Cheetah (computer)

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The cheetah

The Gepard is a plug-in card computer produced by Gepard Computer based on the Motorola 68000 processors, an operating system developed in-house and a Modula 2 compiler. The name was given due to the difference in performance between the computer and other machines available for private users in the early 1980s.

history

Development begins at a time when

  • the address space of 8-bit computers was exhausted with just a few chips due to advances in memory technology (64 kiBit DRAM )
  • with the Motorola 68000 - CPU a 16/32-bit processor was available without the restrictions of the 8-bit world
  • The existing series machine with this CPU (including Apple Lisa , TRS-80 Model 16, HP 9826A, Fortune 32:16) was difficult to obtain for European interested parties and was mostly beyond the (private) financial possibilities
  • Many users were looking for a successor to 8-bit computers such as the Apple II in the form of an open, modularly expandable and completely documented system
  • popular 16/32 bit computers such as Amiga and Atari ST were still a long way off

After the development of the Gepard computer began in 1983, Gepard Computer GmbH & Co. KG, based in Oldenburg, was founded in 1984.

Consequently, the marketing mainly emphasized the modularity and speed of the system, with which every user should be able to put together the appropriate system extension himself.

The concept of the plug-in card computer was already widespread in 8-bit computers, mainly with Z80 processors. The computer magazines mc and c't , founded around the same time as Gepard, presented their own plug-in card computers with a 68000 CPU (MC68000 and c't68000) around 1984, and the NDR small computer also followed this trend.

hardware

The computer consists of individual modules in the format of a Euro card , which are plugged together on a passive bus with up to 16 slots. A three-row 96-pin connector according to DIN 41612 ("VG strip") is used as the bus connector , the assignment of which is based on the ECB bus that was widespread at the time of development, but was proprietary in detail. Long bus cards required the data and address lines to be terminated , which meant that a bus slot was occupied.

The development of the hardware can be divided into three phases:

  • 1st phase (from 1983): dependent 16-bit subsystem for the Apple II . In addition to the bus and power supply, the cheetah consists of cards with a CPU 68000, RAM 128 kiB, ROM and Apple-Link without its own IO. An Apple II equipped for UCSD Pascal (ie 48 kiB RAM + 16 kiB additional RAM, at least one floppy disk drive) is required as the host computer , which also accepts the link card to the Gepard. Towards the end of the phase, a C64 was also supported as a host computer.
  • 2nd phase (from 1985): independent 16-bit card computer system. The Gepard works completely with its own peripherals and consists of a bus, power supply unit and a minimum of cards with CPU 68000/68010, RAM 256 kiB, ROM, floppy controller, multifunction card and a video card. The development of many other plug-in cards also falls into this phase.
  • 3rd phase (from 1987): Host for 32-bit subsystem: With the availability of the successor CPU 68020, the limits of the 16-bit bus were clearly exceeded, so that the capabilities of the CPU could only be used by having a core computer with CPU and RAM on a common plug-in card. The map computer developed in the direction of a 32-bit computer core with surrounding 16-bit peripherals.

Plug-in cards

map description
CPU CPU 68000 or 68010 with adjustable clock 8..12 MHz
CPU32 CPU 68020 with RAM up to 4 Mbytes
ROM / Applelink EPROM with bootloader and 8-bit parallel bus to Apple II
R.A.M. DRAM card 128 kiB (16 × 64 kiBit)
R.A.M. DRAM card 512 kiB (16 × 256 kiBit)
R.A.M. DRAM card 1 MiB (32 × 256 kiBit)
R.A.M. DRAM card 8 MiB (8xSIP 1Mx8)
SRAM / EPROM Static RAM or EPROM up to 512 kiB, data retention with battery is possible with RAM only
Multifunction Keyboard input (serial), timer (CTC), real-time clock (RTC) with battery, 2 × game port connections, 2 × digital-to-analog converters (DAC) 8 bit with multiplexer for audio outputs and low-pass filter control, 1 × analog comparator for audio input (successive approximation Voltage measurement with successive approximation in connection with one of the DAC), single-channel audio amplifier for loudspeakers
Serial / parallel 2 × serial port (can be converted between voltage and 20 mA current interface), 2 × parallel port output ( Centronics )

Variants:

  • Baud rate setting via jumper (up to the beginning of phase 2)
  • Baud rate setting via software
Floppy controller Connection of up to four 5.25 "and 3.5" floppy disk drives via the Shugart bus, data rates 128/256/512 kiBit / s, support for drives with flap switch / eject mechanism
Text video 4 kiB RAM for image data (9 bit character code + 2 bit attributes per character)

