why mips fetch the instruction from 0xbfc00000 (phy:0x1fc000000)

[changeyourname]

why mips cpu fetch the instruction from 0xbfc00000 (phy:0x1fc000000) ranther than 0xa0000000 upon reset,there is some memory space from 0xa0000000 to 0xbfc00000 ，What is this space used to do ，Whether Can I place the hardware code at 0xa0000000

best wishes

[yuri-panchul]

bfc00000 happened because of historical reasons. According to Mike Gupta, a member of early MIPS design teams:

Reset Vector being at non-zero value:
Reset is in ROM space. ROM space is generally associated with I/O space and generally fragmented. In 1985 (or so), we thought 4MB would be more than enough to boot form [and we decided to put it at the end of physical space: 0x1fc00000-0x1fffffff = 0x400000 = 4,194,304 bytes]. Also, it allows for the DRAMs (generally much bigger) to be naturally aligned at address 0. The newer processor do not require BFC00000 / 1FC00000 as this address is programmable.

Addresses between 0 and 1fc00000:
This is roughly 2 GB of space. No it does not have to be fully populated. Address 0 to maybe 1st 64 KB (kilo) had to be for exception vectors. Even that is now not necessary with the advent to Exception base register