This looks like an error in the C_dispatch_irq_handler function from rtems-19990709/c/src/lib/libbsp/powerpc/mcp750/irq/irq.c, lines 357 and 392: outport_byte(PIC_SLAVE_IMR_IO_PORT, ((i8259s_cache & 0xff00) > 8)); Shouldn't ">" be ">>" ?