RISC-V Assembly Programmer’s Manual

This document is in the Development state

Assume everything can change. This draft specification will change before being accepted as standard, so implementations made to this draft specification will likely not conform to the future standard.

List of tables

Table 1. Registers of the RV32I. Based on RISC-V documentation and Patterson and Waterman "The RISC-V Reader" (2017)
Table 2. Assembler Directives
Table 3. Assembler Relocation Functions
Table 4. FLI operands reference table
Table 5. Pseudo Instructions
Table 6. Pseudoinstructions for acccessing control and status registers

Copyright and License Information

It is licensed under the Creative Commons Attribution 4.0 International License (CC-BY 4.0).

The full license text is available at creativecommons.org/licenses/by/4.0/

Scope

This document aims to provide guidance to assembly programmers targeting the standard RISC-V assembly language, which common open-source assemblers like GNU as and LLVM’s assembler support. Other assemblers might not support the same directives or pseudoinstructions; their dialects are outside the scope of this document.

Contributors

See github.com/riscv-non-isa/riscv-asm-manual/graphs/contributors.

1. Command-Line Arguments

I think it’s probably better to beef up the binutils documentation rather than duplicating it here.

2. Registers

Registers are the most important part of any processor. RISC-V defines various types, depending on which extensions are included: The general registers (with the program counter), control registers, floating point registers (F extension), and vector registers (V extension).

2.1. General registers

The RV32I base integer ISA includes 32 registers, named x0 to x31. The program counter PC is separate from these registers, in contrast to other processors such as the ARM-32. The first register, x0, has a special function: Reading it always returns 0 and writes to it are ignored. As we will see later, this allows various tricks and simplifications.

In practice, the programmer doesn’t use this notation for the registers. Though x1 to x31 are all equally general-use registers as far as the processor is concerned, by convention certain registers are used for special tasks. In assembler, they are given standardized names as part of the RISC-V application binary interface (ABI). This is what you will usually see in code listings. If you really want to see the numeric register names, the -M argument to objdump will provide them.

Table 1. Registers of the RV32I. Based on RISC-V documentation and Patterson and Waterman "The RISC-V Reader" (2017)
Register	ABI	Use by convention	Preserved?
x0	zero	hardwired to 0, ignores writes	n/a
x1	ra	return address for jumps	no
x2	sp	stack pointer	yes
x3	gp	global pointer	n/a
x4	tp	thread pointer	n/a
x5	t0	temporary register 0	no
x6	t1	temporary register 1	no
x7	t2	temporary register 2	no
x8	s0 or fp	saved register 0 or frame pointer	yes
x9	s1	saved register 1	yes
x10	a0	return value or function argument 0	no
x11	a1	return value or function argument 1	no
x12	a2	function argument 2	no
x13	a3	function argument 3	no
x14	a4	function argument 4	no
x15	a5	function argument 5	no
x16	a6	function argument 6	no
x17	a7	function argument 7	no
x18	s2	saved register 2	yes
x19	s3	saved register 3	yes
x20	s4	saved register 4	yes
x21	s5	saved register 5	yes
x22	s6	saved register 6	yes
x23	s7	saved register 7	yes
x24	s8	saved register 8	yes
x25	s9	saved register 9	yes
x26	s10	saved register 10	yes
x27	s11	saved register 11	yes
x28	t3	temporary register 3	no
x29	t4	temporary register 4	no
x30	t5	temporary register 5	no
x31	t6	temporary register 6	no
pc	(none)	program counter	n/a

As a general rule, the saved registers s0 to s11 are preserved across function calls, while the argument registers a0 to a7 and the temporary registers t0 to t6 are not. The use of the various specialized registers such as sp by convention will be discussed later in more detail.

2.2. Control registers

(TBA)

2.3. Floating Point registers (RV32F)

(TBA)

2.4. Vector registers (RV32V)

(TBA)

3. Addressing

Addressing formats like %pcrel_lo(). We can just link to the RISC-V PS ABI document to describe what the relocations actually do.

4. Instruction Set

Official Specifications webpage:

riscv.org/specifications/

Latest Specifications draft repository:

github.com/riscv/riscv-isa-manual

4.1. Instructions

5. RISC-V ISA Specifications:

riscv.org/specifications/

5.1. Instruction Aliases

ALIAS line from opcodes/riscv-opc.c

To better diagnose situations where the program flow reaches an unexpected location, you might want to emit there an instruction that’s known to trap. You can use an UNIMP pseudoinstruction, which should trap in nearly all systems. The de facto standard implementation of this instruction is:

C.UNIMP: 0000. The all-zeroes pattern is not a valid instruction. Any system which traps on invalid instructions will thus trap on this UNIMP instruction form. Despite not being a valid instruction, it still fits the 16-bit (compressed) instruction format, and so 0000 0000 is interpreted as being two 16-bit UNIMP instructions.
UNIMP : C0001073. This is an alias for CSRRW x0, cycle, x0. Since cycle is a read-only CSR, then (whether this CSR exists or not) an attempt to write into it will generate an illegal instruction exception. This 32-bit form of UNIMP is emitted when targeting a system without the C extension, or when the .option norvc directive is used.

