This directory contains a PIM SYSTEM containing the PROCESS primitives built on top of GNU Pthreads.
The justification for this approach is that it provides a SYSTEM compatible with Programming in Modula-2  and the Logitech 3.0 compiler. It also allows higher level executives to be ported onto GM2 with little effort. The disadvantage with this approach is that IOTRANSFER is not preemptive. IOTRANSFER will only context switch when a call to LISTEN is made or a call to SYSTEM.TurnInterrupts is made.
In practice this limitation can be tolerated as long as processes perform IO at some point (or wait for a timer interrupt) or call SYSTEM.TurnInterrupts. But nevertheless a LOOP END will starve all other processes. However the great advantage is that GNU Modula-2 can offer users the ability to use IOTRANSFER, TRANSFER, NEWPROCESS in user space, on a multi-user operating system and across a range of platforms.
The GNU Modula-2 SYSTEM works by utilizing the user context switching mechanism provided by GNU Pthreads. NEWPROCESS creates a new context, TRANSFER switches contexts. IOTRANSFER is more complex. There is a support module SysVec which provides pseudo interrupt vectors. These can be created from input/output file descriptors or timer events timeval. This vector is then passed to IOTRANSFER which keeps track of which file descriptors and timevals are active. When a call to TurnInterrupts or LISTEN is made the sub system calls pth_select and tests for any ready file descriptor or timeout. A ready file descriptor or timeout will ultimately cause the backwards TRANSFER inside IOTRANSFER to take effect.
See the gm2/examples/executive directory for an executive and timerhandler module which provide higher level process creation, synchronisation and interrupt handling routines. These libraries have been tested with the examples shown in gm2/examples/executive and gm2/gm2-libs-coroutines.
Users of these libraries and the libraries in gm2/examples/executive must link their application against the GNU Pthread library (typically by using -lpth).