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Serial DOS
program KeyboardDemo;
uses
Dos, Crt;
const
EscapeKey = #27;
var
OldKeybrdVector: Procedure;
{$F+}
procedure Keyclick; interrupt;
begin
if Port[$60] < $80 then begin
Sound(5000);
Delay(1);
Nosound;
end;
inline($9C) { PUSHF - Push the flags onto the stack }
OldKeybrdVector;
end;
{$F-}
begin
GetIntVec($9,@OldKeybrdVector);
SetIntVec($9,Addr(Keyclick));
repeat
if KeyPressed then begin
OutputLetter := ReadKey;
Write(OutputLetter);
end; {if}
until OutputLetter = EscapeKey;
SetIntVec($9,@OldKeybrdVector);
end.
There are a number of things that this program does, and we need to explore the realm of
16-bit DOS software as well. The 8086 chip designers had to make quite a few compromises
in order to work with the computer technology that was available at the time it was designed.
Computer memory was quite expensive compared to the overall cost of the computer. Most
of the early microcomputers that the IBM-PC was competing against only had 64K or 128K
of main CPU RAM anyway, so huge programs were not considered important. In fact, the
original IBM-PC was designed to operate on only 128K of RAM although it did become
standard with generally up to 640K of main RAM, especially by the time the IBM PC-XT
was released and the market for PC "clones" turned out what is generally considered the
"standard PC" computer today.
The design came up with what is called segmented memory, where the CPU address is
made up of a memory "segment" pointer and a 64K block of memory. That is why some
early software on these computers could only run in 64K of memory, and created nightmares
for compiler authors on the 8086. Pentium computers don't generally have this issue, as the
memory model in "protected mode" doesn't use this segmented design methodology.
4.6.1 Far Procedure Calls
{$F+}
{$F-}
This program has two "compiler switches" that inform the compiler of the need to use what
are called far procedure calls. Normally for small programs and simple subroutines, you are
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