Executing software interrupts

Simply execute a realintr() call with the desired interrupt number and the supplied register data structure. But some of these interrupts require you to supply them a pointer to a buffer where they can store data to or obtain data from in memory. These interrupts are real mode functions and so they only can access the first Mb of linear address space, not FPC's data segment. For this reason FPC supplies a pre-initialized dos memory location within the GO32 unit. This buffer is internally used for dos functions too and so it's contents may change when calling other procedures. It's size can be obtained with tb_size and it's linear address via transfer_buffer. Another way is to allocate a completely new dos memory area via the global_dos_alloc function for your use and supply its real mode address.

See also

Example

{ Executes a real mode software interrupt

Exactly the interrupt call to get the DOS version.

get DOS version Int 21h / function 30h
Input:
        AH = $30
        AL = $1
Return:
        AL = major version number
        AH = minor version number
}

uses
        go32;

var
        r : trealregs;

begin
        r.ah := $30;
        r.al := $01;
        realintr($21, r);
        Writeln('DOS v', r.al,'.',r.ah, ' detected');
end.

Example

{ This example shows the difference between protected and real mode
interrupts; it redirects the protected mode handler to an own handler
which returns an impossible function result and calls it afterwards.
Then the real mode handler is called directly, to show the difference
between the two.

Used Interrupt:
get DOS version Int 21h / function 30h
     Input: AH = $30
            AL = $1
     Return: AL = major version number
             AH = minor version number
}

uses
        crt,
        go32;

var
        r : trealregs;
        { temporary variable used for the protected mode int call }
        axreg : Word;

        oldint21h : tseginfo;
        newint21h : tseginfo;

{ this is our int 21h protected mode interupt handler. It catches
the function call to get the DOS version, all other int 21h calls
are redirected to the old handler; it is written in assembly
because the old handler can't be called with pascal }
procedure int21h_handler; assembler;
asm
        cmpw $0x3001, %ax
        jne .LCallOld
        movw $0x3112, %ax
        iret

.LCallOld:
        ljmp %cs:oldint21h
end;

{ a small helper procedure, which waits for a keypress }
procedure resume;
begin
        Writeln;
        Write('-- press any key to resume --'); readkey;
        gotoxy(1, wherey); clreol;
end;

begin
        { see the text messages for further detail }
        clrscr;
        Writeln('Executing real mode interrupt');
        resume;
        r.ah := $30; r.al := $01;  realintr($21, r);
        Writeln('DOS v', r.al,'.',r.ah, ' detected');
        resume;
        Writeln('Executing protected mode interrupt without our own',
                ' handler');
        Writeln;
        asm
                movb $0x30, %ah
                movb $0x01, %al
                int $0x21
                movw %ax, axreg
        end;
        Writeln('DOS v', r.al,'.',r.ah, ' detected');
        resume;
        Writeln('As you can see the DPMI hosts default protected mode',
                'handler');
        Writeln('simply redirects it to the real mode handler');
        resume;
        Writeln('Now exchanging the protected mode interrupt with our ',
                'own handler');
        resume;

        newint21h.offset := @int21h_handler;
        newint21h.segment := get_cs;
        get_pm_interrupt($21, oldint21h);
        set_pm_interrupt($21, newint21h);

        Writeln('Executing real mode interrupt again');
        resume;
        r.ah := $30; r.al := $01; realintr($21, r);
        Writeln('DOS v', r.al,'.',r.ah, ' detected');
        Writeln;
        Writeln('See, it didn''t change in any way.');
        resume;
        Writeln('Now calling protected mode interrupt');
        resume;
        asm
                movb $0x30, %ah
                movb $0x01, %al
                int $0x21
                movw %ax, axreg
        end;
        Writeln('DOS v', lo(axreg),'.',hi(axreg), ' detected');
        Writeln;
        Writeln('Now you can see that there''s a distinction between ',
                'the two ways of calling interrupts...');
        set_pm_interrupt($21, oldint21h);
end.