The Intel 80386 and 80486 families of microprocessors are capable of addressing 64TB of virtual memory. At present, OS/2 Version 2.0 has the capability of supporting up to 4GB of virtual memory and 4GB of real memory. In order to support more than 16MB of memory however, certain requirements must be met.
Firstly, the OS/2 V2.0 physical memory manager needs to know how much memory is present. This is done during kernel initialization by calling the BIOS interrupt 15 with subfunction code x'88'. This call will return the total amount of memory present. If the BIOS supports interrupt 15 with subfunction code x'C7', "return memory map information", then this function will be used to return an array giving the layout of the different levels of memory; for example, system board memory and adapter memory available in the machine, together with address ranges for the different memory types. Should the function not be present, the interrupt will return with the carry flag set. Support for this function is not mandatory. Information obtained is used by OS/2 V2.0 to prioritize the areas of physical memory that are allocated first.
The second requirement for supporting more than 16MB is that the paging device fixed disk controller can support data transfers to or from memory locations above the 16MB line. This ability usually requires the availability of a DMA controller with a 32-bit addressing capability, either on the system board or on a busmaster SCSI adapter. To determine whether this DMA is available, the disk device driver level is queried. If it is a level 3 device driver, it is further interrogated to find out if the DMA that the disk controller uses has support for 32-bit addressing. If this second requirement is satisfied and the disk controller can handle 32-bit addresses larger than 16MB, then all of memory is used by OS/2 for programs and data. If it cannot, then the OS/2 page manager allocates all of the memory above 16MB as a paging cache to be used before paging to the disk. This substitution is a good use of the memory above 16MB, but not as good as using it directly.
A final requirement for supporting more than 16MB is that the machine's ABIOS is capable of handling requests with addresses above the 16MB line.
Presently in the IBM PS/2 range, only the Models 90 and 95 have support for more than 16MB of memory and when using 8MB SIMMs will support up to 64MB. Machines using 80386SX, 80386SL or 80386SLC processors are not able to support more than 16MB as they only support 24-bit addressing external to the processor.
If OS/2 V2.0 detects the presence of a device that does not support 32-bit addressing, and if support for more than 16MB is enabled, it will ensure that buffers used by that device's driver are locked below the 16MB line during I/O operations. Level 3 device drivers are then given a permanent buffer below the 16MB line. For level 1 and 2 device drivers the OS/2 V2.0 virtual memory manager, when requested to lock a user buffer for that device, will move the buffer below the 16MB line.
The only device for which 32-bit addressing support is mandatory is the paging device. Pages are written to or read from their exact physical location in memory and are not moved before I/O takes place. Should paging be disabled by specifying MEMMAN=NOSWAP in the CONFIG.SYS, then more than 16MB can be supported even though there is no disk subsystem support for it.
When determining whether an OEM machine will support more than 16MB, the considerations are: