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On Feb 14, 2011, at 12:29 PM, David Daney wrote:
Background:
Current MIPS 32-bit ABIs (both o32 and n32) are restricted to 2GB of user virtual memory space. This is due the way MIPS32 memory space is segmented. Only the range from 0..2^31-1 is available. Pointer values are always sign extended.
Because there are not already enough MIPS ABIs, I present the ...
Proposal: A new ABI to support 4GB of address space with 32-bit pointers.
The proposed new ABI would only be available on MIPS64 platforms. It would be identical to the current MIPS n32 ABI *except* that pointers would be zero-extended rather than sign-extended when resident in registers. In the remainder of this document I will call it 'n32-big'. As a result, applications would have access to a full 4GB of virtual address space. The operating environment would be configured such that the entire lower 4GB of the virtual address space was available to the program.
I have to wonder if it's worth the effort. The primary problem I see is that this new ABI requires a 64bit kernel since faults through the upper 2G will go through the XTLB miss exception vector.
At a low level here is how it would work:
1) Load a pointer to a register from memory:
n32: LW $reg, offset($reg)
n32-big: LWU $reg, offset($reg)
That might be sufficient for userland, but the kernel will need to do similar things (even if a 64bit kernel) when accessing structures supplied by 32-bit syscalls.
It seems to be workable but if you need the additional address space why not use N64?
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