RISC-V汇编指南

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

RISC-V Assembly Programmer’s ManualCopyright 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

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

1. loop:
   
2.         j loop

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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. 1:
   
2.         j 1b

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Absolute addressing

The following example shows how to load an absolute address:

1. .section .text
   
2. .globl _start
   
3. _start:
   
4.             lui a1,       %hi(msg)       # load msg(hi)
   
5.             addi a1, a1,  %lo(msg)       # load msg(lo)
   
6.             jalr ra, puts
   
7. 2:            j 2b
   
8. 
   
9. .section .rodata
   
10. msg:
    
11.             .string "Hello World\n"

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

1. 0000000000000000 <_start>:
   
2.    0:        000005b7                  lui        a1,0x0
   
3.                         0: R_RISCV_HI20        msg
   
4.    4:        00858593                  addi        a1,a1,8 # 8 <.L21>
   
5.                         4: R_RISCV_LO12_I        msg

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Relative addressing

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

1. .section .text
   
2. .globl _start
   
3. _start:
   
4. 1:            auipc a1,     %pcrel_hi(msg) # load msg(hi)
   
5.             addi  a1, a1, %pcrel_lo(1b)  # load msg(lo)
   
6.             jalr ra, puts
   
7. 2:            j 2b
   
8. 
   
9. .section .rodata
   
10. msg:
    
11.             .string "Hello World\n"

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

1. 0000000000000000 <_start>:
   
2.    0:        00000597                  auipc        a1,0x0
   
3.                         0: R_RISCV_PCREL_HI20        msg
   
4.    4:        00858593                  addi        a1,a1,8 # 8 <.L21>
   
5.                         4: R_RISCV_PCREL_LO12_I        .L11

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Load Immediate

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

1. .section .text
   
2. .globl _start
   
3. _start:
   
4. 
   
5. .equ CONSTANT, 0xcafebabe
   
6. 
   
7.         li a0, CONSTANT

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which generates the following assembler output as seen by objdump:

1. 0000000000000000 <_start>:
   
2.    0:        00032537                  lui            a0,0x32
   
3.    4:        bfb50513                  addi        a0,a0,-1029
   
4.    8:        00e51513                  slli        a0,a0,0xe
   
5.    c:        abe50513                  addi        a0,a0,-1346

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Load Address

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

1. .section .text
   
2. .globl _start
   
3. _start:
   
4. 
   
5.         la a0, msg
   
6. 
   
7. .section .rodata
   
8. msg:
   
9.             .string "Hello World\n"

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

1. 0000000000000000 <_start>:
   
2.    0:        00000517                  auipc        a0,0x0
   
3.                         0: R_RISCV_PCREL_HI20        msg
   
4.    4:        00850513                  addi        a0,a0,8 # 8 <_start+0x8>
   
5.                         4: R_RISCV_PCREL_LO12_I        .L11

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Constants

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

1. .equ UART_BASE, 0x40003000
   
2. 
   
3.         lui a0,      %hi(UART_BASE)
   
4.         addi a0, a0, %lo(UART_BASE)

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

1. .equ UART_BASE, 0x40003000
   
2. .equ REG_RBR, 0
   
3. .equ REG_TBR, 0
   
4. .equ REG_IIR, 2
   
5. .equ IIR_TX_RDY, 2
   
6. .equ IIR_RX_RDY, 4
   
7. 
   
8. .section .text
   
9. .globl _start
   
10. _start:
    
11. 1:      auipc a0, %pcrel_hi(msg)    # load msg(hi)
    
12.         addi a0, a0, %pcrel_lo(1b)  # load msg(lo)
    
13. 2:      jal ra, puts
    
14. 3:      j 3b
    
15. 
    
16. puts:
    
17.         li a2, UART_BASE
    
18. 1:      lbu a1, (a0)
    
19.         beqz a1, 3f
    
20. 2:      lbu a3, REG_IIR(a2)
    
21.         andi a3, a3, IIR_TX_RDY
    
22.         beqz a3, 2b
    
23.         sb a1, REG_TBR(a2)
    
24.         addi a0, a0, 1
    
25.         j 1b
    
26. 3:      ret
    
27. 
    
28. .section .rodata
    
29. msg:
    
30.             .string "Hello World\n"

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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:

1. .equ RTC_BASE,      0x40000000
   
2. .equ TIMER_BASE,    0x40004000
   
3. 
   
4. # setup machine trap vector
   
5. 1:      auipc   t0, %pcrel_hi(mtvec)        # load mtvec(hi)
   
6.         addi    t0, t0, %pcrel_lo(1b)       # load mtvec(lo)
   
7.         csrrw   zero, mtvec, t0
   
8. 
   
9. # set mstatus.MIE=1 (enable M mode interrupt)
   
10.         li      t0, 8
    
11.         csrrs   zero, mstatus, t0
    
12. 
    
13. # set mie.MTIE=1 (enable M mode timer interrupts)
    
14.         li      t0, 128
    
15.         csrrs   zero, mie, t0
    
16. 
    
17. # read from mtime
    
18.         li      a0, RTC_BASE
    
19.         ld      a1, 0(a0)
    
20. 
    
21. # write to mtimecmp
    
22.         li      a0, TIMER_BASE
    
23.         li      t0, 1000000000
    
24.         add     a1, a1, t0
    
25.         sd      a1, 0(a0)
    
26. 
    
27. # loop
    
28. loop:
    
29.         wfi
    
30.         j loop
    
31. 
    
32. # break on interrupt
    
33. mtvec:
    
34.         csrrc  t0, mcause, zero
    
35.         bgez t0, fail       # interrupt causes are less than zero
    
36.         slli t0, t0, 1      # shift off high bit
    
37.         srli t0, t0, 1
    
38.         li t1, 7            # check this is an m_timer interrupt
    
39.         bne t0, t1, fail
    
40.         j pass
    
41. 
    
42. pass:
    
43.         la a0, pass_msg
    
44.         jal puts
    
45.         j shutdown
    
46. 
    
47. fail:
    
48.         la a0, fail_msg
    
49.         jal puts
    
50.         j shutdown
    
51. 
    
52. .section .rodata
    
53. 
    
54. pass_msg:
    
55.         .string "PASS\n"
    
56. 
    
57. fail_msg:
    
58.         .string "FAIL\n"

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本篇完

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