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RISC-V Assembly Programmer's Manual

Copyright and License Information

The RISC-V Assembly Programmer's Manual is

© 2017 Palmer Dabbelt palmer@dabbelt.com © 2017 Michael Clark michaeljclark@mac.com © 2017 Alex Bradbury asb@lowrisc.org

It is licensed under the Creative Commons Attribution 4.0 International License (CC-BY 4.0). The full license text is available at https://creativecommons.org/licenses/by/4.0/.

Command-Line Arguments

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

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).

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.

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 6 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

Registers of the RV32I. Based on RISC-V documentation and Patterson and Waterman "The RISC-V Reader" (2017)

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.

Control registers

(TBA)

Floating Point registers (RV32F)

(TBA)

Vector registers (RV32V)

(TBA)

Addressing

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

Instruction Set

Official Specifications webpage:

Latest Specifications draft repository:

Instructions

RISC-V User Level ISA Specification

https://riscv.org/specifications/

RISC-V Privileged ISA Specification

https://riscv.org/specifications/privileged-isa/

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 pseudo-instruction, 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.

Pseudo Ops

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

The following table lists assembler directives:

Directive Arguments Description
.align integer align to power of 2 (alias for .p2align)
.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 {rvc,norvc,pic,nopic,push,pop} RISC-V options
.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
.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
.p2align p2,[pad_val=0],max align to power of 2
.balign b,[pad_val=0] byte align
.zero integer zero bytes

Assembler Relocation Functions

The following table lists assembler relocation expansions:

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

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

Absolute addressing

The following example shows how to load an absolute address:

.section .text
.globl _start
_start:
	    lui a0,       %hi(msg)       # load msg(hi)
	    addi a0, a0,  %lo(msg)       # load msg(lo)
	    jal ra, puts
2:	    j 2b

.section .rodata
msg:
	    .string "Hello World\n"

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

0000000000000000 <_start>:
   0:	000005b7          	lui	a1,0x0
			0: R_RISCV_HI20	msg
   4:	00858593          	addi	a1,a1,8 # 8 <.L21>
			4: R_RISCV_LO12_I	msg

Relative addressing

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

.section .text
.globl _start
_start:
1:	    auipc a0,     %pcrel_hi(msg) # load msg(hi)
	    addi  a0, a0, %pcrel_lo(1b)  # load msg(lo)
	    jal ra, puts
2:	    j 2b

.section .rodata
msg:
	    .string "Hello World\n"

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

0000000000000000 <_start>:
   0:	00000597          	auipc	a1,0x0
			0: R_RISCV_PCREL_HI20	msg
   4:	00858593          	addi	a1,a1,8 # 8 <.L21>
			4: R_RISCV_PCREL_LO12_I	.L11

GOT-indirect addressing

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

.section .text
.globl _start
_start:
1:	    auipc a0, %got_pcrel_hi(msg) # load msg(hi)
	    ld    a0, %pcrel_lo(1b)(a0)  # load msg(lo)
	    jal ra, puts
2:	    j 2b

.section .rodata
msg:
	    .string "Hello World\n"

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

0000000000000000 <_start>:
   0:	00000517          	auipc	a0,0x0
			0: R_RISCV_GOT_HI20	msg
   4:	00053503          	ld	a0,0(a0) # 0 <_start>
			4: R_RISCV_PCREL_LO12_I	.L11

Load Immediate

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

.section .text
.globl _start
_start:

.equ CONSTANT, 0xcafebabe

        li a0, CONSTANT

which generates the following assembler output as seen by objdump:

0000000000000000 <_start>:
   0:	00032537          	lui	    a0,0x32
   4:	bfb50513          	addi	a0,a0,-1029
   8:	00e51513          	slli	a0,a0,0xe
   c:	abe50513          	addi	a0,a0,-1346

Load Address

The following example shows the la psuedo instruction which is used to load symbol addresses:

.section .text
.globl _start
_start:

        la a0, msg

.section .rodata
msg:
	    .string "Hello World\n"

which generates the following assembler output and relocations for non-PIC as seen by objdump:

0000000000000000 <_start>:
   0:	00000517          	auipc	a0,0x0
			0: R_RISCV_PCREL_HI20	msg
   4:	00850513          	addi	a0,a0,8 # 8 <_start+0x8>
			4: R_RISCV_PCREL_LO12_I	.L11

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

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

Constants

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

.equ UART_BASE, 0x40003000

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

This example uses the li pseudoinstruction to load a constant and writes a string using polled IO to a UART:

.equ UART_BASE, 0x40003000
.equ REG_RBR, 0
.equ REG_TBR, 0
.equ REG_IIR, 2
.equ IIR_TX_RDY, 2
.equ IIR_RX_RDY, 4

.section .text
.globl _start
_start:
1:      auipc a0, %pcrel_hi(msg)    # load msg(hi)
        addi a0, a0, %pcrel_lo(1b)  # load msg(lo)
2:      jal ra, puts
3:      j 3b

puts:
        li a2, UART_BASE
1:      lbu a1, (a0)
        beqz a1, 3f
2:      lbu a3, REG_IIR(a2)
        andi a3, a3, IIR_TX_RDY
        beqz a3, 2b
        sb a1, REG_TBR(a2)
        addi a0, a0, 1
        j 1b
3:      ret

.section .rodata
msg:
	    .string "Hello World\n"

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)

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"