5.2. Pseudo Ops

Both the RISC-V-specific and GNU .-prefixed options.

The following table lists assembler directives:

Table 2. Assembler Directives
Directive	Arguments	Description
.align	integer	align to power of 2 (alias for `.p2align` which is preferred - see .align
.p2align	p2,[pad_val=0],max	align to power of 2
.balign	b,[pad_val=0]	byte align
.file	"filename"	emit filename FILE LOCAL symbol table
.globl	symbol_name	emit symbol_name to symbol table (scope GLOBAL)
.local	symbol_name	emit symbol_name to symbol table (scope LOCAL)
.comm	symbol_name,size,align	emit common object to .bss section
.common	symbol_name,size,align	emit common object to .bss section
.ident	"string"	accepted for source compatibility
.section	[{.text,.data,.rodata,.bss}]	emit section (if not present, default .text) and make current
.size	symbol, symbol	accepted for source compatibility
.text		emit .text section (if not present) and make current
.data		emit .data section (if not present) and make current
.rodata		emit .rodata section (if not present) and make current
.bss		emit .bss section (if not present) and make current
.string	"string"	emit string
.asciz	"string"	emit string (alias for .string)
.equ	name, value	constant definition
.macro	name arg1 [, argn]	begin macro definition \argname to substitute
.endm		end macro definition
.type	symbol, @function	accepted for source compatibility
.option	{arch,rvc,norvc,pic,nopic,relax,norelax,push,pop}	RISC-V options. Refer to .option for a more detailed description.
.byte	expression [, expression]*	8-bit comma separated words
.2byte	expression [, expression]*	16-bit comma separated words
.half	expression [, expression]*	16-bit comma separated words
.short	expression [, expression]*	16-bit comma separated words
.4byte	expression [, expression]*	32-bit comma separated words
.word	expression [, expression]*	32-bit comma separated words
.long	expression [, expression]*	32-bit comma separated words
.8byte	expression [, expression]*	64-bit comma separated words
.dword	expression [, expression]*	64-bit comma separated words
.quad	expression [, expression]*	64-bit comma separated words
.float	expression [, expression]*	32-bit floating point values, see Floating-point literals for the value format.
.double	expression [, expression]*	64-bit floating point values, see Floating-point literals for the value format.
.quad	expression [, expression]*	128-bit floating point values, see Floating-point literals for the value format.
.dtprelword	expression [, expression]*	32-bit thread local word
.dtpreldword	expression [, expression]*	64-bit thread local word
.sleb128	expression	signed little endian base 128, DWARF
.uleb128	expression	unsigned little endian base 128, DWARF
.zero	integer	zero bytes
.variant_cc	symbol_name	annotate the symbol with variant calling convention
.attribute	name, value	RISC-V object attributes, more detailed description see .attribute.
.insn	see description	emit a custom instruction encoding, see .insn

6. `.align`

The .align directive for RISC-V is an alias to .p2align, which aligns to a power of two, so .align 2 means align to 4 bytes. Because the definition of the .align directive varies by architecture, it is recommended to use the unambiguous .p2align or .balign directives instead.

7. `.attribute`

The .attribute directive is used to record information about an object file/binary that a linker or runtime loader needs to check for compatibility.

For more information like attribute name, number, value type and description, please refer to attribute section in RISC-V psABI.

.attribute take two arguments. The first argument of .attribute is the symbolic name of attribute or the attribute number, the prefix Tag_RISCV_ can be omitted, the second argument can be string or number.

Syntax for .attribute:

.attribute <NAME_OR_NUMBER>, <ATTRIBUTE_VALUE>

NAME_OR_NUMBER := <attribute-name>
                | [1-9][0-9]*

ATTRIBUTE_VALUE := <string>
                 | <number>

8. `.option`

8.1. `rvc`/`norvc`

This option will be deprecated soon after .option arch has been widely implemented on main stream open source toolchains.

Enable/disable the C-extension for the following code region. This option is equivalent to .option arch, +c/.option arch, -c, but widely supported by older toolchain versions.

Alternative style:

.option push
.option arch, +c   # Alternative of .option rvc
.option pop

.option push
.option arch, -c   # Alternative of .option norvc
.option pop

.option rvc might set the ELF flag EF_RISCV_RVC in some toolchains. That might cause the linker to compress instructions in code regions where that was not intended.

There is a difference between .option rvc/.option norvc and .option arch, +c/.option arch, -c. The latter won’t set EF_RISCV_RVC in the ELF flags.

8.2. `arch`

Enable and/or disable specific ISA extensions for the following code regions, but without changing the arch attribute and EF_RISCV_RVC in the ELF flags, that means it will not raise the minimal execution environment requirement, so the user should take care to the execution of the code regions around .option push/.option arch/.option pop.

Syntax for .option arch:

.option arch, <EXTENSIONS-OR-FULLARCH>

EXTENSIONS-OR-FULLARCH := <EXTENSIONS>
                        | <FULLARCHSTR>

EXTENSIONS             := <EXTENSION> ',' <EXTENSIONS>
                        | <EXTENSION>

FULLARCHSTR            := <full-arch-string>

EXTENSION              := <OP> <EXTENSION-NAME> <VERSION>

OP                     := '+'
                        | '-'

VERSION                := [0-9]+ 'p' [0-9]+
                        | [1-9][0-9]*
                        |

EXTENSION-NAME         := Naming rule is defined in RISC-V ISA manual

Extension version can be omitted, the assembler will use the built-in default version for that extension.
OP can be enable (+) or disable (-).
Format of <full-arch-string> is the same as -march option.

Example:

.attribute arch, rv64imafdc
# You can only use instructions from the i, m, a, f, d and c extensions.
memcpy_general:
    add     a5,a1,a2
    beq     a1,a5,.L2
    add     a2,a0,a2
    mv      a5,a0
.L3:
    addi    a1,a1,1
    addi    a5,a5,1
    lbu     a4,-1(a1)
    sb      a4,-1(a5)
    bne     a5,a2,.L3
.L2:
    ret

.option push     # Push current options to the stack.
.option arch, +v # Enable vector extension, we can use any instruction in imafdcv extension.
memcpy_vec:
    mv a3, a0
.Lloop:
    vsetvli t0, a2, e8, m8, ta, ma
    vle8.v v0, (a1)
    add a1, a1, t0
    sub a2, a2, t0
    vse8.v v0, (a3)
    add a3, a3, t0
    bnez a2, .Lloop
    ret
.option pop   # Pop current option from the stack, restore the enabled ISA extension status to imafdc.

.option push     # Push current option to the stack.
.option arch, -c # Disable compressed extension, we can't use any instruction in extension.
memcpy_norvc:
    add     a5,a1,a2
    beq     a1,a5,.L2
    add     a2,a0,a2
    mv      a5,a0
.L3:
    addi    a1,a1,1
    addi    a5,a5,1
    lbu     a4,-1(a1)
    sb      a4,-1(a5)
    bne     a5,a2,.L3
.L2:
    ret
.option pop   # Pop current option from the stack, restore the enabled ISA extension status to imafdc.

.option push  # Push current option to the stack.
.option arch, rv64imc # Set arch to rv64imc.
    nop
.option pop   # Pop current option from the stack, restore the enabled ISA extension status to imafdc.

A typical use case is with ifunc, e.g. the C library is built with rv64gc, but a few functions like memcpy provide two versions, one built with rv64gc and one built with rv64gcv, and then select between them by ifunc mechanism at run-time. However, we don’t want to change the minimal execution environment requirement to rv64gcv, since the rv64gcv version will be invoked only if the execution environment supports the vector extension, so the minimal execution environment requirement still is rv64gc.

.option arch, + will also enable all required extensions, for example, rv32i + .option arch, +v will also enable f, d, zve32x, zve32f, zve64x, zve64f, zve64d, zvl32b, zvl64b and zvl128b extensions.

We recommend .option arch, + and .option arch, - are used with .option push/.option pop instead of a .option arch, + / .option arch, - pair, because .option arch, + will enable all required extensions, but .option arch, - only disables the specific extension, so the result might be unexpected, for example: rv32i + .option arch, +v + .option arch, -v will result rv32ifd_zve32x_zve32f_zve64x_zve64f_zve64d_zvl32b_zvl64b_zvl128b not rv32i. Another example is .option arch, rv64ifd + .option arch, -f, which results in rv64ifd, because f will be added back when adding the implied extensions of d.

.option arch, +<ext>, -<ext> is accepted and will result in enabling the extensions that depend on ext, e.g. rv32i + .option arch, +v, -v will result rv32ifd_zve32x_zve32f_zve64x_zve64f_zve64d_zvl32b_zvl64b_zvl128b.

8.3. `pic`/`nopic`

Set the code model to PIC (position independent code) or non-PIC. This will affect the expansion of the la pseudoinstruction, refer to listing of standard RISC-V pseudoinstructions.

8.4. `relax`/`norelax`

Enable/disable linker relaxation for the following code region.

A code region followed by .option relax will emit R_RISCV_RELAX/R_RISCV_ALIGN even if the linker does not support relaxation. The suggested usage is using .option norelax with .option push/.option pop if linker relaxation should be disabled for a code region.

Recommended way to disable linker relaxation of specific code region is use .option push, .option norelax and .option pop, that prevent enabled linker relaxation accidentally if user already disable linker relaxation.

8.5. `push`/`pop`

Push/pop current options to/from the options stack.

9. `.insn`

Emit an arbitrary instruction. This is useful for custom instructions or for very new instructions which an assembler may not support.

