What is UMB?
| Upper Memory Block |
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Computer memory is made up of many locations, each holding one 'bit' of data. The computer needs a unique 'address' for each location so that the microprocessor knows exactly where to find it, just as each house in a housing estate needs an address so that the postman can find it. The newer microprocessors, such as the 80386 and 80486, can generate a huge range of different addresses, but of course this doesn't mean that all possible addresses lead to a memory chip, just as knowing all the house numbers on a building site doesn't mean all the houses are built. A 'memory map' is a diagram listing all the addresses that a microprocessor can generate and showing which ones lead to physical devices installed in the system in question. For reasons arising from the original design of Intel's processors, 'holes' often appear in the diagram between 640K and lMb - i.e. between address number 65,535 and number 1,048,575 - and these are called UMBs. They're useful because memory chips with addresses higher than 1 Mb can be made to appear to occupy these holes (rather like redirecting a letter). This allows the micro- processor to operate with more memory in a fast mode, called Virtual mode, where the address range is limited to lMb. |
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