video memory and supported three graphics modes:
A programmable lookup table
with an 8-bit output mapped the pixel value to one of 256 different intensities (composite video) or one of 8 colours (TTL RGB output).
Output from the graphics board was mixed with output from the text-only video interface, allowing text and graphics to be easily overlaid. The graphics output only covered the top 20 lines of the text display and therefore text output could be set to only use the bottom 4 lines if overlap was not desired.
Memory in both the text and graphics video interfaces was accessed by the processor using port-mapped I/O and therefore did not consume memory address space.
was either via cassette tape, floppy disk
, or an external file server
. The cassette interface operated at either 300 bit/s or 1200 bit/s.
ROS 1.2 (see below) and later systems could be connected via a parallel interface to an external single or dual 5¼-inch disk drive unit with a built-in double density
Intelligent Disc Controller (IDC). Disk capacity was 180 KB (48 TPI
drives) or 360 KB (96 TPI drives) per side.
Research Machines also offered plug-in ROM Packs, containing up to 64 KB of ROM. These connected via the parallel interface allowing applications to be quickly loaded into RAM.
that ran over a coaxial cable
in a similar manner to 10BASE2
Ethernet
. Each station on the network required a unique, 8-bit network address that was set by means of a DIP switch
on the rear of the unit. Using the built-in Z-Net firmware a diskless 480Z could be directly booted from a network file server
(typically a Research Machines 380Z
).
contained the processor and up to 128 KB of RAM as well as most of the external interfaces:
The high resolution graphics and memory option board allowed an extra 128 KB of RAM to be added and included a TTL RGB monitor output. The option board could also be fitted with a IEEE-488
interface and an AMD 9511 or 9512 floating point
coprocessor
.
the operating system. ROS also provided a software front panel
allowing providing a display of registers and memory, and supporting breakpoint
s and single-stepping of machine code.
. ROS functions were called by means of the Emulator Trap
(EMT) pseudo-opcode
, which used the Z80 RST 30H instruction to call
the EMT handler function. The EMT handler read the first byte following the RST 30H instruction to determine which EMT function was being requested; all parameters were passed in registers. A call-relative pseudo-opcode was also implemented using RST 28H.
ROS services were largely compatible with the COS services provided on the earlier Research Machines 380Z.
as part of the standard firmware. This was mainly for use on cassette-based systems and was switched out of the address space if the system was booted from disk or the network.
s were CP/M
2.2 if booted from disk or CP/NOS (a network-only version of CP/M) if booted from a file server. Systems running CP/M could access network services using CP/NET.
. Research Machines also produced their own assembler
(ZASM), text editor
(TXED) and BASIC interpreter.
Programs for the earlier Research Machines 380Z written in high-level languages, such as BASIC, or using only basic CP/M and standard firmware functions, could be run directly on the 480Z. Programs that wrote directly to hardware such as the high-resolution graphics cards were generally incompatible.
8-bit
The first widely adopted 8-bit microprocessor was the Intel 8080, being used in many hobbyist computers of the late 1970s and early 1980s, often running the CP/M operating system. The Zilog Z80 and the Motorola 6800 were also used in similar computers...
microcomputer
Microcomputer
A microcomputer is a computer with a microprocessor as its central processing unit. They are physically small compared to mainframe and minicomputers...
produced by Research Machines Limited in Oxford
Oxford
The city of Oxford is the county town of Oxfordshire, England. The city, made prominent by its medieval university, has a population of just under 165,000, with 153,900 living within the district boundary. It lies about 50 miles north-west of London. The rivers Cherwell and Thames run through...
, England
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
, during the early 1980s.
The 480Z used a Z80
Zilog Z80
The Zilog Z80 is an 8-bit microprocessor designed by Zilog and sold from July 1976 onwards. It was widely used both in desktop and embedded computer designs as well as for military purposes...
microprocessor
Microprocessor
A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...
with up to 256 KB
Kilobyte
The kilobyte is a multiple of the unit byte for digital information. Although the prefix kilo- means 1000, the term kilobyte and symbol KB have historically been used to refer to either 1024 bytes or 1000 bytes, dependent upon context, in the fields of computer science and information...
of bank-switched
Bank switching
Bank switching is a technique to increase the amount of usable memory beyond the amount directly addressable by the processor. It can be used to configure a system differently at different times; for example, a ROM required to start a system from diskette could be switched out when no longer...
