CPU Memory Management
MemMan
Memory Management (Pentium).
Logical Address: segment (16bit): offset(32 bit)
Normally part of the code in the software module.
(Also known as the virtual memory space).
Linear Address:
from 0 to (232 –1): 4Giga Bytes
Flat address space of the Pentium CPU.
Physical Address: The actual RAM memory address implemented on the
system.
Example: System runs software program(s) requiring a total of 1GB of RAM
(logical address space) but only 256 MB (physical address space) has been
installed.
Memory Management – Translation between Logical and Physical Address.
Benefits:
Enables programs with large logical address space
requirement to run in a smaller physical memory space
(by storing some of the code on the disk and transferring
to RAM when required - Disk Swapping)
Pentium can run in either :
Real Mode: – i.e.8086 segmentation. Does not allow multi-tasking as there is
no protection for one process to interfere with another one.
Or
Protected Mode: - Each process runs in a separate code segment. Segments
can have different privilege levels preventing the lower privilege process (such
as an application) to run a higher privilege one (e.g. operating system).
Pentium (when running in Protected Mode) has two independent Memory
Management Facilities:
1. Segmentation
2. Paging
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CPU Memory Management
MemMan
SEGMENTATION
Pentium CPU has six 16 bit segment registers called SELECTORS
Logical Address =
SEGMENT + OFFSET
(in SELECTOR)
The information held in SELECTOR register is used by CPU to determine:
(i)
Start (Base) of Segment in the Linear Space
(ii)
Length of Segment
(iii)
Privilege Level of the Segment
CPU Selectors provide index (pointer) to Segment Descriptors stored in RAM
in the form of memory structures called Descriptor Table(s).
Determination of the Linear Address
There are two types of Descriptor Tables:
• Global Descriptor Table : for code belonging to Operating System
Segments Created by OS before CPU switched to
Protected Mode.
• Local Descriptor Table(s): for code belonging to running application(s).
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PAGING.
Paging is a mechanism which helps the Operating System to create VIRTUAL
(faked) address space by swapping code between disk and RAM. This
process is known as Virtual Memory Support.
Paging is an address translation from the Linear to the Physical Address.
The Linear Address Space is divided into fixed length (4kbyte) PAGES.
Similarly, the Physical Address Space is divided into the same fixed length
FRAMES. Within their respective address spaces pages and frames are
numbered sequentially. Paging mechanism assigns a frame number to a
given page number. The pages that have no frames assigned to them are
stored on the disk. When the CPU needs to run the code on any nonassigned page it generates a Page Fault Exception, upon which the Operating
system reassigns a currently non-used frame to that page and copies the
code from that page on the disk to the newly assigned RAM frame.
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CPU Memory Management
Page Translation Mechanism.
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MemMan
CPU Memory Management
MemMan
The page mapping process is achieved by Operating System storing two
types of Translation Tables in RAM:
•
•
Page Directory Table (base in CPU)
Page Table (for each running process)
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- of pointers to Page Tables
- of pointers to Memory Frames