6809/6811/6812 SWI/TRAP system calls
                   ====================================

SWI1  Tube/BBC error                    SBUG breakpoint
SWI2  OS/9 system call
SWI3  Flex system call
TRAP  &F0-&FF: Tube/BBC API system call

BBC Tube API
============
On entry:
  SWI:DEFB errnum:DEFS "error string":DEFB 0
On exit:
  Does not return

6812 Tube API
=============
SWI1 - generate error
~~~~~~~~~~~~~~~~~~~~~
On entry:
  SWI:DEFB errnum:DEFS "error string":DEFB 0
On exit:
  Does not return

TRAP - system call
~~~~~~~~~~~~~~~~~~
On entry:
  TRAP &F0-&FF: OSWRINF, OSRDINF, OSSYS, OSMISC, OSFIND, OSGBPB,
                OSBPUT, OSBGET, OSARGS, OSFILE, OSRDCH, OSWRCH,
                OSWORD, OSBYTE, OSCLI, OSQUIT
  TRAP &00-&EF: Passed to TRAPV

OS/9 system call
================
On entry:
  SWI2:DEFB callnum
On exit:
  CC=ok
  CS=error, B=error number

SBUG breakpoint
===============
On entry:
  SWI1 - Breakpoint

Flex system call
================
On entry:
  SWI3 - used by Flex