RAM
Ram
-Animals:*Ram, an uncastrated male sheep*Ram cichlid, a species of freshwater fish endemic to Colombia and Venezuela-Military:*Battering ram*Ramming, a military tactic in which one vehicle runs into another...
. The system could be used as a stand-alone unit with cassette-based storage and the BASIC
BASIC
BASIC is a family of general-purpose, high-level programming languages whose design philosophy emphasizes ease of use - the name is an acronym from Beginner's All-purpose Symbolic Instruction Code....
programming language run from ROM
Read-only memory
Read-only memory is a class of storage medium used in computers and other electronic devices. Data stored in ROM cannot be modified, or can be modified only slowly or with difficulty, so it is mainly used to distribute firmware .In its strictest sense, ROM refers only...
, or it could boot CP/NOS (a network version of CP/M
CP/M
CP/M was a mass-market operating system created for Intel 8080/85 based microcomputers by Gary Kildall of Digital Research, Inc...
) over a local area network
Local area network
A local area network is a computer network that interconnects computers in a limited area such as a home, school, computer laboratory, or office building...
from a file server
File server
In computing, a file server is a computer attached to a network that has the primary purpose of providing a location for shared disk access, i.e. shared storage of computer files that can be accessed by the workstations that are attached to the computer network...
. When fitted with an optional external floppy disk
Floppy disk
A floppy disk is a disk storage medium composed of a disk of thin and flexible magnetic storage medium, sealed in a rectangular plastic carrier lined with fabric that removes dust particles...
drive the system could boot the CP/M
CP/M
CP/M was a mass-market operating system created for Intel 8080/85 based microcomputers by Gary Kildall of Digital Research, Inc...
operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
directly.
The 480Z was sold mainly to the UK educational market as cassette-based system or as a diskless node which could be connected, via the proprietary CHAIN Network, to a Research Machines 380Z
Research Machines 380Z
The Research Machines 380Z was an early 8-bit microcomputer produced by Research Machines Limited in Oxford, England, from 1978 to 1985....
acting as a file server
File server
In computing, a file server is a computer attached to a network that has the primary purpose of providing a location for shared disk access, i.e. shared storage of computer files that can be accessed by the workstations that are attached to the computer network...
.
Main unit and processor
The LINK 480Z was packaged as an integrated keyboard and system unit. Early systems were supplied with a black sheet-metal case, however this was quickly replaced by a cream-coloured plastic housing. The optional 5¼-inch floppy diskFloppy disk
A floppy disk is a disk storage medium composed of a disk of thin and flexible magnetic storage medium, sealed in a rectangular plastic carrier lined with fabric that removes dust particles...
drive unit was external.
The only microprocessor offered was a 4 MHz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
Z80A
Zilog Z80
The Zilog Z80 is an 8-bit microprocessor designed by Zilog and sold from July 1976 onwards. It was widely used both in desktop and embedded computer designs as well as for military purposes...
.
Memory
Although some early systems had only 32 KBKilobyte
The kilobyte is a multiple of the unit byte for digital information. Although the prefix kilo- means 1000, the term kilobyte and symbol KB have historically been used to refer to either 1024 bytes or 1000 bytes, dependent upon context, in the fields of computer science and information...
(32 × 1024 byte
Byte
The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...
s) of RAM
Ram
-Animals:*Ram, an uncastrated male sheep*Ram cichlid, a species of freshwater fish endemic to Colombia and Venezuela-Military:*Battering ram*Ramming, a military tactic in which one vehicle runs into another...
, most 480Zs were fitted with at least 64 KB of memory. Bank switching
Bank switching
Bank switching is a technique to increase the amount of usable memory beyond the amount directly addressable by the processor. It can be used to configure a system differently at different times; for example, a ROM required to start a system from diskette could be switched out when no longer...
allowed memory to be extended to 128 KB on the main board, and up to 256 KB by using the option board (which also included the high resolution graphics hardware). As the Z80 processor could only directly address 64 KB without software support the additional memory was typically used as a RAM disk
RAM disk
A RAM disk or RAM drive is a block of RAM that a computer's software is treating as if the memory were a disk drive...
