Members of Intel's 80x86 and some other processor families have two distinct address spaces through which they can communicate with these memories and peripherals. The first address space is called the memory space and is intended mainly for memory devices; the second is reserved exclusively for peripherals and is called the I/O space. However, peripherals can also be located within the memory space, at the discretion of the hardware designer. When that happens, we say that those peripherals are memory-mapped.
from the processor's point of view, memory-mapped peripherals look and act very much like memory devices. However, the function of a peripheral is obviously quite different from that of a memory. Instead of simply storing the data that is provided to it, a peripheral might instead interpret it as a command or as data to be processed in some way. If peripherals are located within the memory space, we say that the system has memory-mapped I/O.
The designers of embedded hardware often prefer to use memory-mapped I/O exclusively, because it has advantages for both the hardware and software developers. It is attractive to the hardware developer because he might be able to eliminate the I/O space, and some of its associated wires, altogether. This might not significantly reduce the production cost of the board, but it might reduce the complexity of the hardware design. Memory-mapped peripherals are also better for the programmer, who is able to use pointers, data structures, and unions to interact with the peripherals more easily and efficiently.
5.2.1 Memory Map
As you are reading about a new board, create a table that shows the name and address range of each memory device and peripheral that is located in the memory space. Organize the table so that the lowest address is at the bottom and the highest address is at the top. Each time you add a device to the memory map, place it in its approximate location in memory and label the starting and ending addresses, in hexadecimal. After you have finished inserting all of the devices into the memory map, be sure to label any unused memory regions as such.
if you look back at the block diagram of the Arcom board in Figure 5-1, you will see that
there are three devices attached to the address and data buses. These devices are the RAM and ROM and a mysterious device labeled " zilog 85230 serial controller." The documentation provided by Arcom says that the RAM is located at the bottom of memory and extends upward for the first 128 KB of the memory space. The ROM is located at the top of memory and extends downward for 256 KB. But this area of memory actually contains two ROMs-an EPROM and a Flash memory device each of size 128 KB. The third device, the Zilog 85230 Serial Communications Controller, is a memory-mapped peripheral whose registers are accessible between the addresses 70000h and 72000h.
The memory map in Figure 5-2 shows what these devices look like to the processor. In a sense, this is the processor's "address book." Just as you maintain a list of names and addresses in your personal life, you must maintain a similar list for the processor. The memory map contains one entry for each of the memories and peripherals that are accessible from the processor's memory space. This diagram is arguably the most important piece of information about the system and should be kept up-to-date and as part of the permanent records associated with the project.
Figure 5-2. Memory map for the Arcom board
For each new board, you should create a header file that describes its most important features. This file provides an abstract interface to the hardware. In effect, it allows you to refer to the various devices on the board by name, rather than by address. This has the added benefit of making your application software more portable. If the memory map ever changes for example, if the 128 KB of RAM is moved you need only change the affected lines of the board-specific header file and recompile your application.
As this chapter progresses, I will show you how to create a header file for the Arcom board. The first section of this file is listed below. The part of the header file below describes the memory map. The most notable difference between the memory map in the header file and that in Figure 5-2 is the format of the addresses. Pointers Versus Addresses explains why.
/**********************************************************************
*
* Memory Map
*
* Base Address Size Description
* -------------- ----- -----------------------------------
* 0000:0000h 128K SRAM
* 2000:0000h Unused
* 7000:0000h Zilog SCC Registers
* 7000:1000h Zilog SCC Interrupt Acknowledge
* 7000:2000h Unused
* C000:0000h 128K Flash