Segmentation

Logical View of Segmentation

Normally, the user program is compiled, and the compiler automatically constructs segments reflecting the input program. A Pascal compiler might create separate segments for the following:

• the global variables;
• the procedure call stack, to store parameters and return addresses;
• the code portion of each procedure or function;
• the local variables of each procedure and function.

A Fortran compiler might create a separate segment for each common block. Arrays might be assigned separate segments. The loader would take all these segments and assign them segment numbers.

3.5.2 Segmentation Hardware

A segmented address space can be implemented by using address mapping hardware. Segment table is used for mapping the two dimensional user defined address into one dimensional physical address.

• Segment table – maps two-dimensional physical addresses; each table entry has:

• Base – contains the starting physical address where the segments reside in memory

• Limit – specifies the length of the segment

Segmentation hardware.

The use of a segment table is illustrated in Figure. A logical address consists of two parts: a segment number, s, and an offset into that segment, d. The segment number is used as an index into the segment table. The offset d of the logical address must be between 0 and the segment limit. If it is not, we trap to the operating system (logical addressing attempt beyond end of segment). If this offset is legal, it is added to the segment base to produce the address in physical memory of the desired byte. The segment table is thus essentially an array of base-limit register pairs.

• Segment-table base register (STBR) points to the segment table‘s locationin memory

• Segment-table length register (STLR) indicates number of segments usedby a program;

Segment number‗s‘ is legal, if s < STLR

Example of segmentation:

consider the situation shown in above figure. We have five segments numbered from 0 through 4. The segments are stored in physical memory as shown. The segment table has a separate entry for each segment, giving the beginning address of the segment in physical memory (or base) and the length of that segment (or limit). For example, segment 2 is 400 bytes long and begins at location 4300. Thus, a reference to byte 53 of segment 2 is mapped onto location 4300 + 53 = 4353. A reference to segment 3, byte 852, is mapped to 3200 (the base of segment 3) + 852 = 4052. A reference to byte 1222 of segment 0 would result in a trap to the operating system, as this segment is only 1,000 bytes long.

3.5.3 Protection and Sharing
Protection:

The advantage of segmentation is the association of protection with the segments. In a modern architecture, instructions are non-self-modifying, so instruction segments can be defined as read only or execute only. The memory mapping hardware will check the protection bits associated with each segment table entry to prevent illegal accesses to memory, such as attempts to write into a read-only segment, or to use an execute-only segment as data. By placing an array in its own segment, the memory-management hardware will automatically check that array indexes are legal and do not stray outside the array boundaries.

With each entry in segment table associate:
ü validation bit = 0                    illegal segment
ü validation bit = 1                      legal segment
ü read/write/execute privileges
Sharing:

o Another advantage of segmentation involves the sharing of code or data.

o Each process has a segment table associated with it, which the dispatcher uses to define the hardware segment table when this process is given the CPU.

o Segments are shared when entries in the segment tables of two different processes point to the same physical location.

For example, consider the use of a text editor in a time-sharing system. A complete editor might be quite large, composed of many segments. These segments can be shared among all users, limiting the physical memory needed to support editing tasks. Rather than n copies of the editor, we need only one copy.