Memory Address map

  • The addressing of memory can establish by means of a table that specifies the memory address assigned to each chip.
  •  The table, called a memory address map, is a pictorial representation of assigned address space for each chip in the system, shown in the table.
  • To demonstrate with a particular example, assume that a computer system needs 512 bytes of RAM and 512 bytes of ROM.
    The RAM and ROM chips to be used specified in figures.
  • The component column specifies whether a RAM or a ROM chip used.
  • Moreover, The hexadecimal address column assigns a range of hexadecimal equivalent addresses for each chip.
  •  The address bus lines listed in the third column.
  • Although there 16 lines in the address bus, the table shows only 10 lines because the other 6 not used in this example and assumed to be zero.
  • The small x‟s under the address bus lines designate those lines that must connect to the address inputs in each chip.
  •  Moreover, The RAM chips have 128 bytes and need seven address lines. The ROM chip has 512 bytes and needs 9 address lines.
  • The x‟s always assigned to the low-order bus lines: lines 1 through 7 for the RAM. And lines 1 through 9 for the ROM.
  • It is now necessary to distinguish between four RAM chips by assigning to each a different address. For this particular example, we choose bus lines 8 and 9 to represent four distinct binary combinations.
  •  Also, The table clearly shows that the nine low-order bus lines constitute a memory space for RAM equal to 29 = 512 bytes.
  • The distinction between a RAM and ROM address done with another bus line. Here we choose line 10 for this purpose.
  •  When line 10 0, the CPU selects a RAM, and when this line equal to 1, it selects the ROM.

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