homework questions from stallings book

a.why is there so man开发者_开发百科y wait states in the in the vms/vax process states ?

All of the waits except one have to do with memory swapping or thread swapping.

The VAX architecture had virtual addressing. A program could access up to 1 gigabyte of address space, which was huge in 1977. If I remember correctly, 32 or 64 megabytes of memory was the standard. This meant that programs could access more memory than the machine actually had. VAX managed this virtual memory by paging memory to and from a disk drive.

Multiple users could use the VAX. This was accomplished with multiple user threads. Since the processor could only execute one instruction at a time, only one thread could be active at a time. Generally, a thread would run until an I/O instruction was encountered. The thread would be swapped out, and other threads allowed to execute, while the I/O instruction completed.

If you want to really feel what it was like back in the olden days, read Tracy Kidder's "Soul of a New Machine". It's the story of the team that developed the Data General Eclipse MV/8000.

Because each one of them has its own purpose...