The problem of the page frame

For expanded memory to work there has to be a 64KByte area above conventional memory that can be used as a page frame. In many cases you can leave the automatic installation routine of the EMM that you have chosen to pick the location of the page frame for you. If your system is fairly standard then this should work without any problems. However, if your system is even slightly non-standard the automatic location method is likely to fail. In this case it is up to you to find a free 64 KByte area and enter its address as a parameter in the line that loads the EMM in the CONFIG.SYS file. There are two main problems with doing this. The first is that you have to use hexadecimal notation to specify the address and this frightens many users. The second is actually finding a free area. The first problem can be solved by reading the technical box on Hex addressing later in this chapter. The second problem can be solved by either reading your machine’s manual, or using a memory mapping utility (for example the one included with System Sleuth) or, if all else fails, a trial and error procedure testing each possible 64KByte block in turn.

If you can’t find a 64KByte page frame in high memory then many EMMs will allocate a 64KByte area in conventional memory to be used as a page frame. The main disadvantage of this is the loss of 64KBytes of valuable conventional memory. A true LIM 4.0 application can manage without a page frame by simply swapping any section of conventional memory with expanded memory blocks. In this case you can set the LIMulator not to allocate a page frame and so avoid the loss of conventional memory.

The IBM PS/2 386 machines (models 70 and 80) are particularly awkward to set up when it comes to finding a page frame. The reason is partly that larger BIOS ROMs are installed but is mainly because the adapter cards take larger amounts of address space. The solution is to reconfigure the machine using the setup disk so as to move the adapter cards to free a 64KByte region of memory. The LIMulator 386MAX includes copious instructions about how this can be achieved for a range of systems, but sometimes it is simply impossible.