Question

The IBM System/370 architecture uses a two-level memory structure and refers to the two levels as segments and pages, although the segmentation approach lacks many of the features described earlier in this chapter. For the basic 370 architecture, the page size may be either 2 Kbytes or 4 Kbytes, and the segment size is fixed at either 64 Kbytes or 1 Mbyte. For the 370/XA and 370/ESA architectures, the page size is 4 Kbytes and the segment size is 1 Mbyte. Which advantages of segmentation does this scheme lack? What is the benefit of segmentation for the 370?

 

Answer 1

Initial consideration: Considering the architecture for the IBM System/370 that is capable of providing higher processing power and more storage capacity since it uses two level of memory structure which are known as the segments and caches and the given scheme is as follows:

 

• In segmentation the address space is divided into numbers of segments of variable sizes and on the other hand in paging the address space is divided into numbers of fixed sized units that is known as pages.

• It is more useful for better use of memory.

• The basic architecture for the IBM system/370 architecture the size of the page are generally either 2KiloBytes (KB) or 4KB and the size of the segments will be vary from 64KiloBytes to 1MegaBytes.

• The two different types of the architecture are existed such as 370/XA that is Extended Architecture and another is the 370/ESA known as the Enterprise Systems Architecture.

• The page size is 4KB for Extended version and 1MB for the Enterprise Systems Architecture (ESA).

 

Advantages of segmentation that given scheme lacks: 

• In the architectural configuration for IBM System/370 the segments are of fixed size which is not visible for the programmer and the programmer will be know the internal details and hence internal architecture is hidden from outside world.

• In IBM system/370 the reference bit is zero for the processor hardware and whenever a new page is loaded into the frame, the reference bit is changed to one upon referencing a particular location within the frame.

• The number of queues of the page frame table is managed by the Operating System and a page frame table entry is moved from one queue to another based on the length of duration of the referenced bit from the page frame which is sets to zero.

• Whenever there is a need to replace the page then the page chosen from the queue of the longest life non referenced frames.

 

Benefit of segmentation for the IBM system /370 Architecture: 

• The main benefit for using the segmentation for the IBM system architecture S/270 is the protection.

• Another benefit is the availability of the dynamic address translation facility that allows the control program to efficiently maintain and implement the group of functions which are collectively known as virtual storage.

The protection for the whole segment is achieved because the "P" bits which are present in the each segment table provide the protection and security for the entire segments.

• There is no other benefit of the segmentation which are listed for the segmentation and realized in the IBM S/370 model.

Initial consideration: Considering the architecture for the IBM System/370 that is capable of providing higher processing power and more storage capacity since it uses two level of memory structure which are known as the segments and caches and the given scheme is as follows: