MEMORY MANAGEMENT

Data and programs are stored, usually in binary form, in a memory subsystem

On early computers, memory subsystem was a single main memory

MEMORY MANAGEMENT

Dynamic real memory management OS/360

magnetic tape was the principal medium for secondary storage, but its sequential nature limited its possibilities for memory management

the arrival of disk storage as an economical second-level storage with good random access capabilities was the catalyst for a new approach to memory management.

MEMORY MANAGEMENT

Dynamic real memory management OS/360 taking advantage of random access disk storage,

OS360 provide execution-time binding(In OS360 a set of services provide the link between the application program and the program modules on direct access storage)

For example, during execution, a program can request the appropriate subroutine; only then will it be brought from secondary storage. This defers main memory occupancy until it is actually needed and also allows the selection of the subroutine to be based on the data at hand.

MEMORY MANAGEMENT

Dynamic real memory management OS/360

execution-time binding is important because: this capability offers the potential of reducing the

maximum amount of storage required by a program during its execution without the preplanning becoming hopelessly complex

the operating system itself can use it to great advantage

MEMORY MANAGEMENT

Overview of Memory Management Techniques

Component # Tasks Memory Management Comment

PCP(Primary Control Program),

1 1 partition Batch-processing model – One jobhad all available memory untilcomplete

MFT(Multiprocessing with a Fixed number of Tasks.)

4 to 15 Fixed partitions Number of partitions = number oftasks

MVT(Multiprogramming with a Variable (limitless) number of Tasks)

unlimited Variablepartitions

Created high degree of externalfragmentation

MEMORY MANAGEMENT

Memory Structure and Implementation 3 techniques used:

Planned overlay structure Dynamic serial structure Dynamic parallel structure

MEMORY MANAGEMENT

Memory Structure and Implementation Planned overlay structure - was utilized when not

all of the elements of a program needed to be actively loaded on the system simultaneously

- the programmer was able to segment the program into load modules that need to be present simultaneously in main memory

MEMORY MANAGEMENT

Memory Structure and Implementation Dynamic serial structure - was useful when jobs

became more complex - load modules can

be called dynamically

MEMORY MANAGEMENT

Memory Structure and Implementation Dynamic parallel structure - It creates a task

that can proceed in parallel with other tasks, but since it uses the ATTACH and thus requires the processor to go into kernel, or supervisor, mode, its use needs to be limited.