DIRECT MEMORY AND PORT ACCESS AND COMMUNICATION.
         ฿฿฿฿฿฿฿฿฿฿฿฿฿฿฿฿฿฿฿฿฿฿฿฿฿฿฿฿฿฿฿฿฿฿฿฿฿฿฿฿฿฿฿฿฿฿฿฿

 Memory access.
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 Mem  {byte}        )
 MemW {word}        )   are used to directly access memory.
 MemL {longint}     )

 e.g.  Mem[$0040:$0049] := 7;

       stores the value 7 in the byte of memory at $0040:$0049.

       Data := MemW[Seg(V):Ofs(V)];

       moves the word value stored in the first two bytes of the
       variable V into the variable Data.

       MemLong := MemL[64:3*4];

       MemLong variable takes the value stored at $0040:$000C.


 Port access.
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 Port  {byte}    )   are one-dimensional arrays, each element
                 )   representing a data port, whose port address
 PortW {word}    )   corresponds to its index.

 When a value is assigned to a component of Port or PortW, the value is
 sent as output to the selected port.
 When a component of Port or PortW is referenced in an expression, its
 value is received as input from the selected port.

 e.g.  port[data_port_low] := low_order_byte;

       where data_port_low and low_order_byte have been previously evaluated.

       test := port[data_port_low + 2] div 128;


 Communication.
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 A Unit called 'AuxInOut' implements a text file device driver for the
 communication ports (serial) of an IBM PC. This unit is shown on pages
 213-5 of the Reference Guide (version 5.5) and itself uses procedures
 and functions which call short 'in-line' machine code. This unit also
 uses the DOS unit, which defines TestRec used by AuxInOut.

 The inline machine code procedures and functions use the four BIOS
 interrupts $14/$00, $14/$01, $14/$02 and $14/$03 (see pages 38-40 of 'DOS
 and BIOS Functions' (Que Corporation). 

 Interrupt $14/$00, Initialize Communications Port, requires that the AL
 register contains  a parameter evaluated as follows: 

      Bits 7,6,5        Bits 4,3        Bit2          Bits 1,0 
      Baud rate         Parity          Stop bits     Word length 
      ---------         -------         ----------    ------------ 
      000 = 110 baud    x0 = none       0 = 1 bit     10 = 7 bits 
      001 = 150         01 = odd        1 = 2 bits    11 = 8 bits 
      010 = 300         10 = none 
      011 = 600         11 = even 
      100 = 1200   
      101 = 2400 
      110 = 4800 
      111 = 9600 


 The following program uses the AuxInOut unit to write a string to one of
 the communication ports. Through the AssignAux procedure, the Com1 file is
 associated with the COM1 port, because the first parameter is COM1 and the
 second parameter is 0, which must be placed in the DX register to identify
 the port, COM1.                          

 In this case the following values apply: 1200 baud, no parity, 1 stop bit
 and 8 data bits. Thus the value in AL must be 100 00 0 11 or 1000 0011 or 
 $83

 program TestAux;

 uses AuxInOut;

 var
    Com1 : text;

 begin
    AssignAux(Com1,0,$83);
    Rewrite(Com1);
    Writeln(Com1,'Good morning. Any messages?');
    Close(Com1);
 end.


 where the procedure AssignAux(var F:Text;Port,Params:word); indicates that
 Text file = COM1, Port:=0 and Params:=$83 as evaluated above. 

 The unit AuxInOut is not mentioned in the manuals for version 6.0, but
 Borland now provide the Async coms routines on a Pascal Utilities diskette.
 The unit ASYNC4U has a function Async_Open, which has the following
 parameters:    (ComPort  : Integer; 
                 BaudRate : Integer; 
                 Parity   : Char; 
                 WordSize : Integer; 
                 StopBits : Integer;)             


 BLAISE Turbo Asynch Plus.
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 Blaise Computing Inc., 2560 Ninth Street, Suite 316, Berkeley, CA 94710
 produce Blaise Computing Power Tools Plus, which include Norton Guides,
 and also Blaise Turbo Asynch Plus, which provides a set of communication
 utilities.

 When communicating by means of the serial data ports, the data and the
 control information are transmitted over a single line, one bit at a time.
 With asynchronous communication, start and stop bits synchronise the
 transmission. This compares with synchronous transmission of data which is
 according to a definite time schedule.

 The Blaise utilities allow:  Open COM port, Close, Write, Read COM port,
 Set and Return COM port options ( baud, parity, etc ).

 The standard IBM COM port addresses are as follows:

    Port           Base address.
    ----           -------------

    COM1           $03F8
    COM2           $02F8

 These addresses can be verified by use of the DOS command DEVICE (with some
 versions of DOS). 



 COMPORT.TXT  
 31.1.90 
 10.10.92