On 02/14/2011 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. 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) 2) Load an address constant into a register: n32: LUI $reg, high_part ORI $reg, low_part
That is not reality. Really it is: LUI $reg, R_MIPS_HI16 ADDIU $reg, R_MIPS_LO16
n32-big: ORI $reg, high_part DSLL $reg, $reg, 16 ORI $reg, low_part
This one would really be: ORI $reg, R_MIPS_HI16 DSLL $reg, $reg, 16 ADDIU $reg, R_MIPS_LO16
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 example).
