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Motherboard
2.9
Hardware Addresses
How Memory
The memory address area of a Pentium CPU has a capacity of 4 Gbyte.
Decoding Works
Together with the 64-bit-wide CPU data bus, the CPU is equipped with
29 address lines (A3...A31) and 8 bus enable lines (BE0...BE7) which encode
the non-existent byte address lines A0, A1 and A2. The CPU address bus is
mapped via system controller (TSC) on the PCI address bus. Memory
addresses from 0000 0000h to 0009 FFFFh (640 Kbytes) and from 0010
0000h to 07FF FFFFh (127 Mbytes) are not included.
The ISA bridge maps the ISA address bus exactly once on the PCI address
bus via the PIIX (PCI ISA IDE accelerator) block. The ISA address bus for 8
bit modules covers the address area from A0 to A19 which corresponds to the
CPU addresses from 0000 0000h to 000F FFFh (1 Mbyte). The address bus is
extended with the address lines A20...A23 for 16-bit ISA modules and
therefore addresses from 0000 0000h to 00FF FFFFh (16 Mbytes).
Special memory read/write signals which are only activated in case of a logic
zero level of the address lines A20, A21, A22 and A23 make the distinction
between the 1 Mbyte and the 16 Mbyte ISA address areas.
If the CPU references address areas which are occupied by the main memory,
ISA bus control signals do not occur; that is, ISA bus modules within this
memory area are not referenced. In the reverse case, an ISA bus master
cannot reach addresses higher than 16 Mbytes. In order to gain a larger
address area than the memory address area with a maximum range of
640kByte to 1 Mbyte, different decoding holes in the Pentium programming
device motherboard are provided for dual port RAM extensions:
During the division of the address areas, a distinction is made between the:
Different read/write signals (I/O, WR, I/O RD, MEMR, MEMW) are used to
reference these areas. The following tables will give you the descriptions of
the individual functional groups for more details.
2-26
The CPU address area FFF8 0000h to FFFD FFFFh (512 k-128 k BIOS =
384 Kbyte) is mapped in the ISA address area 00F8 0000h to
00FD FFFFh and is always referenced in the CPU address area. Decoding
the address lines A24 to A 31 which do not exist on the ISA bus is
fulfilled by special hardware located on the motherboard.
1 Mbyte of memory address area can be assigned to the ISA bus in the 16th
Mbyte. This option can be activated/deactivated in the SETUP procedure.
Memory address area and
I/O address area
SIMATIC PC RI25/45, Technical Description
C79000-G7076-C808-01
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