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Memory map example for a 68HC12
4K RAM starting at address 0
starting at address 2000H
starting at address 8000H
end address = start address +
chip size – 1
remember that 1K = 400H
In this memory map example, three types of memory, RAM, EEPROM and ROM are used. The size of each type of memory is determined first based on the requirement of the system. Then the starting address of each type of memory is determined. In theory, the starting address of a memory chip can be at any memory address. However, if the starting address is at an integer multiple of the size of the memory chip, the address decoding circuit of the memory component can be greatly simplified. In this example, the starting addresses of all memory chips are integer multiple of the sizes of the corresponding memory chips.
Once the size and the starting address of a memory chip is known, the ending address of that memory chip can be calculated using the following fomula
ending address = starting address + memory chip size - 1.
For example, if the starting address of EEPROM is 2000H and the EEPROM size is 1K (400H), the ending address of EEPROM is 23FFH.
Exercise 9.2 : It is assumed that the starting address of ROM module is 8000H and 4 ROM chips of size 8K are used to make the 32K ROM module. What is the starting and ending addresses of each ROM in hexadecimal?