Upper Memory Blocks

Even though a machine may have many megabytes of memory it is still possible for it not to be able to run particular real mode applications programs due to a shortage of conventional memory! The reason for this is that while a program may be able to make use of expanded or extended memory to increase its data storage, it generally needs a certain minimum amount of conventional memory to work at all. In most cases the 640KBytes of conventional memory is a precious resource that needs to be used with care. For example, the 640KBytes

 RAM or other device then there is no reason why it shouldn’t be used to store a device driver that would normally be stored down is conventional memory. in other words there is no reason why the ‘holes’ in the UMA shouldn’t be used as island outposts of conventional memory. There are a number of problems to putting this scheme into practice though. The major problem is that the holes in the UMA are in fact unallocated regions of the memory map-that is they don’t contain any RAM