8 kiB RAM as a character generator for 512 characters

Display modes for standard definition with TV BAS signal :

  • Text mode 80 × 25 characters with 8 × 11 pixels / characters, attributes high / low, normal / inverse
  • Graphics mode 320 × 192 pixels
  • Pseudographic mode via character generator: 640 × 275 pixels
Graphic video Graphics subsystem consisting of base map + GDP maps (see description below)
HD controller For MFM hard disks with ST506 interface
SCSI controller For SCSI hard drives, scanners, etc.
Mouse / trackball Inputs for two-phase incremental encoders . Connection to the game port inputs of the multifunction card is also possible, but there without interrupt support.
prototype Connection adapter for your own developments, contains the bus logic and decoding of address areas

CPU

Due to the asynchronous data bus of the 68000, the CPU clock is a bus signal, but not relevant for other plug-in cards. Each plug-in card controls the timing of memory access in its own responsibility via handshake signals and thus works with different CPU clocks without adaptation.

R.A.M.

As software development progressed, the memory requirement for a functional system increased to around 1 MiB. In addition to being used as main memory, RAM and ROM cards could also be used as reset-proof RAM disks (under GDOS) or as memory for code modules (OS-9). The latter supports the direct start of code from a ROM memory, ie when a stored program is started, only space is required in the main memory for the data segments and the stack, but no space for the program code. This also enables operation without a mass storage device.

RAM cards with DIL- ICs that had become too small could easily be doubled in their capacity and further usefully used by soldering an identical IC piggyback on each RAM-IC. All pins except for the Chip Select input (CS) were connected 1: 1 and the CS pins were wired on the fly with a modified address decoding. There were similar modifications to almost all computers of the time.

Mass storage

Hard disks (MFM and SCSI) were not part of the usual equipment until phase 3, in the early days they were mainly for private users beyond the financial possibilities. For this reason, the standard equipment of the computer from phase 2 onwards consisted of at least one Sony 3.5 "floppy disk drive.

Drives with two special features were used:

  • the double-sided version with 2 × 80 tracks (in contrast to the single-sided drives of the later Apple Mac) together with a sector size of 1 kByte and 5 sectors / track resulted in a capacity of 800 kiB, which is significantly higher than anything common up to then Floppy disk formats.
  • the variant of the floppy disk drive with double spindle speed (600 / min instead of 300 / min), which is still very rare today, enabled reading and writing of DD disks at 500 kiBit / s instead of the usual 250 kiBit / s. The net data rate of 30 kiB / s that could be achieved when reading related data was without competition until it was replaced by hard drives.

The data rate of hard disks exceeded the transfer rate between the HD controller and main memory that could be realized with the CPU, the processors were not fast enough to fetch the data of one sector from the controller before the data of the next sector was available, and the controllers lacked large buffer memories, to temporarily store the data of a complete track. It was therefore not possible to read all sectors of a track within a single revolution of the spindle in ascending order, as with floppy disks. Instead, the sectors were nested -> see Interleave . In the cheetah, an interleave factor of 3 was required.

Graphics subsystem

The cheetah graphics card

The graphics subsystem is a stack of plug-in cards, consisting of a base card and 1 to 8 cards with a graphics display processor (GDP) attached to it.

The base card establishes the bus connection, controls the timing, contains the palette RAM (max. 256 from a palette of 4096 colors) and analog and digital video outputs. On the GDP cards there is 128 kiB RAM for the image data. The NEC µPD7220 is used as GDP .

On the basic map, the color depth of the display is set with 1, 2, 4 or 8 color bits per pixel . The GDP cards share the generation and processing of these color bits, ie each GDP contributes to the color information for each pixel with 1, 2, 4 or 8 bits, whereby each GDP must have the same number of bits and the total number of bits determines the setting on the Base map cannot exceed. Since the 16-bit RAM of each GDP can be read out cyclically with a maximum of 6 MiB / s, the number of color bits per GDP determines the maximum achievable pixel clock . The maximum limit of 48 MHz pixel clock with a full 8-bit color depth can only be reached when fully expanded with eight GDPs (then each GDP processes one of the planar bit planes ). With a single GDP card, the pixel clock of 48 MHz is only possible with two colors and decreases with increasing color depth to 6 MHz with 256 colors.

The 128 kiB screen memory per GDP is sufficient, so that no limitation of the resolution is necessary in any mode due to insufficient memory. In many modes less than half the memory size is used for the display, so it is possible to hold more than one screen page in the memory of the video card.

