From: mutazilah@nospicedham.gmail.com   
      
   On Wednesday, April 12, 2023 at 3:25:01 AM UTC+8, Terje Mathisen wrote:   
      
   > at this point I re-enable all other interrupts   
      
   Actually, that made me rethink - maybe it is more straightforward to:   
      
   Do this from C code, to signal that I am interested in TBE:   
      
   outb port2, tbemask ; enable transmit buffer empty (only)   
      
   Disable all interrupts (cli) from C code as well   
      
   Do this from C code:   
      
   outb port1, transmit_byte   
      
   And at this point, if the byte is transmitted very quickly,   
   the UART knows that it needs to do an interrupt, but   
   interrupts are temporarily disabled, so it knows that it   
   needs to queue the interrupt, not discard it.   
      
   And then I call this assembler:   
      
   xxx:   
   hlt ; this could get interrupted by timer interrupts and   
    then processing continues so we need a jmp   
   jmp xxx   
      
   And actually, even that can go into a function called halt_loop(),   
   called by C.   
      
   And then I can have this generic assembler routine:   
      
   gotint: ; this is the interrupt address installed by C caller   
   add esp, 12 ; we don't return to the previous instruction, which was hlt   
    instead we skip over the return address, segment and flags   
      
   Actually, no I can't. They need to be combined.   
      
   sti   
   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   
   add esp, 12 ; we don't return to the previous instruction, which was hlt   
    instead we skip over the return address, segment and flags   
   sti ; interrupts will have been disabled automatically. need to reenable   
   ret   
      
   Possibly take that sti out and do it from C, as C will have   
   done a call to disable interrupts, so this would be a good match.   
      
   And this is put back into C code:   
      
   outb port2, oldmask ; restore previous interrupt mask (everything disabled)   
      
   So down to just 5 assembler instructions.   
      
   That's probably neat. And yes, I know it is inefficient.   
      
   BFN. Paul.   
      
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