There are three overloads:

.insn <value> - emit a raw instruction with the given value
.insn <insn_length>, <value> - the same, but also verify that the instruction length has the given value in bytes
.insn <type> <fields>

<type> is the instruction type (e.g. r, i, s, cj, …). These types are specified in the RISC-V ISA specification.

<fields> is a comma-separated list of the instruction fields. The order of the fields is achieved by grouping them and listing them from LSB to MSB. The groups are:

opcode fields
function fields
register fields
immediates and symbols

E.g. an instruction with the fields (sorted from LSB to MSB):

opcode7, rd, func3, rs1, rs2, func7

Gets listed as follows:

opcode7, func3, func7, rd, rs1, rs2

For more examples, refer to the Binutils documentation.

10. Assembler Relocation Functions

The following table lists assembler relocation expansions:

Table 3. Assembler Relocation Functions
Assembler Notation	Description	Instruction/Macro
%hi(symbol)	Absolute (HI20)	lui
%lo(symbol)	Absolute (LO12)	load, store, add
%pcrel_hi(symbol)	PC-relative (HI20)	auipc
%pcrel_lo(label)	PC-relative (LO12)	load, store, add
%tprel_hi(symbol)	TLS LE "Local Exec"	lui
%tprel_lo(symbol)	TLS LE "Local Exec"	load, store, add
%tprel_add(symbol)	TLS LE "Local Exec"	add
%tls_ie_pcrel_hi(symbol) *	TLS IE "Initial Exec" (HI20)	auipc
%tls_gd_pcrel_hi(symbol) *	TLS GD "Global Dynamic" (HI20)	auipc
%got_pcrel_hi(symbol) *	GOT PC-relative (HI20)	auipc

* These reuse %pcrel_lo(label) for their lower half

11. Labels

Text labels are used as branch, unconditional jump targets and symbol offsets. Text labels are added to the symbol table of the compiled module.

loop:
        j loop

Numeric labels are used for local references. References to local labels are suffixed with 'f' for a forward reference or 'b' for a backwards reference.

1:
        j 1b

12. Absolute addressing

The following example shows how to load an absolute address:

  lui a0, %hi(msg + 1)
  addi  a0, a0, %lo(msg + 1)

Which generates the following assembler output and relocations as seen by objdump:

0000000000000000 <.text>:
   0: 00000537            lui a0,0x0
      0: R_RISCV_HI20 msg+0x1
   4: 00150513            addi  a0,a0,1 # 0x1
      4: R_RISCV_LO12_I msg+0x1

13. Relative addressing

The following example shows how to load a PC-relative address:

1:
  auipc a0, %pcrel_hi(msg + 1)
  addi  a0, a0, %pcrel_lo(1b)

Which generates the following assembler output and relocations as seen by objdump:

0000000000000000 <.text>:
   0: 00000517            auipc a0,0x0
      0: R_RISCV_PCREL_HI20 msg+0x1
   4: 00050513            mv  a0,a0
      4: R_RISCV_PCREL_LO12_I .L1

14. GOT-indirect addressing

The following example shows how to load an address from the GOT:

1:
  auipc a0, %got_pcrel_hi(msg + 1)
  ld  a0, %pcrel_lo(1b)(a0)

Which generates the following assembler output and relocations as seen by objdump:

0000000000000000 <.text>:
   0: 00000517            auipc a0,0x0
      0: R_RISCV_GOT_HI20 msg+0x1
   4: 00050513            mv  a0,a0
      4: R_RISCV_PCREL_LO12_I .L1

15. Load Immediate

The following example shows the li pseudoinstruction which is used to load immediate values:

  .equ  CONSTANT, 0xdeadbeef

  li  a0, CONSTANT

Which, for RV32I, generates the following assembler output, as seen by objdump:

00000000 <.text>:
   0: deadc537            lui a0,0xdeadc
   4: eef50513            addi  a0,a0,-273 # deadbeef <CONSTANT+0x0>

16. Load Upper Immediate’s Immediate

The immediate argument to lui is an integer in the interval [0x0, 0xfffff]. Its compressed form, c.lui, accepts only those in the subintervals [0x1, 0x1f] and [0xfffe0, 0xfffff].

17. Signed Immediates for I- and S-Type Instructions

All I- and S-type instructions with 12-bit signed immediates --- e.g., addi but not slli --- accept their immediate argument as an integer in the interval [-2048, 2047]. Integers in the subinterval [-2048, -1] can also be passed by their (unsigned) associates in the interval [0xfffff800, 0xffffffff] on RV32I, and in [0xfffffffffffff800, 0xffffffffffffffff] on both RV32I and RV64I.

18. Floating-point literals

The assembler supports the same floating-point literal formats as those defined in the C and C++ standards (i.e., decimal floating-point literals with decimal exponents as well as hexadecimal floating-point literals with binary exponents).

Here are some examples:

3.14159
0.271828e1
0x0.3p-4

The detailed format of the floating point immediate value can be referenced on this page.

