CI and segment formats

The format of DEDB control intervals (CIs) and segments are shown in the following tables and figures.

This topic contains Diagnosis, Modification, and Tuning information.

The following series of diagrams show the following formats:

  • CI format
  • Root segment format
  • Sequential dependent segment format
  • Direct dependent segment format

The tables that follow each figure describe the sections of the CI and segments in the order that the sections appear in the graphic.

Figure 1. CI format
CI has 3 sections: CI prefix, segments and FSEs, and CI suffix. CI prefix contains the following: FSE AP, CI TYP, RAP. CI suffix contains the following: CUSN, RBA, RDF, CIDF.
Table 1. CI format
CI section Number of bytes Explanation
FSE AP 2 bytes Offset to the first free space element. These 2 bytes are unused if the CI is in the sequential dependent part.
CI TYP 2 bytes Describes the use of this CI and the meaning of the next 4 bytes.
RAP 4 bytes Root anchor point if this CI belongs to the base section of the root addressable area. All root segments randomizing to this CI are chained off this RAP in ascending key sequence. Only one RAP exists per CI.

Attention: In the dependent and independent overflow parts, these 4 bytes are used by Fast Path control information. No RAP exists in sequential dependent CIs.
CUSN 2 bytes CI Update Sequence Number (CUSN). A sequence number maintained in each CI. It is increased with each update of the particular CI during the synchronization process.
RBA 4 bytes Relative byte address of this CI.
RDF 3 bytes Record definition field (contains VSAM control information).
CIDF 4 bytes CI definition field (contains VSAM control information).
Figure 2. Root segment format (with sequential and direct dependent segments with subset pointers)
Root segment format has two sections: prefix and data. Prefix contains the following: SC, PD, PTF, SPCF, PCF, PCL, SSP. Data section contains the following: LL and user data.
Table 2. Root segment format
Segment section Number of bytes Explanation
SC 1 byte Segment code.
PD 1 byte Prefix descriptor.
PTF 4 bytes Physical twin forward pointer. Contains the RBA of the next root in key sequence.
SPCF 8 bytes Sequential physical child first pointer. Contains the cycle count and RBA of the last inserted sequential dependent under this root. This pointer will not exist if the sequential dependent segment is not defined.
PCF 4 bytes Physical child first pointer. PCF points to the first occurrence of a direct dependent segment type. There can be up to 126 PCF pointers or 125 PCF pointers if there is a sequential dependent segment. PCF pointers will not exist if direct dependent segments are not defined.
PCL 4 bytes Physical child last pointer. PCL is an optional pointer that points to the last physical child of a segment type. This pointer will not exist if direct dependent segments are not defined.
SSP 4 bytes Subset pointer. For each child type of the parent, up to eight optional subset pointers can exist.
LL 2 bytes Variable length of this segment.
Figure 3. Sequential dependent segment format
Sequential dependent segment has three parts: prefix, data and suffix: Prefix: SC, UN, SPTF. Data: LL and user data. Suffix: TIMESTAMP.
Table 3. Sequential dependent segment format
Segment section Number of bytes Explanation
SC 1 byte Segment code.
UN 1 byte Prefix descriptor.
SPTF 8 bytes Sequential physical twin forward pointer. Contains the cycle count and the RBA of the immediately preceding sequential dependent segment under the same root.
LL 2 bytes Variable length of this segment.
Figure 4. Direct dependent segment format
Direct dependent segment has two parts: prefix and data. Prefix contains the following: SC, UN, PTF, PCF, PCL, SSP. Data contains the following: LL and user data.
Table 4. Direct dependent segment format
Segment section Number of bytes Explanation
SC 1 byte Segment code.
UN 1 byte Unused.
PTF 4 bytes Physical twin forward pointer. Contains the RBA of the next occurrence of this direct dependent segment type.
PCF 4 bytes Physical child first pointer. PCF points to the first occurrence of a direct dependent segment type. In a direct dependent segment there can be up to 125 PCF pointers or 124 PCF pointers if there is a sequential dependent segment. PCF pointers will not exist if direct dependent segments are not defined.
PCL 4 bytes Physical child last pointer. PCL is an optional pointer that points to the last physical child of a segment type. This pointer will not exist if direct dependent segments are not defined.
SSP 4 bytes Subset pointer. For each child type of the parent, up to eight optional subset pointers can exist.
LL 2 bytes Variable length of this segment.