, specifically the Silicon Disk System
Silicon Disk System
The Silicon Disk System was the first commercially available RAM disk for microcomputers.It was written by Jerry Karlin in 1979/80. Karlin was joined by Peter Cheesewright and their company Microcosm Research Ltd marketed the product for a number of years. The product was available as a standalone...
. The silicon disk could be automatically loaded with software and data when the 480Z booted to the network. This saved considerable time in a classroom setting, where software could be loaded in advance of the children arriving. The silicon disc retained its contents if the system underwent a soft reset.
The 480Z was also fitted with up to 32 KB of firmware
Firmware
In electronic systems and computing, firmware is a term often used to denote the fixed, usually rather small, programs and/or data structures that internally control various electronic devices...
that could also be bank-switched out of the normal address space when not in use, leaving a total of 58 KB of RAM directly available to the user, with 2 KB of RAM reserved for system use and 4 KB inaccessible because of the firmware ROM
Read-only memory
Read-only memory is a class of storage medium used in computers and other electronic devices. Data stored in ROM cannot be modified, or can be modified only slowly or with difficulty, so it is mainly used to distribute firmware .In its strictest sense, ROM refers only...
.
Video
All 480Zs, with the exception of some very early units, were fitted with software-switchable 40×24 or 80×24 character text-onlyText mode
Text mode is a kind of computer display mode in which the content of the screen is internally represented in terms of characters rather than individual pixels. Typically, the screen consists of a uniform rectangular grid of character cells, each of which contains one of the characters of a...
monochrome video hardware
Video card
A video card, Graphics Card, or Graphics adapter is an expansion card which generates output images to a display. Most video cards offer various functions such as accelerated rendering of 3D scenes and 2D graphics, MPEG-2/MPEG-4 decoding, TV output, or the ability to connect multiple monitors...
. Composite video
Composite video
Composite video is the format of an analog television signal before it is combined with a sound signal and modulated onto an RF carrier. In contrast to component video it contains all required video information, including colors in a single line-level signal...
output was provided for an external monitor, and an internal RF modulator
RF modulator
An RF modulator is a device that takes a baseband input signal and outputs a radio frequency-modulated signal....
provided a separate output to drive a television set. The text-mode display had its own dedicated memory.
In addition to the text-mode video interface the system could be enhanced with an option board providing a high-resolution
Image resolution
Image resolution is an umbrella term that describes the detail an image holds. The term applies to raster digital images, film images, and other types of images. Higher resolution means more image detail....
graphics capability and an additional TTL RGB interface for a colour monitor. The board was fitted with a dedicated bank of 16 KB of video memory and supported three graphics modes:
- Extra high resolution: 640×192 pixelPixelIn digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....
s, 1 bit per pixelColor depthIn computer graphics, color depth or bit depth is the number of bits used to represent the color of a single pixel in a bitmapped image or video frame buffer. This concept is also known as bits per pixel , particularly when specified along with the number of bits used...
, 1 pagePage (computing)A page, memory page, or virtual page is a fixed-length contiguous block of virtual memory that is the smallest unit of data for the following:* memory allocation performed by the operating system for a program; and...
. - High resolution: 320×192 pixels, 2 bits per pixel, 1 page.
- Medium resolution: 160×96 pixels, 4 bits per pixel, 2 pages.
A programmable lookup table
Lookup table
In computer science, a lookup table is a data structure, usually an array or associative array, often used to replace a runtime computation with a simpler array indexing operation. The savings in terms of processing time can be significant, since retrieving a value from memory is often faster than...
with an 8-bit output mapped the pixel value to one of 256 different intensities (composite video) or one of 8 colours (TTL RGB output).
Output from the graphics board was mixed with output from the text-only video interface, allowing text and graphics to be easily overlaid. The graphics output only covered the top 20 lines of the text display and therefore text output could be set to only use the bottom 4 lines if overlap was not desired.
Memory in both the text and graphics video interfaces was accessed by the processor using port-mapped I/O and therefore did not consume memory address space.
Storage
Mass storageMass storage
In computing, mass storage refers to the storage of large amounts of data in a persisting and machine-readable fashion. Devices and/or systems that have been described as mass storage include tape libraries, RAID systems, hard disk drives, magnetic tape drives, optical disc drives, magneto-optical...
was either via cassette tape, floppy disk
Floppy disk
A floppy disk is a disk storage medium composed of a disk of thin and flexible magnetic storage medium, sealed in a rectangular plastic carrier lined with fabric that removes dust particles...
, or an external file server
File server
In computing, a file server is a computer attached to a network that has the primary purpose of providing a location for shared disk access, i.e. shared storage of computer files that can be accessed by the workstations that are attached to the computer network...
. The cassette interface operated at either 300 bit/s or 1200 bit/s.
ROS 1.2 (see below) and later systems could be connected via a parallel interface to an external single or dual 5¼-inch disk drive unit with a built-in double density
Double density
Double density, often shortened DD, is a capacity designation on magnetic storage, usually floppy disks. It describes the use of an encoding of information, which can encode on average twice as many bits per time unit compared to single density...
Intelligent Disc Controller (IDC). Disk capacity was 180 KB (48 TPI
Tracks per inch
Tracks per inch is a measure of magnetic resolution, in particular the number of individual tracks a floppy disk controller can use within a linear one-inch space.* 48 tpi* 96/100 tpi...
drives) or 360 KB (96 TPI drives) per side.
Research Machines also offered plug-in ROM Packs, containing up to 64 KB of ROM. These connected via the parallel interface allowing applications to be quickly loaded into RAM.
Networking
The LINK 480Z supported a proprietary 800 kbit/s CHAIN local area networkLocal area network
A local area network is a computer network that interconnects computers in a limited area such as a home, school, computer laboratory, or office building...
that ran over a coaxial cable
Coaxial cable
Coaxial cable, or coax, has an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield. The term coaxial comes from the inner conductor and the outer shield sharing the same geometric axis...
in a similar manner to 10BASE2
10BASE2
10BASE2 is a variant of Ethernet that uses thin coaxial cable , terminated with BNC connectors...
Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
. Each station on the network required a unique, 8-bit network address that was set by means of a DIP switch
DIP switch
DIP switches are manual electric switches that are packaged in a group in a standard dual in-line package...
on the rear of the unit. Using the built-in Z-Net firmware a diskless 480Z could be directly booted from a network file server
File server
In computing, a file server is a computer attached to a network that has the primary purpose of providing a location for shared disk access, i.e. shared storage of computer files that can be accessed by the workstations that are attached to the computer network...
(typically a Research Machines 380Z
Research Machines 380Z
The Research Machines 380Z was an early 8-bit microcomputer produced by Research Machines Limited in Oxford, England, from 1978 to 1985....
).
Interfaces and options
The 480Z motherboardMotherboard
In personal computers, a motherboard is the central printed circuit board in many modern computers and holds many of the crucial components of the system, providing connectors for other peripherals. The motherboard is sometimes alternatively known as the mainboard, system board, or, on Apple...
contained the processor and up to 128 KB of RAM as well as most of the external interfaces:
- Two serial interfaces (SIO-4 and SIO-2) – providing full and cut-down RS-232RS-232In telecommunications, RS-232 is the traditional name for a series of standards for serial binary single-ended data and control signals connecting between a DTE and a DCE . It is commonly used in computer serial ports...
interfaces, respectively. - Parallel I/O – used either as a high-speed link for the external disk drives or ROM Packs, or as a Centronics printer port.
- Cassette interface.
- Accessory input – two channel analogue joystick interface.
- Composite video and audio.
- RF TV output.
- Network connection (optional).
The high resolution graphics and memory option board allowed an extra 128 KB of RAM to be added and included a TTL RGB monitor output. The option board could also be fitted with a IEEE-488
IEEE-488
IEEE-488 is a short-range digital communications bus specification. It was created for use with automated test equipment in the late 1960s, and is still in use for that purpose. IEEE-488 was created as HP-IB , and is commonly called GPIB...
interface and an AMD 9511 or 9512 floating point
Floating point
In computing, floating point describes a method of representing real numbers in a way that can support a wide range of values. Numbers are, in general, represented approximately to a fixed number of significant digits and scaled using an exponent. The base for the scaling is normally 2, 10 or 16...
coprocessor
Coprocessor
A coprocessor is a computer processor used to supplement the functions of the primary processor . Operations performed by the coprocessor may be floating point arithmetic, graphics, signal processing, string processing, or encryption. By offloading processor-intensive tasks from the main processor,...
.
