Object records that follow the SEGDEF record can refer to it to identify a particular segment. The SEGDEF records are ordered by occurrence and are referenced by segment indexes (origin 1) in subsequent records.

   1 byte  2 bytes
  ÚÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂ---
  ³  99   ³ Record         ³
  ³       ³  Length        ³
  ÀÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁ---

          <variable>   4 bytes      1 or 2   1 or 2   1 or 2  1 byte
       --ÚÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄ¿
         ³ Segment    ³ Segment    ³ Segment³ Class  ³Overlay³Chk Sum³
         ³ Attribute  ³ Length     ³ Name   ³ Name   ³Name   ³or 0   ³
         ³            ³            ³ Index  ³ Index  ³Index  ³       ³
       --ÀÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÙ

The segment attribute is bit-significant; the layout is:

    <-3 bits-> <-3 bits->  <-1 bit ->  <-1 bit ->   2 bytes    1 byte
   ÚÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄ¿
   ³ A        ³ C         ³ B         ³ P         ³ Frame     ³ Offset    ³
   ³          ³           ³           ³           ³   Number  ³           ³
   ÀÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÙ
                                                    <cond>      <cond>

The fields have the following meaning:

A

alignment, a 3-bit field, which specifies the alignment required when this program segment is placed within a logical segment. Values are:

absolute segment

relocatable, byte aligned

relocatable, word (2 byte, 16-bit) aligned

relocatable, paragraph (16 byte) aligned

relocatable, aligned on page (4K byte) boundary.

relocatable, aligned on double word (4 byte) boundary

not supported

not defined

combination, a 3-bit field, which determines the way the program segment is mapped into a logical segment. Values are:

private, do not combine with any other program segment

reserved

public, combine by appending at an offset which meets the alignment requirement

reserved

same as C=2 (public)

stack, combine as for C=2.

common, combine by overlay using maximum size

same as C=2 (public)

big, used as the high order bit of the segment length field. If this bit is set the segment length value must be zero and the segment's size is 2ª32 or 4Gbytes long.

Holds the descriptor table B/D bit value (this is the descriptor table D bit for code segments and the B bit for data segments).

If zero, then segment is no larger than 64K (if data) and 16-bit addressing and operands are the default (if code). This is a Use16 segment.

if not zero, this is a 4Gbyte segment Use32 segment.

Note that this is the only method for defining Use32 segments.

Segment Length

The Segment Length is a 4 byte numeric quantity and specifies the number of bytes in this program segment.
NOTE: If segment is exactly 4Gbytes in size, segment length should be set to zero and the B field in the ACBP byte should be set to 1.
Complete Segment Name

The three name indices refer to names that appeared in previous LNAMES record(s). The linker ignores the overlay name index. The full name of a segment consists of the segment and class names. Segments in different object modules are normally combined according to the A and C values if their full names are identical. These indices must be nonzero, although the name itself may be null.

The segment name index identifies the segment with a name. the name need not be unique -- other segments of the same name will be concatenated onto the first segment with that name. The name may have been assigned by the programmer, or it may have been generated by the compiler.

The class name index identifies the segment with a class name (such as CODE, DATA or STACK). The linker places segments with the same class name into a contiguous area of memory in the run-time memory map.

The overlay index is ignored by the linker.
notes:

The linker imposes a limit of 255 SEGDEF records per object module.

The following name/class combinations are reserved: