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CHIP-SELECT UNIT
6.4.2
Start Address
The START register of each chip-select defines its starting (base) address. The start address value
is compared to the ten most-significant address bits of the bus cycle. A bus cycle whose ten most-
significant address bits are equal to or greater than the start address value causes the chip-select
to go active. Table 6-2 defines the address bits that are compared with the start address value for
memory and I/O bus cycles.
It is not possible to have a chip-select start on any arbitrary byte boundary. A chip-select config-
ured for memory accesses can start only on multiples of 1 Kbyte. A chip-select configured for I/O
accesses can start only on multiples of 64 bytes. The equations below calculate the physical start
address for a given start address value.
For memory accesses:Start Value (Decimal) × 1024 = Physical Start Address (Decimal)
For I/O accesses:Start Value (Decimal) × 64= Physical Start Address (Decimal)
Address Space
Address Range
Memory
I/O
6.4.3
Stop Address
The STOP register of each chip-select defines its ending address. The stop address value is com-
pared to the ten most-significant address bits of the bus cycle. A bus cycle whose ten most-sig-
nificant bits of address are less than the stop address value causes the chip-select to go active.
Table 6-2 defines the address bits that are compared with the stop address value for memory and
I/O bus cycles.
It is not possible to have a chip-select end on any arbitrary byte boundary. A chip-select config-
ured for memory accesses can end only on multiples of 1 Kbyte. A chip-select configured for I/O
accesses can end only on multiples of 64 bytes. The equations below define the ending address
for the chip-select.
For memory accesses:(Stop Value (Decimal) × 1024) – 1= Physical Ending Address (Decimal)
For I/O accesses:(Stop Value (Decimal) × 64) – 1= Physical Ending Address (Decimal)
6-10
Table 6-2. Memory and I/O Compare Addresses
Number of Bits
1 Mbyte
64 Kbyte
Comparator Input
20
A19:A10
16
A15:A6
Resolution
1 Kbyte
64 Bytes
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