From: mutazilah@nospicedham.gmail.com   
      
   Hi.   
      
   As far as I can tell from testing, the BIOS INT 14H doesn't   
   work on most real hardware I have tried (computers made   
   maybe 10 years ago that still have a serial port).   
      
   So I am gearing up to replace that with 80386 PM32 code.   
      
   I have some comms routines for MSDOS (pdcomm) that I   
   wrote decades ago, but I want to do it a different way this time.   
      
   Less sophisticated, but more straightforward.   
      
   And designed for a single-tasking system with only one   
   CPU enabled (PDOS/386).   
      
   Most of the code can be done in C, but the last bit I want   
   to do in assembler.   
      
   Prior to hitting the assembler, I will have installed the   
   new interrupt address (which is assembler code, gotint,   
   below).   
      
   I'm just trying to confirm the sequence in the final assembler.   
      
   outb port1, transmit_byte   
   outb port2, tbemask ; enable transmit buffer empty (only)   
   xxx:   
   hlt ; this could get interrupted by timer interrupts and   
    then processing continues so we need a jmp   
   jmp xxx   
   gotint: ; this is the interrupt address installed by C caller   
   outb port2, oldmask ; restore previous interrupt mask (everything disabled)   
   add esp, 12 ; we don't return to the previous instruction, which was hlt   
    instead we skip over the return address, segment and flags   
      
   ret ; return to C caller which will do the EOI or whatever   
    via separate calls to individual simple assembler functions   
    like outportb()   
      
   Note that this was inspired by something similar I wrote   
   for S/370.   
      
   It's basically quite minimal assembler and straightforward.   
      
   There is a loop in the assembler, which I didn't have in my   
   old routines, but it's not really processing logic.   
      
   After transmit is working I'll try a variation of the above for receive.   
      
   I especially don't know if these two need to be swapped:   
      
   outb port1, transmit_byte   
   outb port2, tbemask ; enable transmit buffer empty (only)   
      
   I don't want to miss an interrupt. I want the order to   
   guarantee that I will get the interrupt. ie if I have already   
   attempted to write the transmit byte, will the interrupt   
   pend until I enable it, or will it be skipped?   
      
   Or is the other way around? If I haven't enabled interrupts   
   will it just transmit the byte and not bother interrupting?   
      
   Assume that the transmit is very fast, or the CPU is very   
   slow, so that there is a gap between the two outb   
   instructions where a decision is made on whether to   
   interrupt or not.   
      
   Thanks. Paul.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)