What Is a Map Register?
Computers use map registers to make it easier to access computer memory without placing demands on the computer's processor. Instead of having the processors take data in and out of memory, Direct Memory Access allows the hard disk to go directly to the computer memory. The map register is a tool for finding information in memory.
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Address Spaces
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A computer relies on a driver, a software program that interacts with hardware, to operate DMA. DMA drivers employ three different memory spaces: The virtual memory supported by the processor; the logical address space in which the driver finds information; and the actual physical storage space in the computer's random access memory. Logical address space doesn't always match the physical layout in the RAM, so the driver needs the register to map the relationship between the two spaces.
Logical Addresses
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Computers use logical addresses because the available RAM space may be scattered across the physical memory address. That doesn't work for computer processes that need a contiguous block of memory to run. Logical addresses provide contiguous blocks of memory space, even if the physical storage is scattered. The DMA driver can't connect directly to the physical address, so it has to go through logical or virtual memory. Physical addresses are still indispensable as the processes need physical memory to work.
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Map Registers
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Each computer's Hardware Abstraction Layer, or HAL, which mediates between hardware and software, stores one or more map registers. Each map register connects a range of logical addresses to the corresponding physical addresses. A single map register maps a minimum of 1 byte of data and a maximum of a full physical page of data: An operation that involves storing three pages of data will need three map registers. Each adapter object -- a tool the driver uses to interact with the system -- has at least one map register available.
Size
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Logical addressing may seem more complicated than using physical memory directly, but in some ways, it's simpler. Finding information in the physical addresses requires searching through 4 gigabytes or more of RAM, something beyond the ability of many drivers. Logical addresses encompass a much smaller space, which makes it easier and quicker for drivers to search for data there, leaving the map register to locate the data in the corresponding physical address.
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