The CPU can address the GDPs individually via the base card or, in any combination, supply several GDPs with one access at the same time with commands. The CPU does not have direct access to the image memory; all write and read access to the image data must be carried out by or via the respective GDP. The disadvantage here is the 8-bit access width of the CPU to the GDPs, but a FIFO in the data path ensures that the CPU does not have to wait for the execution of every command.

Depending on the screen geometry, this graphics system can generate video signals up to a maximum resolution of around 1000 × 800 points at 35 kHz line frequency.

At a time when computers usually only used the television standard for display on the screen and the product “color computer monitor with high resolution” did not yet exist, it was a considerable effort to find a display device suitable for this subsystem. It was only with the spread of EGA graphics for the IBM PC AT that such monitors became affordable .

software

Pointing the way for the development of the Gepard operating system GDOS were the UCSD-Pascal available on the Apple II as well as the new development of the Modula-2 language presented by Niklaus Wirth . The focus of the development was the construction of the Modula-2 compiler, which was developed in assembler.

Standard operating system

In contrast to the compiler, the user interface of the operating system was adopted almost identically from UCSD-Pascal. A system menu line at the top of the screen and a file manager called a "filer" are typical. A "monitor" was added for processing memory areas. The operating system initially called "GDOS" was later renamed "OS Science".

Only a few programs achieved widespread use; B. The game BOLO by Meinolf Schneider was originally developed on the cheetah.

Alternative operating system

OS-9 /68000 was ported to the computer from a third party . This Unix-like multi-user operating system made standard programs available, in addition to many system programs that are still common today, as well as editors such as Emacs and C compilers.

OS-9 could be set very flexibly to many disk formats using descriptors, and reading and writing of FAT12 disks on the emerging MS-DOS computers was also possible with little effort. The OS-9 standard format (DD disks with an SD-formatted first track), on the other hand, was not supported, which made the exchange with other systems much more difficult. Such mixed formatted floppy disks can only be read today with great effort and historical hardware. Cumana, which ported OS-9 to the Atari ST , gave its name to a floppy disk format with 800 kiB, which also perfectly matched the Gepard's drives.

Applications

The cheetah computer was also used in research.

Quantities

At the end of 1985, the manufacturer stated that 300 systems had been sold.

User support

The Gepard Computer company sent out a newsletter at random, in which they informed about the current state of development. Buyers received an address list from other users to exchange experiences, this was organizationally promoted in 1985 by a monthly diskette dispatch, in which everyone could participate with and without their own text or program contributions (Gepard Forum).

There was a similar facility for OS 9 users (EFFO).

Journal articles about the system appeared e.g. B. in the 68000er , but were generally rare and unhelpful.

Weaknesses and cessation of production

The performance gap compared to other computers was still huge at the beginning of the development, but shrank in the course of the technical development already at the beginning of phase 2.

As a lead over the emerging competing computers, the following remained longer:

  • high CPU speed
  • fast floppy disk access
  • Modularity of all system components
  • very flexible memory configurations and consistent separation of system and video memory
  • powerful graphics system
  • Multi-user capable under OS-9

But the disadvantages also became increasingly important:

  • Use of inexpensive 68xx peripheral components (only 8 bit, 1 MHz access clock, only one auto interrupt vector for a whole card)
  • Serial interfaces without FIFO , baud rate setting not by software, only by jumpers
  • no DMA , audio processor and other CPU support for audio and video, although provided on the bus side
  • the quality of the high-frequency properties of the bus board was marginal (only two-sided layout, no multilayer ). Not every card combination could run in every slot, RAM cards only worked stably close to the CPU.
  • Operating system supplied without multiuser and multitasking properties and without a graphical user interface
  • very expensive color graphics system

The characteristic of complete modularity, which is decisive for the system, was only actually implemented for a relatively short period of time in phase 2. In the later phases, other computers offered comparable expandability and the system became increasingly uninteresting for purely private users compared to graphically oriented computers and the emerging IBM computer world and the user forums fell apart.

The manufacturer went bankrupt at the end of 1986.

Individual evidence

  1. [1] c't68000 computer with main board that can be divided into plug-in cards
  2. [2] Mega-byte in self-construction
  3. [3] Who Is Thomas Tempelmann?
  4. [4] bolo-and-bolo-workshop-for-atari-st
  5. [5] OS-9 Operating System User's Guide
  6. [6] OS9FAQ
  7. [7] Reading OS9 Disks in Linux
  8. Gepard Computer in the BR SpaceNight on Youtube
  9. [8] European Forum for OS-9
  10. [9] See section "Annual report of the board"
  11. ^ [10] Bundesanzeiger Archive, announcement of October 23, 1986; search company "Gepard", address "Oldenburg"

Web links

Commons : Cheetah  - Collection of images, videos and audio files