HP OpenVMS Systems Documentation
OpenVMS Linker Utility Manual
The linker puts both the DATA and $CHAR_STRING_CONSTANTS program sections in the same image section because they both have the same settings of significant attributes. Within the image section, the linker organizes the program sections alphabetically, so the $CHAR_STRING_CONSTANTS program section appears before the DATA program section. The linker creates a separate image section for the $CODE program section.
The linker performs similar processing of the program sections in the default cluster. The Image Section Synopsis section of the map file lists the clusters the linker created and lists the image sections it created for each cluster. This section also describes the layout of the image in memory, including the base address of each image section. Example 3-6 illustrates an excerpt of the Image Section Synopsis section from the map file produced with the sample link. The listing includes clusters for contributions for the VAX C Run-Time Library.
For more information about the image section synopsis section of a map file, see Section 5.2.3.
To find out which program sections the linker placed in each image section, look at the Program Section Synopsis section of the map file. This section lists all the program sections in each cluster and lists the contributions (the number of bytes) to each program section from each object module. By comparing the base-address of the program sections with the base-addresses of the image sections in the Image Section Synopsis section, you can tell in which image section the program sections appear. Example 3-7 is an excerpt from the Program Section Synopsis section of the map file produced by the sample link operation.
The linker aligns image sections on CPU-specific page boundaries. Within an image section, the linker assigns to each program section a virtual address relative to the base address of the image section.
If the program sections have the concatenated (CON) attribute set, the linker positions the program sections one after the other within an image section, inserting padding bytes between the program sections if necessary to achieve the alignment requirement of a particular contribution to a program section. The linker retains the alignment specified for each program section contribution but uses the largest alignment of a contributing module as the alignment of the whole program section.
If the program sections have the overlaid (OVR) attribute set, the linker uses the same start address for the program sections so that they occupy the same virtual memory (that is, the program sections overlay each other). For overlaid program sections, the linker allocates enough space to accommodate the largest of all the program section contributions. Note that the linker does not generate a warning message if the contributions specify different size allocations.
Any module can initialize the contents of an overlaid program section. However, the final contents of the program section are determined by the last contributing module. Therefore, the order in which you specify the input modules is important.
The linker keeps track of free (available) virtual addresses by maintaining a free virtual memory list. For each cluster, the linker determines the number of pages required, searches the list beginning at the lowest virtual address for a contiguous number of pages large enough to contain the cluster, allocates those addresses to the cluster, then removes those addresses from the list.
The linker allocates virtual memory to the first cluster beginning at a page size boundary for executable images in the P0 region of the user's virtual address space, unless the cluster is based, in which case it allocates virtual memory beginning at the specified address. For VAX linking, the default is 512 (200 hexadecimal). However, you can specify the page size using the /BPAGE qualifier. (For information about the /BPAGE qualifier, see Part 2.)
On its first pass through the cluster list, the linker allocates virtual addresses to any based user clusters or based shareable image clusters on the cluster list, removing the allocated addresses from the free virtual memory list as it proceeds. On its second pass, it repeats this procedure for nonbased user clusters. (Remember that nonbased shareable image clusters will have memory allocated for them at run time.)
Because the linker processes clusters in the order of their appearance on the cluster list, the virtual address space of the final image will generally contain contiguous image sections of consecutive clusters on the basis of their order in the cluster list. The presence of based clusters, however, may prevent such an outcome, and for this reason they are not recommended.
3.3.5 Image Section Attributes
When it creates image sections, the linker assigns attributes to the image section based on the attributes of the program sections it contains. The image section attributes describe certain characteristics of the portion of memory they represent, for example, the protection characteristics. For example, an image section that contains program sections with the writability attribute also has the writability attribute set. Table 3-2 and Table 3-3 include the image section attributes associated with an image section that contains program sections with a particular set of attributes. Table 3-5 lists all the image section attributes. Image section attributes, like program section attributes, are Boolean values that are either on or off.
The linker uses type designations instead of image section attributes to propagate the SHR and PIC program section attributes. The linker assigns the type designation [E]ISD$K_NORMAL for image sections in executable images. Image sections in shareable images can be any of the following types:
The Image Section Synopsis section of a map file lists the attributes of each image section created in the Protection and Paging column. See Example 3-6 for an illustration. You can also get a listing of all the image sections created by the linker by using the ANALYZE/IMAGE utility. The output generated by this utility includes a list of all the image sections that make up the image, with their attributes. An excerpt from the analysis of the image file MYTEST.EXE is shown in Example 3-8.
The items in the following list correspond to the numbers in Example 3-8:
3.3.6 Controlling Image Section Creation
188.8.131.52 Modifying Program Section Attributes
The linker combines program sections in the same cluster into the same image section if they have the same settings for the significant program section attributes. To force the linker to put the program sections into different image sections, change the attributes of one of the program sections by using the PSECT_ATTR= option.
For example, in the sample link operation, the DATA program section and the $CHAR_STRING_CONSTANTS program section are combined into the same image section. If you want the $CHAR_STRING_CONSTANTS program section to appear in a different image section, change one of the significant attributes. For example, in the following link of the sample programs, the writability attribute is set to NOWRT. (For Alpha linking, you do not need to explicitly specify the C Run-Time Library in the link operation because it resides in the default system shareable image library [IMAGELIB.OLB], which the linker processes by default.)
Example 3-9 presents an excerpt from the Image Section Synopsis section of the map file produced by this link.
Note that the default cluster contains one additional image section, a
read-only image section beginning at virtual address 0x00000600, than
the default cluster in the original link, illustrated in Section 3.3.1.
In general, the linker creates image sections on a per-cluster basis; that is, only program sections within a particular cluster can contribute to image section creation. (The linker can collect program sections with the global attribute from all clusters into a single image section.) To ensure that a program section appears in a particular image section, put the program section in a specific cluster.
For example, in the sample link operation illustrated in Example 3-5, the linker puts all the program sections in the object module MYSUB.OBJ in the cluster named MYSUB_CLUS because the CLUSTER= option is specified. If you wanted to group all of the program sections that contain code from all the other clusters into the MYSUB_CLUS cluster, you could specify the COLLECT= option, as in the following example. (By convention, VAX language processors put the code they generate into program sections named $CODE. Program section naming conventions are architecture specific.)