19. Load Floating-point Immediate

The Zfa extension introduces fli.{h|s|d|q} instructions for loading a specific set of floating-point immediates, supported values can be found in the RISC-V ISA specification but are also listed below.

The fli instruction is used to load a floating point immediate into a floating register, the accepted immediate is defined in Floating-point literals and the reference table can be found in FLI operands reference table.

  fli.s fa0, 0x1p-15
  fli.s fa1, 0.00390625
  fli.s fa2, 6.25e-02

The tool should reject any value that does not exactly match a floating-point immediate operand for the 'fli' instruction.

RISC-V does not offer a generic pseudoinstruction to load an arbitrary floating point immediate value. Instead, a programmer can use the .float/.double directive to declare a floating point immediate value in the source code, and then load it into a floating point register using the load global pseudoinstruction (fl{h|w|d|q}).

  .data
.VAL:
  .float .0x1p+17
  .text
  flw fa0, .VAL, t0

Table 4. FLI operands reference table
Value	Example legal input values
-1.0	-0x1p+0, -1.0, -1.0e+0
Minimum positive normal	min
1.0 x 2 ^ -16	0x1p-16, 0.0000152587890625, 1.52587890625e-05
1.0 x 2 ^ -15	0x1p-15, 0.000030517578125, 3.0517578125e-05
1.0 x 2 ^ -8	0x1p-8, 0.00390625, 3.90625e-03
1.0 x 2 ^ -7	0x1p-7, 0.0078125, 7.8125e-03
0.0625 (2 ^ -4)	0x1p-4, 0.0625, 6.25e-02
0.125 (2 ^ -3)	0x1p-3, 0.125, 1.25e-01
0.25	0x1p-2, 0.25, 2.5e-01
0.3125	0x1.4p-2, 0.3125, 3.125e-01
0.375	0x1.8p-2, 0.375, 3.75e-01
0.4375	0x1.cp-2, 0.4375, 4.375e-01
0.5	0x1p-1, 0.5, 5.0e-01
0.625	0x1.4p-1, 0.625, 6.25e-01
0.75	0x1.8p-1, 0.75, 7.5e-01
0.875	0x1.cp-1, 0.875, 8.75e-01
1.0	0x1p+0, 1.0, 1.0e+00
1.25	0x1.4p+0, 1.25, 1.25e+00
1.5	0x1.8p+0, 1.5, 1.5e+00
1.75	0x1.cp+0, 1.75, 1.75e+00
2.0	0x1p+1, 2.0, 2.0e+00
2.5	0x1.4p+1, 2.5, 2.5e+00
3	0x1.8p+1, 3.0, 3.0e+00
4	0x1p+2, 4.0, 4.0e+00
8	0x1p+3, 8.0, 8.0e+00
16	0x1p+4, 16.0, 1.6e+01
128 (2 ^ 7)	0x1p+7, 128.0, 1.28e+02
256 (2 ^ 8)	0x1p+8, 256.0, 2.56e+02
2 ^ 15	0x1p+15, 32768.0, 3.2768e+04
2 ^ 16	0x1p+16, 65536.0, 6.5536e+04
Positive infinity	inf
Canonical NaN	nan

A value can be expressed in various forms within the same format. For example, 6.5536e+04 can be alternatively written as 6553.6e+01 or 65.536e+03. The table provides one possible representation, but any equivalent exact value may be used.

20. Load Address

The following example shows the la pseudoinstruction which is used to load symbol addresses using the correct sequence based on whether the code is being assembled as PIC:

  la  a0, msg + 1

For non-PIC this is an alias for the lla pseudoinstruction documented below.

For PIC this is an alias for the lga pseudoinstruction documented below.

The la pseudoinstruction is the preferred way for getting the address of variables in assembly unless explicit control over PC-relative or GOT-indirect addressing is required.

21. Load Local Address

The following example shows the lla pseudoinstruction which is used to load local symbol addresses:

  lla a0, msg + 1

This generates the following instructions and relocations as seen by objdump:

0000000000000000 <.text>:
   0: 00000517            auipc a0,0x0
      0: R_RISCV_PCREL_HI20 msg+0x1
   4: 00050513            mv  a0,a0
      4: R_RISCV_PCREL_LO12_I .L0

22. Load Global Address

The following example shows the lga pseudoinstruction which is used to load global symbol addresses:

  lga a0, msg + 1

This generates the following instructions and relocations as seen by objdump (for RV64; RV32 will use lw instead of ld):

0000000000000000 <.text>:
   0: 00000517            auipc a0,0x0
      0: R_RISCV_GOT_HI20 msg+0x1
   4: 00053503            ld  a0,0(a0) # 0 <.text>
      4: R_RISCV_PCREL_LO12_I .L0

