Segmentation¶

• Segments are a logical unit such as
  • main program
  • function, object

Segmentation table¶

• Logical address (seg#, offset)

  • Offset has the same length as physical address

• Segmentation table, each entry has

  • Base : start of physical address
  • Limit : the length of the segment

• Segment-table bse register (STBR): the physical address of the segmentation table

• Segement-table length register (STLR): the number of segments

Segmentation hardware¶

• Limit register is used to check offset length
• MMU allocate memory by assigning an appropriate base address for each segment

Protection and Sharing¶

• Protection bits associated with segments

  • Readonly segment (code)
  • Read-write segments (data,heap,stack)

• Code sharing occurs at segment level

  • Shared memory communication
  • Shared library

Segmentation with Paging¶

• Apply segmentation in logical address space
• Apply paging in physical address space

Address Translation¶

Segmentation calculations¶

Linear address to physical address¶

Example

• Physical memory size: 512 B
• Page size: 32 B
• Logical address can have 8 segments

Given a 12 bits logical address 0x448, page and segment tables, translate it to address physical address

tables

Segment table	Page table

Solution

• Segment offset = \(\log_2(\text{#segments})=3\) bits
• Page number = \(\log_2(\text{#pages})=\log_2\left(\dfrac{2^9}{2^5}\right)=4\) bits
• 0x448 is 010001001000 in binary
• 010001001000 010 is segment offset, so we take 001110110 and add 001001000 to it
• Linear address 001110110+001001000=010111110.
• First \(4\) bits is page offset, so the page number is \(2\)
• Page offset is the remaining bits in linear address, i.e., 11110
• Physical address is {page number}{page offset}={0010}{11110}

IMPORTANT NOTES ON NAMING¶