Craig:
Your ideas I think have hit the nail on the head. I went to look more carefully at the serial I/O interrupt routines. Indeed the buffers are messed with by loading DPH and DPL separately. I had not noticed this before!! As a matter of fact the interrupt routines are looking like assembly language written in C syntax!! It looks to me that the original programmer was unfamiliar with how to do a serial interrupt into a circular queue and had found some sample assembler code that did it....and so copied it into C code. Mind you it is C code...not inline 'asm' code. It looks pathetic to see the interrupt routine with....

DPH = buffer page
DPL = buffer offset
ACC = SBUF0 //get uart
<some kind of macro that emits 0xF0> //harc code a movx @dptr,a
DPL++
buffer offset = DPL

man oh man.......I guess I need to re-write about 25 of those source files.

Michael Karas