23. Load and Store Global

The following pseudoinstructions are available to load from and store to global objects:

l{b|h|w|d} <rd>, <symbol>: load byte, half word, word or double word from global ^[1]
l{bu|hu|wu} <rd>, <symbol>: load unsigned byte, half word, or word from global ^[1]
s{b|h|w|d} <rd>, <symbol>, <rt>: store byte, half word, word or double word to global ^[2]
fl{h|w|d|q} <rd>, <symbol>, <rt>: load half, float, double or quad precision from global ^[2]
fs{h|w|d|q} <rd>, <symbol>, <rt>: store half, float, double or quad precision to global ^[2]

The following example shows how these pseudoinstructions are used:

  lw  a0, var1
  fld fa0, var2, t0
  sw  a0, var3, t0
  fsd fa0, var4, t0

Which generates the following assembler output and relocations as seen by objdump:

0000000000000000 <.text>:
   0: 00000517            auipc a0,0x0
      0: R_RISCV_PCREL_HI20 var1
   4: 00052503            lw  a0,0(a0) # 0 <.text>
      4: R_RISCV_PCREL_LO12_I .L0
   8: 00000297            auipc t0,0x0
      8: R_RISCV_PCREL_HI20 var2
   c: 0002b507            fld fa0,0(t0) # 8 <.text+0x8>
      c: R_RISCV_PCREL_LO12_I .L0
  10: 00000297            auipc t0,0x0
      10: R_RISCV_PCREL_HI20  var3
  14: 00a2a023            sw  a0,0(t0) # 10 <.text+0x10>
      14: R_RISCV_PCREL_LO12_S  .L0
  18: 00000297            auipc t0,0x0
      18: R_RISCV_PCREL_HI20  var4
  1c: 00a2b027            fsd fa0,0(t0) # 18 <.text+0x18>
      1c: R_RISCV_PCREL_LO12_S  .L0

24. Constants

The following example shows loading a constant using the %hi and %lo assembler functions.

  .equ  UART_BASE, 0x40003080

  lui a0, %hi(UART_BASE)
  addi  a0, a0, %lo(UART_BASE)

Which generates the following assembler output as seen by objdump:

0000000000000000 <.text>:
   0: 40003537            lui a0,0x40003
   4: 08050513            addi  a0,a0,128 # 40003080 <UART_BASE>

25. Far Branches

The assembler will convert conditional branches into far branches when necessary, via inserting a short branch with inverted conditions past an unconditional jump. For example

target:
  bne a0, a1, target
.rep 1024
  nop
.endr
  bne a0, a1, target

ends up as

       0: 00b51063            bne a0,a1,0 <target>
...
    1004: 00b50463            beq a0,a1,100c <target+0x100c>
    1008: ff9fe06f            j 0 <target>

26. Function Calls

The following pseudoinstructions are available to call subroutines far from the current position:

call <symbol>: call away subroutine ^[3]
call <rd>, <symbol>: call away subroutine ^[4]
tail <symbol>: tail call away subroutine[ ^[5]
jump <symbol>, <rt>: jump to away routine ^[6]

The following example shows how these pseudoinstructions are used:

  call  func1
  tail  func2
  jump  func3, t0

Which generates the following assembler output and relocations as seen by objdump:

0000000000000000 <.text>:
   0: 00000097            auipc ra,0x0
      0: R_RISCV_CALL func1
   4: 000080e7            jalr  ra # 0x0
   8: 00000317            auipc t1,0x0
      8: R_RISCV_CALL func2
   c: 00030067            jr  t1 # 0x8
  10: 00000297            auipc t0,0x0
      10: R_RISCV_CALL  func3
  14: 00028067            jr  t0 # 0x10

27. Floating-point rounding modes

For floating-point instructions with a rounding mode field, the rounding mode can be specified by adding an additional operand. e.g. fcvt.w.s with round-to-zero can be written as fcvt.w.s a0, fa0, rtz. If unspecified, the default dyn rounding mode will be used.

Supported rounding modes are as follows (must be specified in lowercase):

rne: round to nearest, ties to even
rtz: round towards zero
rdn: round down
rup: round up
rmm: round to nearest, ties to max magnitude
dyn: dynamic rounding mode (the rounding mode specified in the frm field of the fcsr register is used)

28. Control and Status Registers

The following code sample shows how to enable timer interrupts, set and wait for a timer interrupt to occur:

.equ RTC_BASE,      0x40000000
.equ TIMER_BASE,    0x40004000

# setup machine trap vector
1:      auipc   t0, %pcrel_hi(mtvec)        # load mtvec(hi)
        addi    t0, t0, %pcrel_lo(1b)       # load mtvec(lo)
        csrrw   zero, mtvec, t0

# set mstatus.MIE=1 (enable M mode interrupt)
        li      t0, 8
        csrrs   zero, mstatus, t0

# set mie.MTIE=1 (enable M mode timer interrupts)
        li      t0, 128
        csrrs   zero, mie, t0

# read from mtime
        li      a0, RTC_BASE
        ld      a1, 0(a0)

# write to mtimecmp
        li      a0, TIMER_BASE
        li      t0, 1000000000
        add     a1, a1, t0
        sd      a1, 0(a0)

