highc.txt

一个C语言写的读入位置跟踪器数据的源程序

Metaware High C CBIRD notes 

The High C compiler is a protected mode compiler which is layered on top
of a DOS Extender.  The CBIRD.exe software has be designed to be compatible
with the Pharlap DOS 386 Extender.  

Notes:
1)      In order for a HIGHC interrupt routine to be functional, an
        intermediate ASSEMBLY routine (located HCINT.asm) has to be 
        used.  The reason for this is that it is possible for an
        interrupt to occur while executing a DOS command.  The Phar Lap 
        Extender switches from Protected to Real mode when executing a 
        DOS call, but DOS switches the DS (data segment) to something other 
        then what was being used by the CBIRD protected mode application.
        Therefore, when an interrupt occurs, the assembly routine must
        set the DS to the application's DS and proceed to call the
        interrupt service routine. Note that the interrupt service routine 
        is really just a function called by the assembly interrupt service 
        routine.

2)      Since the DOS handles IRQ0, the PC timer interrupt, there would
        be a large overhead to handle an IRQ0 Tick, which occurs 18.2
        times per second.  CBIRD circumvents this from occurring, by
        taking over the IRQ0 interrupt and handling the interrupt in
        protected mode. When data collection has been completed, CBIRD
        restores DOS time via multiple call (software interrupts) to 
        the DOS IRQ0 interrupt.

3)      Phar Lap handles normally intercepts all interrupts from vector
        8 through F, which correspond to IRQ0 through IRQ7 given the
        default programmable interrupt controller configuration.  Phar Lap
        does this because vectors 8 through F overlap CPU exceptions when
        in protected mode.  The Extender inserts an 'Umbrella' routine
        into all vectors 8 through F and when an interrupt or exception
        occurs, it determines the proper course of action. In the case
        of CBIRD, we write the Interrupt Descriptor Table (IDT) directly
        to remove the overhead of the 'Umbrella' routine on each serial
        character received.