VULCAN Technical Manual
Detailed hardware description
© 2007 Eurotech Ltd Issue D
20
When the configuration register is set to logic 1, the expansion bus occupies the lowest
256MB of address space. When it is set to logic 0, the SDRAM occupies the lowest
256MB of address space. In both cases, the SDRAM occupies the 768MB immediately
following the lowest 256MB and the expansion bus can be accessed starting at the
address 5000_0000.
Upon reset Expansion bus on VULCAN starts at address 0000_0000 and SDRAM can
be accessed starting from address 1000_0000. RedBoot reconfigures the memory
map so that Expansion bus starts at address 5000_0000 and SDRAM starts at address
0000_0000.
The largest SDRAM memory size supported by the Intel
®
IXP4XX product line
processors is 256MB.
The actual memory implemented in any given configuration is aliased (repeated) to fill
the 1GB SDRAM address space. Due to aliasing, all of the SDRAM is accessible even
when the expansion bus occupies the lowest 256MB of address space. On reset, the
configuration register in the expansion bus is set to logic 1. This setting is required
because the dedicated boot memory is Flash memory located on the expansion bus.
Details of the internal registers are in the IXP425 Intel Developer Manual on the
Development Kit CD.