# loop
loop:
        wfi
        j loop

# break on interrupt
mtvec:
        csrrc  t0, mcause, zero
        bgez t0, fail       # interrupt causes are less than zero
        slli t0, t0, 1      # shift off high bit
        srli t0, t0, 1
        li t1, 7            # check this is an m_timer interrupt
        bne t0, t1, fail
        j pass

pass:
        la a0, pass_msg
        jal puts
        j shutdown

fail:
        la a0, fail_msg
        jal puts
        j shutdown

.section .rodata

pass_msg:
        .string "PASS\n"

fail_msg:
        .string "FAIL\n"

29. A listing of standard RISC-V pseudoinstructions

Table 5. Pseudo Instructions
Pseudoinstruction	Base Instruction(s)	Meaning	Comment
la rd, symbol	auipc rd, symbol[31:12] addi rd, rd, symbol[11:0]	Load address	With `.option nopic` (Default)
la rd, symbol	auipc rd, symbol@GOT[31:12] l{w\|d} rd, symbol@GOT[11:0](rd)	Load address	With `.option pic`
lla rd, symbol	auipc rd, symbol[31:12] addi rd, rd, symbol[11:0]	Load local address
lga rd, symbol	auipc rd, symbol@GOT[31:12] l{w\|d} rd, symbol@GOT[11:0](rd)	Load global address
l{b\|h\|w\|d} rd, symbol	auipc rd, symbol[31:12] l{b\|h\|w\|d} rd, symbol[11:0](rd)	Load global
l{bu\|hu\|wu} rd, symbol	auipc rd, symbol[31:12] l{bu\|hu\|wu} rd, symbol[11:0](rd)	Load global, unsigned
s{b\|h\|w\|d} rd, symbol, rt	auipc rt, symbol[31:12] s{b\|h\|w\|d} rd, symbol[11:0](rt)	Store global
fl{w\|d} rd, symbol, rt	auipc rt, symbol[31:12] fl{w\|d} rd, symbol[11:0](rt)	Floating-point load global
fs{w\|d} rd, symbol, rt	auipc rt, symbol[31:12] fs{w\|d} rd, symbol[11:0](rt)	Floating-point store global
nop	addi x0, x0, 0	No operation
li rd, immediate	*Myriad sequences^[7]	Load immediate
mv rd, rs	addi rd, rs, 0	Copy register
not rd, rs	xori rd, rs, -1	Ones’ complement
neg rd, rs	sub rd, x0, rs	Two’s complement
negw rd, rs	subw rd, x0, rs	Two’s complement word
sext.b rd, rs	slli rd, rs, XLEN - 8 srai rd, rd, XLEN - 8	Sign extend byte	This is a single instruction when `Zbb` extension is available.
sext.h rd, rs	slli rd, rs, XLEN - 16 srai rd, rd, XLEN - 16	Sign extend halfword	This is a single instruction when `Zbb` extension is available.
sext.w rd, rs	addiw rd, rs, 0	Sign extend word
zext.b rd, rs	andi rd, rs, 255	Zero extend byte
zext.h rd, rs	slli rd, rs, XLEN - 16 srli rd, rd, XLEN - 16	Zero extend halfword	This is a single instruction when `Zbb` extension is available.
zext.w rd, rs	slli rd, rs, XLEN - 32 srli rd, rd, XLEN - 32	Zero extend word	This is a single instruction when `Zba` extension is available.
seqz rd, rs	sltiu rd, rs, 1	Set if = zero
snez rd, rs	sltu rd, x0, rs	Set if != zero
sltz rd, rs	slt rd, rs, x0	Set if < zero
sgtz rd, rs	slt rd, x0, rs	Set if > zero
fmv.s rd, rs	fsgnj.s rd, rs, rs	Copy single-precision register
fabs.s rd, rs	fsgnjx.s rd, rs, rs	Single-precision absolute value
fneg.s rd, rs	fsgnjn.s rd, rs, rs	Single-precision negate
fmv.d rd, rs	fsgnj.d rd, rs, rs	Copy double-precision register
fabs.d rd, rs	fsgnjx.d rd, rs, rs	Double-precision absolute value
fneg.d rd, rs	fsgnjn.d rd, rs, rs	Double-precision negate
beqz rs, offset	beq rs, x0, offset	Branch if = zero
bnez rs, offset	bne rs, x0, offset	Branch if != zero
blez rs, offset	bge x0, rs, offset	Branch if ≤ zero
bgez rs, offset	bge rs, x0, offset	Branch if ≥ zero
bltz rs, offset	blt rs, x0, offset	Branch if < zero
bgtz rs, offset	blt x0, rs, offset	Branch if > zero
bgt rs, rt, offset	blt rt, rs, offset	Branch if >
ble rs, rt, offset	bge rt, rs, offset	Branch if ≤
bgtu rs, rt, offset	bltu rt, rs, offset	Branch if >, unsigned
bleu rs, rt, offset	bgeu rt, rs, offset	Branch if ≤, unsigned
j offset	jal x0, offset	Jump
jal offset	jal x1, offset	Jump and link
jr rs	jalr x0, rs, 0	Jump register
jalr rs	jalr x1, rs, 0	Jump and link register
ret	jalr x0, x1, 0	Return from subroutine
call offset	auipc x1, offset[31:12] jalr x1, x1, offset[11:0]	Call far-away subroutine
tail offset	auipc x6, offset[31:12] jalr x0, x6, offset[11:0]	Tail call far-away subroutine Tail call far-away subroutine	It will use `x7` as scratch register when `Zicfilp` extension is available.
fence	fence iorw, iorw	Fence on all memory and I/O
pause	fence w, 0	PAUSE hint

