Operating Systems

Start Lecture #17

Remark: Lab 3 (banker) assigned. It is due in 2 weeks.

Midterm Exam Answers Reviewed:

Running Multiple Programs Without a Memory Abstraction

This can be done via swapping if you have only one program loaded at a time. A more general version of swapping is discussed below.

One can also support a limited form of multiprogramming, similar to MFT (which is described next). In this limited version, the loader relocates all relative addresses, thus permitting multiple processes to coexist in physical memory the way your linker permitted multiple modules in a single process to coexist.

**Multiprogramming with Fixed Partitions

Two goals of multiprogramming are to improve CPU utilization, by overlapping CPU and I/O, and to permit short jobs to finish quickly.

• This scheme was used by IBM for system 360 OS/MFT (multiprogramming with a fixed number of tasks).
• An alternative would have a single input list instead of one queue for each partition.
  • With this alternative, if there are no big jobs, one can use the big partition for little jobs.
  • The single list is not a queue since would want to remove the first job for each partition.
  • I don't think IBM did this.
• You can think of the input lists(s) as the ready list(s) with a scheduling policy of FCFS in each partition.
• Each partition was monoprogrammed, the multiprogramming occurred across partitions.
• The partition boundaries are not movable (must reboot to move a job).
  • So the partitions are of fixed size.
  • MFT can have large internal fragmentation, i.e., wasted space inside a region of memory assigned to a process.
• Each process has a single segment (i.e., the virtual address space is contiguous). We will discuss segments later.
• The physical address space is also contiguous (i.e., the program is stored as one piece).
• No sharing of memory between process.
• No dynamic address translation.
• OS/MFT is an example of address translation during load time.
  • The system must establish addressability.
  • That is, the system must set a register to the location at which the process was loaded (the bottom of the partition). Actually this is done with a user-mode instruction so could be called execution time, but it is only done once at the very beginning.
  • This register (often called a base register by ibm) is part of the programmer visible register set. Soon we will meet base/limit registers, which, although related to the IBM base register above, have the important difference of being outside the programmer's control or view.
  • Also called relocation.
  • In addition, since the linker/assembler allow the use of addresses as data, the loader itself relocates these at load time.
• Storage keys are adequate for protection (the IBM method).
• An alternative protection method is base/limit registers, which are discussed below.
• An advantage of the base/limit scheme is that it is easier to move a job.
• But MFT didn't move jobs so this disadvantage of storage keys is moot.

3.2 A Memory Abstraction: Address Spaces

3.2.1 The Notion of an Address Space

Just as the process concept creates a kind of abstract CPU to run programs, the address space creates a kind of abstract memory for programs to live in.

This does for processes, what you so kindly did for modules in the linker lab: permit each to believe it has its own memory starting at address zero.

Base and Limit Registers

Base and limit registers are additional hardware, invisible to the programmer, that supports multiprogramming by automatically adding the base address (i.e., the value in the base register) to every relative address when that address is accessed at run time.

In addition the relative address is compared against the value in the limit register and if larger, the processes aborted since it has exceeded its memory bound. Compare this to your error checking in the linker lab.

The base and limit register are set by the OS when the job starts.

3.2.2 Swapping

Moving an entire processes between disk and memory is called swapping.