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?