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
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.
At a low level here is how it would work:
1) Load a pointer to a register from memory:
LW $reg, offset($reg)
LWU $reg, offset($reg)
2) Load an address constant into a register:
LUI $reg, high_part
ORI $reg, low_part
ORI $reg, high_part
DSLL $reg, $reg, 16
ORI $reg, low_part
Q: What would have to change to make this work?
o A new ELF header flag to denote the ABI.
o Linker support to use proper library search paths, and linker scrips
to set the INTERP program header, etc.
o GCC has to emit code for the new ABI.
o Could all existing n32 relocation types be used? I think so.
o Runtime libraries would have to be placed in a new location
(/lib32big, /usr/lib32big ...)
o The C library's ld.so would have to use a distinct LD_LIBRARY_PATH
for n32-big code.
o What would the Linux system call interface be? I would propose
using the existing Linux n32 system call interface. Most system
calls would just work. Some, that pass pointers in in-memory
structures, might require kernel modifications (sigaction() for