Memory System Design II - Fundamentals of Computer Organization and Architecture - page 154

Segment Number Offset

Segment Table

Displacement

Σ

Physical Segment Base Address Σ

Physical Address

Figure 7.18 Segment address translation

physical segment base address. Adding the latter to the offset yields the required
physical address. Figure 7.18 illustrates the segment address translation process.
Possible additional information included in the segment table includes:

1. Segment length
2. Memory protection (read-only, execute-only, system-only, and so on)
3. Replacement algorithm (similar to those used in the paged systems)
4. Placement algorithm (finding a suitable place in the main memory to hold the
incoming segment). Examples include
(a) First fit
(b) Best fit
(c) Worst fit

7.2.9. Paged Segmentation

Both segmentation and paging are combined in most systems. Each segment is
divided into a number of equal sized pages. The basic unit of transfer of data
between the main memory and the disk is the page, that is, at any given time, the
main memory may consist of pages from various segments. In this case, the virtual
address is divided into a segment number,apage number, and displacement within
the page. Address translation is the same as explained above except that the physical
segment base address obtained from the segment table is now added to the virtual
page number in order to obtain the appropriate entry in the page table. The output
of the page table is the page physical address, which when concatenated with
the word field of the virtual address results in the physical address. Figure 7.19
illustrates the paged segmentation address translation.