Understanding and programming the toc
The TOC is used to find objects in an XCOFF file.
The Table of Contents (TOC) of an XCOFF file is analogous to the table of contents of a book. The TOC is used to find objects in an XCOFF file. An XCOFF file is composed of sections that contain different types of data to be used for specific purposes. Some sections can be further subdivided into subsections or csects. A csect is the smallest replaceable unit of an XCOFF file. At run time, the TOC can contain the csect locations (and the locations of labels inside of csects).
The three sections that contain csects are:
| Item | Description |
|---|---|
| .text | Indicates that this csect contains code or read-only data. |
| .data | Indicates that this csect contains read-write data. |
| .bss | Indicates that this csect contains uninitialized mapped data. |
The storage class of the csect determines the section in which the csect is grouped.
The TOC is located in the .data section of an XCOFF object file and is composed of TOC entries. Each TOC entry is a csect with storage-mapping class of TC or TD.
A TOC entry with TD storage-mapping class contains scalar data which can be directly accessed from the TOC. This permits some frequently used global symbols to be accessed directly from the TOC rather than indirectly through an address pointer csect contained within the TOC. To access scalar data in the TOC, two pieces of information are required:
- The location of the beginning of the TOC ( i.e. the TOC anchor).
- The offset from the TOC anchor to the specific TOC entry that contains the data.
A TOC entry with TC storage-mapping class contains the addresses of other csects or global symbols. Each entry can contain one or more addresses of csects or global symbols, but putting only one address in each TOC entry is recommended.
When a program is assembled, the csects are sorted such that the .text csects are written first, followed by all .data csects except for the TOC. The TOC is written after all the other .data csects. The TOC entries are relocated, so that the TOC entries with TC storage-mapping class contain the csect addresses after the sort, rather than the csect addresses in the source program.
When an XCOFF module is loaded, TOC entries with TC storage-mapping class are relocated again so that the TOC entries are filled with the real addresses where the csects will reside in memory. To access a csect in the module, two pieces of information are required:
- The location of the beginning of the TOC.
- The offset from the beginning of the TOC to the specific TOC entry that points to the csect. If a TOC entry has more than one address, each address can be calculated by adding (0...(n-1))*4 to the offset, where n is the position of the csect address defined with the .tc pseudo-op.