ABI (Application Binary Interface) Standards

The ABI defines the following rules for how applications should exchange binary data:

• Data types and alignment methods
• Register exchange methods for function call arguments and results
• System call invocation methods
• Program code initialization and data initialization methods
• File exchange methods (ELF, etc.)

EABI Standards

EABI (Embedded ABI) deals with ABI for embedded environments. In ARM architecture, the way ABI is used differs based on the Linux version, divided into the following two approaches:

• arm/OABI
  • ABI method used before kernel v2.6.15 (mainline v2.6.16) (also called Old ABI or legacy ABI)
  • Used up to glibc 2.3.6
  • gcc: linux-arm-none-gnu
• arm/EABI
  • ARM EABI method used since kernel v2.6.16
  • Used from glibc v2.3.7 and v2.4
  • gcc: linux-arm-none-gnueabi

Differences between arm/OABI and arm/EABI

Software interrupt calling method

• OABI
  • swi __NR_SYSCALL_BASE(==0x900000)+1
• EABI
  • mov r7, #1 (system call index)
  • swi 0

Structure packaging

• OABI
  • Structures are aligned in 4-byte units
• EABI
  • Uses structure size as-is

Stack argument alignment

• OABI
  • Stored in 4-byte units when saving to stack
• EABI
  • Stored in 8-byte units when saving to stack

64-bit type argument alignment

• OABI
  • Aligned in 4-byte units
• EABI
  • Aligned in 8-byte units

Enum type size

• OABI
  • 4-byte units
• EABI
  • Can be specified as variable

Number of registers for argument passing

• OABI
  • 4 registers (r0~r3)
• EABI
  • 7 registers (r0~r6)

Example of differences

Software interrupt + 64-bit type argument alignment

Example: long sum64(unsigned int start, size_t size); syscall no=100

• arm/OABI
  • Assign start to r0
  • Assign size as 64-bit to r1 and r2
  • swi #(0x900000 + 100)
• arm/EABI
  • Assign start to r0
  • Assign size as 64-bit to r2 and r3
  • Assign 100 to r7
  • swi 0