The following is a brief summary of LIM EMS Version 4.0 functions. Note that it is a summary of the EMS specification itself, and not of its implementation under OS/2 Version 2.0.
Allocating/Reallocating/Deallocating Expanded Memory
An allocation request can be made for a number of expanded memory pages and, if successful, a handle is returned. This handle is then used by the application to reallocate or deallocate memory.
Mapping Expanded Memory
Logical pages in an object can be mapped into physical address ranges addressable by the 8086 processor. A mapping indicates the relation between EMS physical pages and <EMS Handle, EMS Logical Page> pairs that the application requires. One example would be to map an expanded memory video buffer (EMS logical pages) to the mappable window (EMS physical pages) to create a video image in expanded memory. An EMS service request can then be used to move the image from expanded memory to screen memory.
A single logical page can be mapped to multiple physical pages. This is used by programs such as Lotus 1-2-3. When a single logical page is mapped to multiple physical pages, a write to any of these physical pages writes to the same expanded memory. An application can write to one address and then read the results from another address. This feature can be used to provide independent mappings to a shared structure for multiple modules. To implement this aliasing, multiple page registers must all point to the same memory. This leads to a requirement for the memory manager to support aliasing of page table entries.
A physical page can be unmapped. Reads/writes to unmapped memory do not kill the application, but an application cannot depend on reading what it has previously written. LIM EMS specifies that a program must unmap mappable windows before allowing another program to run, in order to protect the memory mapped by the first program.
The specification does not stipulate what happens to programs that touch unmapped memory, only that the physical pages are "inaccessible for reading and writing". One possible implementation is to map unmapped pages to nonexistent physical pages (the Microsoft Windows/386 product does this). Another alternative is to map them to physical ROM. This can lead to problems, however, on some machines that cache ROM. The safest alternative is to use a single physical page of memory.
Information Calls
An application may obtain information about the EMS resources available, current mappings, and handle usage. In a multiprogramming environment or where TSRs are loaded, however, this information may be out of date by the time it is used. For instance, an application may determine the amount of LIM memory available, but before getting the opportunity to request an allocation, a TSR may request EMS memory. The application would find less memory available than expected.
Saving/Restoring Mappings
Several calls are available for an application to obtain data that can later be returned to EMS to restore a mapping. For some calls, this information is saved internally by the EMS driver. For others, it is returned to the application.
One type of save operation automatically saves the registers pointing to the first 64KB of the mappable window to an internal EMS buffer associated wtih an EMS handle supplied by the user. Typically, this is used by a TSR to save and restore the current mappable window by saving to an EMS buffer associated with a handle owned by the TSR. Other save operations return either complete or partial information to the application, which the application can later provide to restore memory mappings. Still other calls allow EMS the option to store register information on the application's stack. There are five different ways to save and restore registers in addition to techniques for setting the registers.
Alternate Register Sets
This is an optional feature of EMS. Mapping can be done to any of a number of register sets. The application can then switch the active register set. The effect is similar to switching page tables under OS/2 Version 2.0. Alternate Register Sets can be protected by the first application to claim a protection key. Only the application with the key will be allowed to claim a register set or switch the active one unless no one claims the key or the process with the key permits others to use the alternate sets.
This feature is typically used by DOS extenders such as Microsoft Windows. Switching memory during a task switch can be accomplished by turning on permission for changing register sets using the permission key, switching the current register set, and turning alternate set permission off. Even when alternate register sets are not supported, save and restore operations for register set 0 are simulated with data passed to and from the application.
DMA Register Sets
This is an optional feature of EMS. These register sets allow association of a DMA channel with a register set. All DMA on that channel is remapped through the associated DMA register set allowing EMS remapping during DMA. When this feature is not supported, remapping of register sets may be delayed until DMA completes.
Program Execution
This function allows an application to execute a procedure or subroutine which lies in an expanded memory area not currently mapped into the 8086 address space. EMS will perform a remap to bring the procedure or routine into the 8086 address space, and pass control to the specified entry point. This may be accomplished in either of two ways:
This function allows applications to store code in expanded memory.
Data Movement
Copy and exchange services provide data movement between any combination of conventional or expanded memory. The start of a region of expanded memory is indicated by handle, logical page and offset. The memory being affected need not be currently mapped into the 8086 address space. Overlapping copies succeed without corrupting data, and a return code indicates overlap to the application. Exchange operations may not overlap.
This function allows applications to conveniently move portions of expanded objects around in expanded memory, or move them to or from conventional memory, without having to first remap the objects into the 8086 address space.
EMM Protection
Limited protection is available. The first application that requests a key can turn enable or disable access to alternate and DMA register sets. There is no protection for memory objects. Any application can determine all handles in use and perform any operations on them. Within a single EMS implementation, a badly behaved application can wreak havoc on any application using EMS.
For example, one Windows application may write over the memory of any other. This is consistent with the general lack of protection in the DOS environment, where applications have free access to the machine's physical memory.
OS Support
On power up, an EMM implementation which supplies mappable conventional memory allocates all mappable conventional memory to handle 0 and maps it in. This is typically all memory above the memory on the system board up to 640KB. This occurs before the operating system starts up and allows programs like Windows to remap conventional memory. Programs that remap conventional memory are required to reset the mapping before returning to the operating system. EMS does not enforce this, however.