Firmware
The 480Z was fitted with up to 32 KB of firmware:- ROM monitor (ROS) – 8 KB.
- Z-Net network firmware – 4 KB.
- BASICBASICBASIC is a family of general-purpose, high-level programming languages whose design philosophy emphasizes ease of use - the name is an acronym from Beginner's All-purpose Symbolic Instruction Code....
in ROM – 20 KB.
ROM monitor
ROS (standing for Resident Operating System) provided a monitor program and a set of basic system services. The monitor could be used to start BASIC from ROM, load application programs from cassette, or bootBooting
In computing, booting is a process that begins when a user turns on a computer system and prepares the computer to perform its normal operations. On modern computers, this typically involves loading and starting an operating system. The boot sequence is the initial set of operations that the...
the operating system. ROS also provided a software front panel
Front panel
A front panel was used on early electronic computers to display and allow the alteration of the state of the machine's internal registers and memory. The front panel usually consisted of arrays of indicator lamps, toggle switches, and push buttons mounted on a sheet metal face plate...
allowing providing a display of registers and memory, and supporting breakpoint
Breakpoint
In software development, a breakpoint is an intentional stopping or pausing place in a program, put in place for debugging purposes. It is also sometimes simply referred to as a pause....
s and single-stepping of machine code.
ROS services
ROS provided a number of basic hardware control functions, such as keyboard input, writing text to the video memory and disk input/outputInput/output
In computing, input/output, or I/O, refers to the communication between an information processing system , and the outside world, possibly a human, or another information processing system. Inputs are the signals or data received by the system, and outputs are the signals or data sent from it...
. ROS functions were called by means of the Emulator Trap
Trap (computing)
In computing and operating systems, a trap, also known as an exception or a fault, is typicallyThere is a wide variation in the nomenclature...
(EMT) pseudo-opcode
Opcode
In computer science engineering, an opcode is the portion of a machine language instruction that specifies the operation to be performed. Their specification and format are laid out in the instruction set architecture of the processor in question...
, which used the Z80 RST 30H instruction to call
Subroutine
In computer science, a subroutine is a portion of code within a larger program that performs a specific task and is relatively independent of the remaining code....
the EMT handler function. The EMT handler read the first byte following the RST 30H instruction to determine which EMT function was being requested; all parameters were passed in registers. A call-relative pseudo-opcode was also implemented using RST 28H.
ROS services were largely compatible with the COS services provided on the earlier Research Machines 380Z.
Z-Net
The Z-Net firmware was used to allow the system to network boot from a file server over the local area network. It was switched out of the main address space once the system had booted.BASIC in ROM
Research Machines provided a full version of their BASIC interpreterInterpreter (computing)
In computer science, an interpreter normally means a computer program that executes, i.e. performs, instructions written in a programming language...
as part of the standard firmware. This was mainly for use on cassette-based systems and was switched out of the address space if the system was booted from disk or the network.
Main ROS versions
ROS versions were:- ROS 1.0
- ROS 1.1
- ROS 1.2 – added floppy disk support.
- ROS 2.2
Operating systems
The main operating systemOperating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
s were CP/M
CP/M
CP/M was a mass-market operating system created for Intel 8080/85 based microcomputers by Gary Kildall of Digital Research, Inc...
2.2 if booted from disk or CP/NOS (a network-only version of CP/M) if booted from a file server. Systems running CP/M could access network services using CP/NET.
Application software
Many standard CP/M applications were available, such as WordStarWordStar
WordStar is a word processor application, published by MicroPro International, originally written for the CP/M operating system but later ported to DOS, that enjoyed a dominant market share during the early to mid-1980s. Although Seymour I...
. Research Machines also produced their own assembler
Assembly language
An assembly language is a low-level programming language for computers, microprocessors, microcontrollers, and other programmable devices. It implements a symbolic representation of the machine codes and other constants needed to program a given CPU architecture...
(ZASM), text editor
Text editor
A text editor is a type of program used for editing plain text files.Text editors are often provided with operating systems or software development packages, and can be used to change configuration files and programming language source code....
(TXED) and BASIC interpreter.
Programs for the earlier Research Machines 380Z written in high-level languages, such as BASIC, or using only basic CP/M and standard firmware functions, could be run directly on the 480Z. Programs that wrote directly to hardware such as the high-resolution graphics cards were generally incompatible.