DOS/32 Advanced DOS Extender - Technical Reference



7.0 - DOS/32 Advanced - Memory Management

Under Clean, XMS and VCPI systems DOS/32 Advanced will always allocate extended memory at startup and set up its own memory heap, from which your protected mode program will receive its memory. This is done so to minimize the amount of calls to the present system software the DOS Extender otherwise would have to do. Instead, the memory blocks, also called Extended Memory Blocks or EMBs, will be dynamically allocated, resized and deleted on your program's requests, internally by the DOS Extender accessing only its own memory heap. Under DPMI system, it will be up to the present, "external" DPMI host to provide your program with extended memory, as DOS/32 Advanced built-in DPMI server will never be installed and used.

 

Under VCPI system, the allocated physical memory will be represented by (mapped into) linear memory which in its turn is described by Page Tables. The Page Tables will be dynamically allocated by DOS/32 Advanced at startup in the conventional (DOS) memory, and the number of allocated Page Tables will directly depend on how much physical memory is present in the computer and is accessible to the DOS Extender through the installed VCPI server. Each allocated Page Table will map 4MB of linear memory, but will also take up 4KB of DOS memory. Thus, if a computer has 256MB of memory installed, the DOS Extender will allocate 256MB / 4MB = 64 Page Tables which will take up as much as 64 * 4KB = 256KB of DOS memory. Consequently, if a computer has 1GB of memory installed, the allocated Page Tables will consume 1MB of DOS memory, which is way too much. To prevent such situations, DOS/32 Advanced will limit the number of Page Tables to a certain value, VCPI PageTables, which can be manually adjusted by the user using the SUNSYS Setup Utility program. The default value of VCPI PageTables is 64, which allows DOS/32 Advanced to allocate the maximum of 256MB of memory.

Please note that DOS/32 Advanced DOS Extender of version 6.00 and higher will not allow allocation of more than 64 Page Tables, limiting the maximum amount of possible allocated memory to 256MB. Also, if the Page Tables take too much of valuable DOS memory (the case when you have alot of physical memory installed in the computer) you can manually lower the VCPI PageTables value to turn down DOS memory requirements.


DOS/32 Advanced DOS Extender also has another feature, VCPI+XMS Allocation Scheme, which allows to control the amount of allocated extended memory under VCPI. Due to the design of VCPI, the system will have two different pools of memory: one reserved for EMS and the other for XMS. As the VCPI hosts are installed on top of XMS servers, they will by the rule allocate the memory from XMS and reserve it for EMS pages. An example would be a very popular VCPI/EMS server, "EMM386" by Microsoft, which is required to be installed on top of the XMS, "HIMEM.SYS". Despite the two different system memory pools, DOS/32 Advanced will be able to allocate all accessible memory from the both VCPI and XMS when VCPI+XMS Allocation Scheme feature is turned on (see SUNSYS Setup Utility documentation for further explanation).


When calling DOS/32 Advanced DPMI memory management functions (DPMI functions 05xxh), the DOS Extender will always perform an internal check of all Extended Memory Blocks to see whether their integrity is in good shape and none has been overwritten or corrupted. Although this feature can be disabled with the SUNSYS Setup Utility program for the "retail" versions of your protected mode programs, it is recommended that you keep it enabled when developing applications as this is a good way to make sure that none of the routines in your program is corrupting the memory. As soon as DOS/32 Advanced detects that one or more of the Extended Memory Blocks are overwritten or corrupted in some way, it will terminate your application with an error. This protection against Extended Memory Blocks overwriting is more than 99 per cent secure, and if this error is triggered by the DOS Extender, you will know for sure what exactly is wrong.

 


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