4 RV64I Base Integer Instruction Set, Version 2.1

This chapter describes the RV64I base integer instruction set, which builds upon the RV32I variant described in rv32. This chapter presents only the differences with RV32I, so should be read in conjunction with the earlier chapter.

4.1 Register State

RV64I widens the integer registers and supported user address space to 64 bits (XLEN=64 in gprs).

4.2 Integer Computational Instructions

Most integer computational instructions operate on XLEN-bit values. Additional instruction variants are provided to manipulate 32-bit values in RV64I, indicated by a 'W' suffix to the opcode. These "*W" instructions ignore the upper 32 bits of their inputs and always produce 32-bit signed values, sign-extending them to 64 bits, i.e. bits XLEN-1 through 31 are equal.

note

The compiler and calling convention maintain an invariant that all 32-bit values are held in a sign-extended format in 64-bit registers. Even 32-bit unsigned integers extend bit 31 into bits 63 through 32. Consequently, conversion between unsigned and signed 32-bit integers is a no-op, as is conversion from a signed 32-bit integer to a signed 64-bit integer. Existing 64-bit wide SLTU and unsigned branch compares still operate correctly on unsigned 32-bit integers under this invariant. Similarly, existing 64-bit wide logical operations on 32-bit sign-extended integers preserve the sign-extension property. A few new instructions (ADD[I]W/SUBW/SxxW) are required for addition and shifts to ensure reasonable performance for 32-bit values.

4.2.1 Integer Register-Immediate Instructions

ADDIW is an RV64I instruction that adds the sign-extended 12-bit immediate to register rs1 and produces the proper sign extension of a 32-bit result in rd. Overflows are ignored and the result is the low 32 bits of the result sign-extended to 64 bits. Note, ADDIW rd, rs1, 0 writes the sign extension of the lower 32 bits of register rs1 into register rd (assembler pseudoinstruction SEXT.W).

Shifts by a constant are encoded as a specialization of the I-type format using the same instruction opcode as RV32I. The operand to be shifted is in rs1, and the shift amount is encoded in the lower 6 bits of the I-immediate field for RV64I. The right-shift type is encoded in bit 30. SLLI is a logical left shift (zeros are shifted into the lower bits); SRLI is a logical right shift (zeros are shifted into the upper bits); and SRAI is an arithmetic right shift (the original sign bit is copied into the vacated upper bits).

SLLIW, SRLIW, and SRAIW are RV64I-only instructions that are analogously defined but operate on 32-bit values and sign-extend their 32-bit results to 64 bits. SLLIW, SRLIW, and SRAIW encodings with imm[5] ≠ 0 are reserved.

note

Previously, SLLIW, SRLIW, and SRAIW with imm[5] ≠ 0 were defined to cause illegal-instruction exceptions, whereas now they are marked as reserved. This is a backwards-compatible change.

LUI (load upper immediate) uses the same opcode as RV32I. LUI places the 32-bit U-immediate into register rd, filling in the lowest 12 bits with zeros. The 32-bit result is sign-extended to 64 bits.

AUIPC (add upper immediate to pc) uses the same opcode as RV32I. AUIPC is used to build pc-relative addresses and uses the U-type format. AUIPC forms a 32-bit offset from the U-immediate, filling in the lowest 12 bits with zeros, sign-extends the result to 64 bits, adds it to the address of the AUIPC instruction, then places the result in register rd.

note

Note that the set of address offsets that can be formed by pairing LUI with LD, AUIPC with JALR, etc. in RV64I is [−2³¹−2¹¹, 2³¹−2¹¹−1].

4.2.2 Integer Register-Register Operations

ADDW and SUBW are RV64I-only instructions that are defined analogously to ADD and SUB but operate on 32-bit values and produce signed 32-bit results. Overflows are ignored, and the low 32-bits of the result is sign-extended to 64-bits and written to the destination register.

SLL, SRL, and SRA perform logical left, logical right, and arithmetic right shifts on the value in register rs1 by the shift amount held in register rs2. In RV64I, only the low 6 bits of rs2 are considered for the shift amount.

SLLW, SRLW, and SRAW are RV64I-only instructions that are analogously defined but operate on 32-bit values and sign-extend their 32-bit results to 64 bits. The shift amount is given by rs2[4:0].

4.3 Load and Store Instructions

RV64I extends the address space to 64 bits. The execution environment will define what portions of the address space are legal to access.

The LD instruction loads a 64-bit value from memory into register rd for RV64I.

The LW instruction loads a 32-bit value from memory and sign-extends this to 64 bits before storing it in register rd for RV64I. The LWU instruction, on the other hand, zero-extends the 32-bit value from memory for RV64I. LH and LHU are defined analogously for 16-bit values, as are LB and LBU for 8-bit values. The SD, SW, SH, and SB instructions store 64-bit, 32-bit, 16-bit, and 8-bit values from the low bits of register rs2 to memory respectively.

4.4 HINT Instructions

All instructions that are microarchitectural HINTs in RV32I (see rv32) are also HINTs in RV64I.# The additional computational instructions in RV64I expand both the standard and custom HINT encoding spaces.

rv64i-h lists all RV64I HINT code points. 91% of the HINT space is reserved for standard HINTs. The remainder of the HINT space is designated for custom HINTs; no standard HINTs will ever be defined in this subspace.

Instruction	Constraints	Code Points	Purpose
LUI	rd=x0	220	Designated for future standard use
AUIPC	rd=x0	220
ADDI	rd=x0, and either rs1≠x0 or imm≠0	217−1
ANDI	rd=x0	217
ORI	rd=x0	217
XORI	rd=x0	217
ADDIW	rd=x0	217
ADD	rd=x0, rs1≠x0	210−32
ADD	rd=x0, rs1=x0, rs2≠x2-x5	28
ADD	rd=x0, rs1=x0, rs2=x2-x5	4	(rs2=x5) NTL.ALL
SLLI	rd=x0, rs1=x0, shamt=31	1	Semihosting entry marker
SRAI	rd=x0, rs1=x0, shamt=7	1	Semihosting exit marker
SUB	rd=x0	210	Designated for future standard use
AND	rd=x0	210
OR	rd=x0	210
XOR	rd=x0	210
SLL	rd=x0	210
SRL	rd=x0	210
SRA	rd=x0	210
ADDW	rd=x0	210
SUBW	rd=x0	210
SLLW	rd=x0	210
SRLW	rd=x0	210
SRAW	rd=x0	210
FENCE	rd=x0, rs1≠x0, fm=0, and either pred=0 or succ=0	210−63
FENCE	rd≠x0, rs1=x0, fm=0, and either pred=0 or succ=0	210−63
FENCE	rd=rs1=x0, fm=0, pred=0, succ≠0	15
FENCE	rd=rs1=x0, fm=0, pred≠W, succ=0	15
FENCE	rd=rs1=x0, fm=0, pred=W, succ=0	1	PAUSE
SLTI	rd=x0	217	Designated for custom use
SLTIU	rd=x0	217
SLLI	rd=x0, and either rs1≠x0 or shamt≠31	211−1
SRLI	rd=x0	211
SRAI	rd=x0, and either rs1≠x0 or shamt≠7	211−1
SLLIW	rd=x0	210
SRLIW	rd=x0	210
SRAIW	rd=x0	210
SLT	rd=x0	210
SLTU	rd=x0	210

note

slli x0, x0, 0x1f and srai x0, x0, 7 were previously designated as custom HINTs, but they have been appropriated for use in semihosting calls, as described in ecall-ebreak. To reflect their usage in practice, the base ISA spec has been changed to designate them as standard HINTs.