II. Hardware
3. Bridge Boards
A2088 - PC-XT-Bridge Board
"Itīs not just a PC connected to The Amiga host computer system, but a complete
new device that utilizes both processors. The PC can be considered at as just
another coprocessor in the Amiga world. Itīs a completly new designed machine,
in other words a hybrid. This will lead us to a complete new approach what computer
design is concerned -a multiprocessor design. Thatīs the path into the future."
R.J. Mical
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Already by summer 1986 the PC coprocessor computer emulator of the Amiga 1000 host computer system based completely on software until then, called "Transformer" was substituted by a hardware emulator, named A 1060-Sidecar. From the beginning Commodore was very keen on keeping the Amiga host computer system compatible to IBM-software. The A 1060 is a complete PC-XT compatible coprocessor computer system comprising a 8088 CPU, 256 (512)kB RAM, and a 360 kB 5,25 inch floppydisk drive with controller and ISA-slots. Inside the A1060 host computer there are two boards: A bridgeboard with Zorro-I interface as well as a complete PC-XT coprocessor computer system board as mentioned above. The connectors for mouse and joystick for operating on the PC coprocessor computer system are passed through. Data input and output of the sidecar is achieved via Amiga hardware. |
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The communication between PC-XT co-processor computer system and Amiga host computer system is managed by a software program developed by R.J. Mical. This specific software program working as a real Amiga task comprises a multitasking kernel named "Janus" (the two-faced). It is responsible of transferring keyboard inputs from Amiga keyboard to the PC co-processor system and of changing data output on the video screen corresponding to the processes of the PC co-processor computer system. Additionally the software program also implements routines for handling data exchange between an internal floppy disk drive (and possible one or more external ones) and Amiga A1060.
The hardware of the communication-interface (bridgeboard) was developed in a German subsidary of Commodore at Braunschweig by Frank-Thomas Ullmann and Dieter Preiss. The hereto related patent application WO 88/08564 was filed worldwide by Commodore. Basically the result of these German developments were:
A1060 - Sidecar (frontview)
A2088 - XT- Emulator (AMIGA A2088 KIT)
A2286 - AT- Emulator (AMIGA A2286 KIT)
A2386SX (Bridge Board A2386SX)
The hardware emulators A2088-PC-XT bridgeboard coprocessor system with Intel 8088 CPU (clock frequency 4,77 MHz) as well as the predecessor A2088T were delivered as so-called Commodore Amiga Kits. Such a kit comprised the bridge board, a 360-kB-(A2088) or 360kB/1,2 MB-(A2286) 5,25 inch -disk drive, the operating System MS-DOS v 3.x and the Janus-software on two floppy disks for PC-XT or AT and Amiga. The PC co-processor computer system can be enhanced by several harddisks. Also booting the PC from an Amiga harddisk is possible. The A2088(T) also features an interface for an external floppy-disk drive.
Although the A1060-sidecar being a completely stand-alone coprocessor computer system, it is not able to work on ist own without the Amiga 1000 host computer system. Therefore hardware emulation of the PC coprocessor computer system in the Amiga host computer system A 2000, A 3000 and A 4000 is achieved by the following bridge boards or bridge cards: A 2088(T) (see Fig.2), A 2286 and A 2386 SX (Intel 80386 SX with 16, 20 or 25 MHz clock frequency). These cards plug into corresponding bridge card slots of the Amiga host computer system. The design of these slots are subject of US Patent No. 4,954,949 . The essential contents of this patent document is the connection and communication of the Amiga host computer bus system (202) with Intel 8088 CPU by the so-called Dual Port RAM (76). Commodore designated Henry Rubin as inventor. See the pictures for a complete view and a more detailed view of an installed bridge card in the Amiga 2000 host computer.
Referring now to patent application
WO 88/08564
for a detailed description on how the hardware of bridge board technology has been actually realized.
Referring to Fig.5 above showing schematically the method of communicating data between co-processor system
(70) comprising 8088 CPU (54), BIOS, MS-DOS OS and operating system extension 74 (PC-Janus) and the Amiga host
computer system with complete hardware, OS (Kickstart and Workbench) and operating system extension (Amiga-Janus).
The method includes using a dual port random access memory (DUAL PORT RAM 76) shared between the CPUs (12, 54)
of the two computer systems for interprocessor communication.
Annexed Fig. 9 illustrates the method of the hardware interface in detail.
For achieving this communication between coprocessor Intel 8088 and host processor Motorola 68000 a special interface is needed for realigning bytes in accordance with the data exchange of data-, address- and control bus. For this purpose there are so-called translator chips: DATA BUS TRANSLATOR (98) (DBT) and ADDRESS BUS TRANSLATOR (100) (ABT). DBT and ABT translate data of 68000 CPU as well as 8088 CPU so that the processors can communicate with each other.
DBT contains several functional units. More spefically it contains a data translator (102) which provides three transfer mechanisms: Word-transfer, byte-transfer and graphics transfer. Transceiver (104) of DBT (98) functions as a high-speed data-transceiver between 8 bit data bus of 8088 CPU and 16 bit interface data bus of DBT. Auto config (108) automatically configures the co-processor system into the host computer memory map during system boot. Finally, control unit (110) of DBT functions to provide control signals for driving the various functions performed in DBT. Control unit contains the base address comparator, address decoders and other circuits which support the interface hardware operation.
ADDRESS BUS TRANSLATOR (ABT) provides PC address translation, DUAL PORT RAM control logic, interrupt control logic, and keyboard emulation. Concerning address translation, ABT translates co-processor system memory and I/O addresses into appropriate locations in the memory map of DUAL PORT RAM as shown in Fig.8. Any I/O request recognized by the ABT as being I/O device emulated by the host computer system triggers the translation function. For example, if host computer system on the parallel port emulation and co-processor system attempt to write to the printer data port, ABT will generate the address for the printer data port location in DUAL PORT RAM. ABT also maps various co-processors system memory requests into DUAL PORT RAM, for example graphical data output. ABT also is supposed to prevent conflicts due to different clock frequencies of host computer system and co-processor system.
In the following explanation to further assist the understanding of the interface, the method of communicating a monochrome or color video display from co-processor system to a video monitor on the Amiga host system is discussed. In this example, the Microsoft "Flight Simulator" program is running on the co-processor computer system which is to be displayed in the video monitor on the host system. This program was not chosen arbitrarily, but was used as a "scale" for the system performance during the development of the communication software Janus by R.J. Mical.
When the "Flight Simulator" program outputs data to be read on the host video monitor, the co-processor system using its BIOS screen I/O routines and writes the information into the monochrome video memory or into color video memory of DUAL PORT RAM. This is provided by the operating system extension (PC-Janus). Before, the co-processor system has therefore requested the desired data transfer, which will be acknowledged by the host system. This requesting is controlled by interrupts. After mapping the video data in an appropriate video RAM portion of DUAL PORT RAM the co-processor system informs the host computer system upon completion of this operation. Now the data can be further processed by the host computer system.
(Thomas Unger)