HP OpenVMS Systems Documentation
HP OpenVMS Linker Utility Manual
7.3.4 Allocating Memory for Image Sections
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.
7.3.5 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 7-2 and Table 7-3 include the image section attributes associated with an image section that contains program sections with a particular set of attributes. Table 7-5 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 7-6 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 7-8.
The items in the following list correspond to the numbers in Example 7-8:
7.3.6 Controlling Image Section Creation
126.96.36.199 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 7-9 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 7.3.1.
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 7-5, 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 $CODE. Program section naming conventions are architecture specific.)
188.8.131.52 Isolating a Program Section into an Image Section
To isolate a program section into an image section, specify the SOLITARY attribute of the program section using the PSECT_ATTR= option. For example, to isolate the GLOBAL_DATA program section in the sample link into its own image section, specify the following:
For Alpha linking, when mapping data into an existing location in the
virtual memory of your program using the Create and Map Global Section
($CRMPSC) system service or the Map Global Section ($MGBLSC) system
service, you must specify an address range (in the
inadr argument) that is aligned on a CPU-specific page
boundary. Because the linker aligns image sections on CPU-specific page
boundaries and the program section in which the section is to be mapped
is the only program section in the image section, you ensure that the
start address of the location is page aligned. In addition, because
Alpha systems must map at least an entire page of memory at a time,
using the SOLITARY attribute allows you to ensure that no other data in
the image section is inadvertently overwritten by the mapping. By
default, the linker creates the next image section on the next page
boundary so that no data can be overwritten.
After allocating memory for the image, the linker initializes the image
by writing the binary contents of the image sections by processing text
information and relocation (TIR) records in the object modules. These
records direct the linker in the initialization of the image section by
telling it what to store in the image section buffers. In addition, the
linker inserts the addresses of symbols within the image wherever they
A TIR record contains object language commands, such as stack and store commands. Stack commands direct the linker to put information on its stack, and store commands direct the linker to write the information from its stack to the buffer for that image section.
During this image section initialization, the linker keeps track of the program section being initialized and the image section to which it has been allocated. The first attempt to initialize part of an image section by storing nonzero data causes the linker to allocate a buffer in its own program region to contain the binary contents of the generated image section. This allocation is achieved by the Expand Region ($EXPREG) system service, and it requires that the linker have available a virtually contiguous region of its own memory at least as large as the image section being initialized.
A buffer is not allocated for an image section until the linker executes a store command (with nonzero data) within that image section.
Debugger information (DBG) records and traceback information (TBT) records are processed only if the debugger was requested and traceback information was not excluded by the /NOTRACE qualifier in the LINK command. Otherwise, these records are ignored. The records contain stack and store object language commands (TIR records), but they are stored in the debugger symbol table (DST) instead of in an image section. (The linker expands its memory region to accommodate the DST, unless the /NOTRACEBACK qualifier was specified in the LINK command.)
When the linker processes end-of-module (EOM) records, it checks that its internal stack has been collapsed to its initial state. When this processing is complete, the linker has written the binary contents of all image sections to image section buffers in its own address space.
The linker writes the contents of its buffers in the following order: