HITACHI 115
8.3
Address Space Subdivision
8.3.1
Address Spaces and Areas
Figure 8.3 shows the address format used in this LSI.
A0
A21
A23,A22
A26–A24
A31–A28 A27
Output address:
Output from address pins
A21–A0
Ignore: Only valid when the address multiplex
function is being used in the DRAM space (area 1);
not output in other cases. When not output,
becomes shadow.
Area selection:
Decoded to become chip select signals
CS0
–
CS7
for areas 0–7
Basic bus width selection:
Not output externally, but used for basic bus width selection
When 0, (H'0000000–H'7FFFFFF), the basic bus width is 8 bits.
When 1, (H'8000000–H'FFFFFFF) the basic bus width is 16 bits.
Ignore: Always ignore, not output externally
4 Gbyte space
16 Mbyte space
4 Mbyte space
128 Mbyte space
Figure 8.3 Address Format
Since this LSI uses a 32-bit address, 4 Gbytes of space can be accessed on the architecture;
however, the upper 4 bits (A31–A28) are always ignored and not output. Bit A27 is basically only
used for switching the bus width. When the A27 bit is 0 (H'0000000–H'7FFFFFF), the bus width
is 8 bits; when the A27 bit is 1 (H'8000000–H'FFFFFFF), the bus width is 16 bits. Of the
remaining 27 bits (A26–A0), a total 128 Mbyte can thus be accessed.
The 128 Mbyte space is subdivided into 8 areas (areas 0–7) of 16 Mbytes each according to the
values of bits A26–A24. The space with bits A26–A24 as 000 is area 0 and the space 111 is area 7.
The A26–A24 bits are decoded and become the chip select signals (
CS0
–
CS7
) of the
corresponding areas 0–7 and output. Table 8.6 shows how the space is divided.