DOIO Read System Call

Initial semaphore settings:

req_serviced := 0; req_pending := 0; device := 0; op_complete := 0;

USER DOIO Device Handler Device

DOIO(stream, mode, identify device; wait(req_pending); wait(device);

amount, destination, perform error checks; pick IORB from locate device

semaphore); assemble IORB; request queue; descriptor;

place IORB on device initiate I/O; satisfy read request;

. request queue; signal(device); signal(op_complete);

. signal(req_pending); wait(op_complete);

. exit; perform error checks;

wait(req_serviced); perform housekeeping;

test error location; signal(req_serviced);

delete IORB;

Operating System Boundary Hardware Boundary