C!T ROM 2/ctrom_ii_b.zip/ctrom_ii_b/PROGRAM/PASCAL/PASTUT34/COMPAT.TXT

Text File | 1993-06-12 | 8KB | 153 lines

                    MACHINE COMPATIBILITY.
                    ▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀

Incompatibilities between microcomputers occur for the following reasons

1. Different microprocessors - Intel 8088, 8086, 80286, 80386, 80486
                             - Zilog Z80, Z80000
                             - MOS Technology 6502
                             - Motorola 6800, 68000, 68020

2. Different operating systems - MS-DOS, Apple-DOS, CP/M, UCSD p system
                               - OS/2, Macintosh Finder, UNIX (XENIX)

3. Different architecture - IBM PC/AT bus, Micro Channel Architecture
                          - Extended Industry Standard Architecture.

4. Different diskette drives - PC & XT 5.25" 360 KB
                             - PC/AT   5.25" 1.2 MB
                             - PS/2 30  3.5" 720 KB
                             - 50/60/80 3.5" 1.44 MB

5. Different display screens - CGA graph 320 x 200 pixels x 16 colours.
                             - Hercules mono graphics 720 x 348 pixels.
                             - EGA graph 640 x 350 pixels x 16 colours.
                             - VGA graph 640 x 480 pixels x 16 colours.

Fortunately most of these potential incompatibilities can be overcome,
but only at extra expense.  It is therefore important to understand the
implications of all these differences.

1. Microprocessors.
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The microprocessors from the various manufacturers originated in two
different ways.  The Intel and Zilog microprocessors are
register-orientated and suitable for control purposes, whilst the MOS
Technology and Motorola ranges are memory-orientated and more suitable
for data processing.  It follows that they have different machine code
and therefore different assembly language instruction sets.

Within each group, the ascending numbers indicate wider data and address
paths to the peripheral devices.  The Intel 8088 processor has 8-bit
paths, the 80286 has 16-bit paths, whilst the 80386 provides 32-bit
paths.  The 80386 machines are usually provided with AT-compatible
expansion bus sockets, which accept circuit boards or adapter cards
intended for the 16-bit AT (80286) or 8-bit PC (8088), so permitting the
continued use of existing peripherals.

2. Operating Systems.
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The different operating systems imply incompatible disk formats, so that
generally a disk created on an Apple machine cannot be read by a IBM PC,
etc.  There are ways to overcome this problem and the latest Apple
Macintosh machines are able to read IBM 3.5" diskettes, whilst the
installation of a Copy II PC Deluxe Option Board allows the PC to write
Mac-formatted disks (3.5") at a cost approaching £100.

It is also possible to connect two computers by suitable cabling between
the serial ports (COM1, COM2) and to transfer files usually in ASCII
text format using suitable controlling software.  The cable connection
is such that the transmit and receive data lines (pins 2 and 3) are
cross-connected and not all of the remaining 25-way D-type plug
connection are made.  The controlling software includes the shareware
product 'Kermit' and commercial software such as Brooklyn Bridge
(~£100), Lap-Link Plus (~£80), both of which include the cable in the
price.  Further details are available in PC Magazine, Volume 8, Number 8
dated 25 April 1989.

It is also possible to use hardware and software dedicated to moving
data files between diskettes with different data formats.  The Galaxy
system supplied by Gemini Microcomputers Ltd., 18 Woodside Road,
Amersham, Bucks, HP7 0BH is an example.  To avoid the capital cost of
this equipment for only a limited amount of file transfer, it is possible
to use the service provided by the University of Warwick, Computing
Services.  Not all formats are catered for by this system, however.


3. Architecture.
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Fortunately the PC/AT architecture is well established and like MS-DOS
is likely to be used for most microcomputers for some time to come.
However, when IBM engineers were involved with the design of a new
architecture to accommodate multitasking operations and the installation
of large numbers of adapter cards in a single machine, they devised
Micro Channel Architecture (MCA).  When this was announced in April
1987, several manufacturers joined together to offer an alternative
called the Extended Industry Standard Architecture (EISA).  This latter
architecture was designed to provide many of the features of MCA, whilst
retaining backwards compatibility with the PC and AT architectures.
MCA machines are very good as communication servers, but even though
they are recommended for this purpose, they only accounted for 18% of
business-micro sales in the UK in the first quarter of 1989. Clearly
care must be taken if contemplating the purchase of MCA machines to
ensure that any existing peripheral equipment can be used by the new
system and that existing software can easily be adapted.


4. Diskette Drives.
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At the present moment there are two principal sizes of diskette - 5.25"
and 3.5".  This causes obvious incompatibilities, unless the computer
has dual mixed drives, or an external diskette drive is provided, at
extra cost.  Since much existing software is stored on 5.25" diskettes
and newer machines generally have 3.5" drives, there may be problems.
Furthermore, even within a given size, there are different storage
densities.  Currently, single density storage is not much used, but the
conventional double density diskette is being replaced by high density
(or high capacity) storage.  It is probably wise to purchase the high
density disk drive (1.44 MB) for the 3.5" size, as it can reliably read
and write lower density disks (720K).  Unfortunately the downward
compatibility of the high density drive (1.2MB) for the 5.25" drive can
not be guaranteed.

Of course it is possible to connect two machines with different disk
drives and to effect a transfer from one to the other as indicated above
under the heading 'Operating Systems'.


5. Display Screens.
───────────────────

The data given on the previous page indicates the most common graphics
mode for each graphics adapter. There are of course others, such as CGA
640 x 200 in 2 colours and Extended or Super VGA modes. Furthermore
there are different text modes 80 columns x 25 rows being most common,
but 40 x 25 generally available. With extended VGA other column and row
combinations are available such as 132 x 43.

Good applications software should be written to allow for these
different display modes and Turbo Pascal provides the necessary
procedures for checking the hardware and determining which graphics
driver and mode to use. Some software however is either display
specific or else requires the user to indicate which graphics driver is
installed. The Video Graphics Array (VGA) provides for downward
compatibility to the other graphics modes, if necessary.


      ─────────────────────────────────────────────────────────


This brief discussion of machine compatibility should ensure that some
of the problems are anticipated, but it is by no means a complete
analysis of all problems. It is therefore essential to obtain a written
assurance from a hardware vendor that the duties required of any
hardware will be met and that there will be no incompatibilities.


COMPAT.TXT
1.3.90