The base integer calling convention provides eight argument registers, a0-a7, the first two of which are also used to return values.

Scalars that are at most XLEN bits wide are passed in a single argument register, or on the stack by value if none is available. When passed in registers or on the stack, integer scalars narrower than XLEN bits are widened according to the sign of their type up to 32 bits, then sign-extended to XLEN bits. When passed in registers or on the stack, floating-point types narrower than XLEN bits are widened to XLEN bits, with the upper bits undefined.

Scalars that are 2×XLEN bits wide are passed in a pair of argument registers, with the low-order XLEN bits in the lower-numbered register and the high-order XLEN bits in the higher-numbered register. If no argument registers are available, the scalar is passed on the stack by value. If exactly one register is available, the low-order XLEN bits are passed in the register and the high-order XLEN bits are passed on the stack.

Scalars wider than 2×XLEN bits are passed by reference and are replaced in the argument list with the address.

Aggregates whose total size is no more than XLEN bits are passed in a register, with the fields laid out as though they were passed in memory. If no register is available, the aggregate is passed on the stack. Aggregates whose total size is no more than 2×XLEN bits are passed in a pair of registers; if only one register is available, the first XLEN bits are passed in a register and the remaining bits are passed on the stack. If no registers are available, the aggregate is passed on the stack. Bits unused due to padding, and bits past the end of an aggregate whose size in bits is not divisible by XLEN, are undefined.

Aggregates or scalars passed on the stack are aligned to the greater of the type alignment and XLEN bits, but never more than the stack alignment.

Aggregates larger than 2×XLEN bits are passed by reference and are replaced in the argument list with the address, as are C++ aggregates with nontrivial copy constructors, destructors, or vtables.

Empty structs or union arguments or return values are ignored by C compilers which support them as a non-standard extension. This is not the case for C++, which requires them to be sized types.

Bitfields are packed in little-endian fashion. A bitfield that would span the alignment boundary of its integer type is padded to begin at the next alignment boundary. For example, struct { int x : 10; int y : 12; } is a 32-bit type with x in bits 9-0, y in bits 21-10, and bits 31-22 undefined. By contrast, struct { short x : 10; short y : 12; } is a 32-bit type with x in bits 9-0, y in bits 27-16, and bits 31-28 and bits 15-10 undefined.

Bitfields may larger than its integer type, bits excess than its integer type will treat as padding bits, then padding to begin at the next alignment boundary. For example struct { char x : 9; char y; } is a 24 byte type with x in bits 7-0, y in bit 23-16, and bits 15-8 undefined, struct { char x : 9; char y : 2 } is a 16-bit type with x in bits 7-0, y in bit 10-9, and bit 8, bits 15-11 is undefined.

Arguments passed by reference may be modified by the callee.

Floating-point reals are passed the same way as aggregates of the same size; complex floating-point numbers are passed the same way as a struct containing two floating-point reals. (This constraint changes when the integer calling convention is augmented by the hardware floating-point calling convention.)

In the base integer calling convention, variadic arguments are passed in the same manner as named arguments, with one exception. Variadic arguments with 2×XLEN-bit alignment and size at most 2×XLEN bits are passed in an aligned register pair (i.e., the first register in the pair is even-numbered), or on the stack by value if none is available. After a variadic argument has been passed on the stack, all future arguments will also be passed on the stack (i.e. the last argument register may be left unused due to the aligned register pair rule).

Values are returned in the same manner as a first named argument of the same type would be passed. If such an argument would have been passed by reference, the caller allocates memory for the return value, and passes the address as an implicit first parameter.

The stack grows downwards (towards lower addresses) and the stack pointer shall be aligned to a 128-bit boundary upon procedure entry. The first argument passed on the stack is located at offset zero of the stack pointer on function entry; following arguments are stored at correspondingly higher addresses.

In the standard ABI, the stack pointer must remain aligned throughout procedure execution. Non-standard ABI code must realign the stack pointer prior to invoking standard ABI procedures. The operating system must realign the stack pointer prior to invoking a signal handler; hence, POSIX signal handlers need not realign the stack pointer. In systems that service interrupts using the interruptee’s stack, the interrupt service routine must realign the stack pointer if linked with any code that uses a non-standard stack-alignment discipline, but need not realign the stack pointer if all code adheres to the standard ABI.

Procedures must not rely upon the persistence of stack-allocated data whose addresses lie below the stack pointer.

Registers s0-s11 shall be preserved across procedure calls. No floating-point registers, if present, are preserved across calls. (This property changes when the integer calling convention is augmented by the hardware floating-point calling convention.)

The hardware floating-point calling convention adds eight floating-point argument registers, fa0-fa7, the first two of which are also used to return values. Values are passed in floating-point registers whenever possible, whether or not the integer registers have been exhausted.

The remainder of this section applies only to named arguments. Variadic arguments are passed according to the integer calling convention.

ABI_FLEN refers to the width of a floating-point register in the ABI. The ABI_FLEN must be no wider than the ISA’s FLEN. The ISA might have wider floating-point registers than the ABI.

For the purposes of this section, "struct" refers to a C struct with its hierarchy flattened, including any array fields. That is, struct { struct { float f[1]; } g[2]; } and struct { float f; float g; } are treated the same. Fields containing empty structs or unions are ignored while flattening, even in C++, unless they have nontrivial copy constructors or destructors. Fields containing zero-length bit-fields are ignored while flattening. Attributes such as aligned or packed do not interfere with a struct’s eligibility for being passed in registers according to the rules below, i.e. struct { int i; double d; } and struct __attribute__((__packed__)) { int i; double d } are treated the same, as are struct { float f; float g; } and struct { float f; float g __attribute__ ((aligned (8))); }.

A real floating-point argument is passed in a floating-point argument register if it is no more than ABI_FLEN bits wide and at least one floating-point argument register is available. Otherwise, it is passed according to the integer calling convention. When a floating-point argument narrower than FLEN bits is passed in a floating-point register, it is 1-extended (NaN-boxed) to FLEN bits.

A struct containing just one floating-point real is passed as though it were a standalone floating-point real.

A struct containing two floating-point reals is passed in two floating-point registers, if neither real is more than ABI_FLEN bits wide and at least two floating-point argument registers are available. (The registers need not be an aligned pair.) Otherwise, it is passed according to the integer calling convention.

A complex floating-point number, or a struct containing just one complex floating-point number, is passed as though it were a struct containing two floating-point reals.

A struct containing one floating-point real and one integer (or bitfield), in either order, is passed in a floating-point register and an integer register, provided the floating-point real is no more than ABI_FLEN bits wide and the integer is no more than XLEN bits wide, and at least one floating-point argument register and at least one integer argument register is available. If the struct is passed in this manner, and the integer is narrower than XLEN bits, the remaining bits are unspecified. If the struct is not passed in this manner, then it is passed according to the integer calling convention.

Unions are never flattened and are always passed according to the integer calling convention.

Values are returned in the same manner as a first named argument of the same type would be passed.

Floating-point registers fs0-fs11 shall be preserved across procedure calls, provided they hold values no more than ABI_FLEN bits wide.

The ILP32E calling convention is designed to be usable with the RV32E ISA. This calling convention is the same as the integer calling convention, except for the following differences. The stack pointer need only be aligned to a 32-bit boundary. Registers x16-x31 do not participate in the calling convention, so there are only six argument registers, a0-a5, only two callee-saved registers, s0-s1, and only three temporaries, t0-t2.

If used with an ISA that has any of the registers x16-x31 and f0-f31, then these registers are considered temporaries.

The ILP32E calling convention is not compatible with ISAs that have registers that require load and store alignments of more than 32 bits. In particular, this calling convention must not be used with the D ISA extension.

This specification defines the following named ABIs:

The ILP32* ABIs are only compatible with RV32* ISAs, and the LP64* ABIs are only compatible with RV64* ISAs. A future version of this specification may define an ILP32 ABI for the RV64 ISA, but currently this is not a supported operating mode.

The *F ABIs require the *F ISA extension, the *D ABIs require the *D ISA extension, and the LP64Q ABI requires the Q ISA extension.

While various different ABIs are technically possible, for software compatibility reasons it is strongly recommended to use the following default ABIs for specific architectures: