RISC-V汇编指南

原文出处：https://github.com/riscv/riscv-asm-manual/blob/master/riscv-asm.md

RISC-V Assembly Programmer’s Manual

Copyright and License Information

The RISC-V Assembly Programmer’s Manual is

© 2017 Palmer Dabbelt [email protected]
© 2017 Michael Clark [email protected]
© 2017 Alex Bradbury [email protected]

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

ISA and ABI register names for X, F, and CSRs.

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

Links to the various RISC-V ISA manuals that are supported.

Instructions

Here we can just link to the RISC-V ISA manual.

Instruction Aliases

ALIAS line from opcodes/riscv-opc.c

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		8-bit comma separated words
.2byte	expression [, expression]*	16-bit comma separated words (unaligned)
.4byte	expression [, expression]*	32-bit comma separated words (unaligned)
.8byte	expression [, expression]*	64-bit comma separated words (unaligned)
.half	expression [, expression]*	16-bit comma separated words (naturally aligned)
.word	expression [, expression]*	32-bit comma separated words (naturally aligned)
.dword	expression [, expression]*	64-bit comma separated words (naturally aligned)
.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

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”	auipc
%tprel_lo(label)	TLS LE “Local Exec”	load, store, add
%tprel_add(offset)	TLS LE “Local Exec”	add

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 a1,       %hi(msg)       # load msg(hi)
	    addi a1, a1,  %lo(msg)       # load msg(lo)
	    jalr 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 a1,     %pcrel_hi(msg) # load msg(hi)
	    addi  a1, a1, %pcrel_lo(1b)  # load msg(lo)
	    jalr 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

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 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

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"