ABAQ
ABAQ
Written by Perihelion, Ltd.
Hardware Specification
The base machine outline specification is as follows:
- T800-20 Transputer 10MIPS, 1.5 Mflop
- Three 20Mhz links, buffered
- 4Mbyte DRAM
- 1 Mbyte dual-port video RAM
- Colour blitter
- True DMA SCSI port for 40M (minimum) hard disc
- Three internal expansion slots
- 68000 Mega ST as I/O processor (plug in card connects fourth 20Mhz link)
Screen Resolution and Use
The table below lists the screen resolutions and their probable typical use. All the following are at 60Hz with portrait orientation.
Mode Resolution Width Description
---- ---------- ----- -----------
0 1280 x 960 4 bits/pixel 4 bits/colour or monochrome
(Desk Top Publishing, engineering
drawings)
1 1024 x 768 8 bits/pixel 8 bits/colour
(CAD, colour pictures, graphs)
2 640 x 480 8 bits/pixel 8 bits/colour 2 screens
(Animation)
3 512 x 480 32 bits/pixel 24 bits colour, 1 overlay bit, 7 tag
bits (True colour, smooth shading,
3D modelling)
The Blitter
The Perihelion blitter is based on work done by Dr Phil Willis of the University of Bath. It provides meaningful operations with colour and colour look-up tables (CLUTs) and implements very fast 2-D raster graphics operations, such as fast font drawing. It also provides a 32-bit wide pipeline (with four tests on each of eight pixels concurrently), and is synchronised with blanking. Using the blitter, square area fill takes 128 megapixels per second, arbitrary two colour character drawing takes up to 64 megapixels per second, and full 2-D block copy takes 16 megapixels per second.
Expansion Capability
The Perihelion design provides for three expansion cards within the box. These can be memory cards, providing a maximum of 64Mbytes using 4M parts, or various versions of alternative graphics cards. The full transputer bus is brought out so any type of peripheral may be connected.
The expansion sockets also bring out the transputer links and control signals. This means that cards containing extra transputers can be added, and the size of the cards allows for four transputers with up to 1Mbyte of RAM each on a single card. One workstation can therefore contain 13 processors. Other link connections can be made outside the box to parallel processor farms of multiple processors. The link connections can also be made to fast peripherals such as a laser printer or disc server.
The Transputer
The T414 is a 32-bit processor that consists of a RISC style CPU, 2K of fast on-chip RAM, an external memory interface and four serial links which may run at 5, 10 or 20 Mbits/second. The T800 is similar except that it also contains a floating point processor and 4K of RAM.
The programmer's model consists of a three register evaluation stack, a workspace pointer and an instruction pointer. A small number of instructions exist for loading and storing values on the stack and for altering the flow of control, the remainder operate on operands on the stack.
The processor has microcoded support for processes at 2 priority levels.
High priority processes may preempt low priority processes after any instruction and run until they give up the processor. High priority processes are essentially equivalent to interrupt routines on conventional processors. Low priority processes are round-robin scheduled on a timesliced basis. Timeslicing only occurs on particular instructions which are defined so that the minimum of state need be saved; process switching is therefore very fast.
The transputer achieves inter-process communication through channels, which are single words of memory. Two processes that wish to communicate rendezvous at a channel and exchange data by copying from one buffer to another. As this is implemented by the microcode, the cost of copying lies only in the memory accesses for the data and not in instruction fetches. Communication is strictly one-to-one and channels may not be shared by more than one sender or receiver. The inter-processor links are designed to behave exactly like channels, and are used with the same instructions.
Parallel Programming
The unique aspect of the Atari/Perihelion design is that is provides multiple processors within a single workstation. The use of multiple processors means that is is possible to write application programs which make use of the possible parallelism inherent in such systems.
Application programs can run under Helios using three programming philosophies. The first of these is the traditional programming model. A program can be taken from another environment, such as Unix or a PC, and with little or no change converted to run under Helios. C and the Unix C library is provided, and such programs will run as a single process in the machine.
Other programs, again probably from Unix, will run in several sections all of which may be run in different processes and connected by pipes.
Helios encourages the use of many small programs which work together to create a final product. A common example is a pre-processor, a compiler front end, a compiler back end, an assembler and a linker. These can all be run together with intermediate connections made by pipes.
Under other operating systems the different processes are timesliced on the one single processor. Under Helios these different processes can be allocated to different processors, so that the individual parts actually run at the same time.
This type of "per-process" parallelism is easily understood, and many applications are already in this form. Examples include a word processor with background spooling and spelling checking or background jobs such as message systems or archiving. If an application is being altered then the use of extra processes should be kept in mind.
The final way in which parallelism may be exploited is by the use of parallel algorithms. These tend to be hard to find for programmers used to the sequential nature of normal computers, but a look at the real world shows, of course, everything running in parallel.
Applications using parallel algorithms will normally be written from scratch with such ideas in mind. The benefit is that such programs will run much faster when the user provides more power in the form of more processors. Many examples of parallel algorithms exist, such as ray tracing, spreadsheet calculations, even computer chess!
Helios Overview
Helios is a true distributed operating system; there are no central services upon which the whole system relies. This results in increased system reliability since the failure of any processor, or the partitioning of the network, will not cause unrelated parts of the system to fail (although they may continue at a somewhat reduced capacity). The distributed nature of Helios is transparent both to the user at his terminal and to programs running within it which need never be aware of the exact location of any services. This feature differentiates it from a network operating system where the distributed nature is more explicit.
Helios is intended to be an open system architecture in which parts may be added, removed, modified or replaced transparently to suit specific purposes. In many ways Helios is simply a set of conventions, or codes of practice, for the behaviour of programs. It may be thought of as a "software backplane" providing an infrastructure for processes to locate and communicate with each other.
Finally, the emphasis throughout the development of Helios has been on finding practical solutions to the problems of distributed computing.
For this reason many of its features are not new but have been derived from existing research systems. The two most important influences have been the Cambridge Distributed Operating System and another system called Amoeba.
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