The questions

Question: Details

How can we build an efficient virtualization of memory?
How do we provide the flexibility needed by applications?
How do we maintain control over which memory locations an application can access?

General technique

Hardware-based address translation (address translation)
- HW transforms virtual address from memory access into physical address.
The OS gets involved to ensure correct translations take place and manage memory to keep track of and maintain control over free and used memory locations.

Initial assumptions

Details

Early attempt: dynamic relocation

 physical address = virtual address + base
0 <= virtual address <= bound
 

What happen after boot?

What happen during process run?

Summary: Details

Pros:
- Highly efficient through simple translation mechanisms
- Provides protection
Cons:
- Wastage through internal fragmentation due to space inside the allocated (contiguous) memory units are not fully utilized.

Initial assumptions

~~User’s address space must be place contiguously in physical memory.~~

~~The size of the address space is less than the size of physical memory.~~

~~Each address space is exactly the same size.~~

How do we support a large address space with (potentially) a lot of free space between the stack and the heap?

Segmentation: generalized base/bounds

Original: One base/bound pair for one address space.
Segmentation: One base/bound pair per logical segment of an address space:
- Code
- Stack
- Heap

Example: Details

Segment	Virtual segment base	Virtual segment bound	Physical segment base	Physical segment bound	Size
Code	0KB	2KB	32KB	34KB	2KB
Heap	4KB	6KB (can grow up)	34KB	36KB	2KB
Stack	16KB	14KB (can grow down)	28KB	26KB	2KB

Reference is made to virtual address 100 (code segment)
- This is called the offset
- physical address = physical segment base + offset = 32K (32768) + 100 = 32868
Reference is made to virtual address 4200 (heap segment)
- What is the correct offset:
- offset = virtual address - virtual segment base = 4200 - 4096 = 104
- physical address = physical segment base + offset = 34K (34816) + 104 = 34920
Reference is made to virtual address 7000?
- Look likes heap segment
- Is it heap segment?
- offset = virtual address - virtual segment base = 7000 - 4096 = 2904
- physical address = physical segment base + offset = 34K (34816) + 2904 = 37720
- This falls into physical addresses that are markedas not in use.
- Segmentation Fault: AKA segmentation violation
- Illegal virtual address

Summary: Details

Pros:
- Efficient saving of physical memory (avoid internal fragmentation)
- Enable the creation of segments with various sizes
Cons:
- External fragmentation

Memory virtualization mechanism: address translation

The questions