5
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 thisUNIMPinstruction form. Despite not being a valid instruction, it still fits the 16-bit (compressed) instruction format, and so0000 0000is interpreted as being two 16-bitUNIMPinstructions.UNIMP:C0001073. This is an alias forCSRRW x0, cycle, x0. Sincecycleis 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 ofUNIMPis emitted when targeting a system without the C extension, or when the.option norvcdirective 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 |