30. Pseudoinstructions for accessing control and status registers

Table 6. Pseudoinstructions for acccessing control and status registers
Pseudoinstruction	Base Instruction(s)	Meaning
rdinstret[h] rd	csrrs rd, instret[h], x0	Read instructions-retired counter
rdcycle[h] rd	csrrs rd, cycle[h], x0	Read cycle counter
rdtime[h] rd	csrrs rd, time[h], x0	Read real-time clock
csrr rd, csr	csrrs rd, csr, x0	Read CSR
csrw csr, rs	csrrw x0, csr, rs	Write CSR
csrs csr, rs	csrrs x0, csr, rs	Set bits in CSR
csrc csr, rs	csrrc x0, csr, rs	Clear bits in CSR
csrwi csr, imm	csrrwi x0, csr, imm	Write CSR, immediate
csrsi csr, imm	csrrsi x0, csr, imm	Set bits in CSR, immediate
csrci csr, imm	csrrci x0, csr, imm	Clear bits in CSR, immediate
frcsr rd	csrrs rd, fcsr, x0	Read FP control/status register
fscsr rd, rs	csrrw rd, fcsr, rs	Swap FP control/status register
fscsr rs	csrrw x0, fcsr, rs	Write FP control/status register
frrm rd	csrrs rd, frm, x0	Read FP rounding mode
fsrm rd, rs	csrrw rd, frm, rs	Swap FP rounding mode
fsrm rs	csrrw x0, frm, rs	Write FP rounding mode
fsrmi rd, imm	csrrwi rd, frm, imm	Swap FP rounding mode, immediate
fsrmi imm	csrrwi x0, frm, imm	Write FP rounding mode, immediate
frflags rd	csrrs rd, fflags, x0	Read FP exception flags
fsflags rd, rs	csrrw rd, fflags, rs	Swap FP exception flags
fsflags rs	csrrw x0, fflags, rs	Write FP exception flags
fsflagsi rd, imm	csrrwi rd, fflags, imm	Swap FP exception flags, immediate
fsflagsi imm	csrrwi x0, fflags, imm	Write FP exception flags, immediate

1. the first operand is implicitly used as a scratch register.

2. the last operand specifies the scratch register to be used.

3. ra is implicitly used to save the return address.

4. similar to call <symbol>, but <rd> is used to save the return address instead.

5. If the Zicfilp extension is available, t2 is implicitly used as a scratch register. Otherwise,t1 is implicitly used as a scratch register.

6. similar to tail <symbol>, but <rt> is used as the scratch register instead.

7. The compiler can generate different instruction sequences to load a specific numeric value into a register.

RISC-V Assembly Programmer’s Manual

List of tables

Copyright and License Information

Scope

Contributors

1. Command-Line Arguments

2. Registers

2.1. General registers

2.2. Control registers

2.3. Floating Point registers (RV32F)

2.4. Vector registers (RV32V)

3. Addressing

4. Instruction Set

4.1. Instructions

5. RISC-V ISA Specifications:

5.1. Instruction Aliases

5.2. Pseudo Ops

6. .align

7. .attribute

8. .option

8.1. rvc/norvc

8.2. arch

8.3. pic/nopic

8.4. relax/norelax

8.5. push/pop

9. .insn

10. Assembler Relocation Functions

11. Labels

12. Absolute addressing

13. Relative addressing

14. GOT-indirect addressing

15. Load Immediate

16. Load Upper Immediate’s Immediate

17. Signed Immediates for I- and S-Type Instructions

18. Floating-point literals

19. Load Floating-point Immediate

20. Load Address

21. Load Local Address

22. Load Global Address

23. Load and Store Global

24. Constants

25. Far Branches

26. Function Calls

27. Floating-point rounding modes

28. Control and Status Registers

29. A listing of standard RISC-V pseudoinstructions

30. Pseudoinstructions for accessing control and status registers

6. `.align`

7. `.attribute`

8. `.option`

8.1. `rvc`/`norvc`

8.2. `arch`

8.3. `pic`/`nopic`

8.4. `relax`/`norelax`

8.5. `push`/`pop`

9. `.insn`