XVIII Memory Mapping
Memory mapping is the process of reserving blocks of system memory, using hexadecimal address, to place bits of information that the CPU and operating system need to find.
In this particular example, we are memory mapping the I/O. It is important to note that I/O is not stored in RAM, but in different areas of the system memory, such as ROM chips on an expansion card.
By assigning a particular memory map to a communications port, when the CPU is ready, it knows where to find the data (the In part of I/O). When the CPU needs to send data out, it uses this address again. This process is known as I/O Addressing.
A typical example is communications port 1. The standard memory address for Com1 is 3F8h.
I/O addresses are not exactly like IRQ and DMA. IRQ and DMA are typically assigned one per device and are of uniform size.
While a sound card needs an I/O address, since it really is several devices on one card, the I/O address needs can (and do) vary.
The challenge arrives with the fact that when I/O addresses are specified, the only part specified is the beginning of the I/O address. A device may only need as little as one byte, or as much as 64 bytes.
It would seem with 32, 64, or even 256 MB of ram, there is plenty of room to find an address. And there is, however we have one little issue that shoots a hole in that logic. That would be the need for backward compatibility.
Prior to Windows 95, finding I/O overlap was difficult. The routine known as Device Manager in the Control Panel->System of Windows 95 made this a much simpler process to discover I/O overlap.
Of course it isn't the simplest of tasks to say, my modem is on IRQ 3 with an I/O address of 2F8-2FFh. What to do? Read on for the answer.
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