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# riscv
keywords: rv32, rv64
- ISA type: `RISC`
- Endianness: `little`, `big`
## Registers
- riscv32 => `XLEN=32`
- riscv64 => `XLEN=64`
### General purpose registers
```markdown
[XLEN-1:0] abi name desc
---------------------------------------------
x0 zero zero register
x1 ra return addr
x2 sp stack ptr
x3 gp global ptr
x4 tp thread ptr
x5-x7 t0-t2 temp regs
x8-x9 s0-s1 saved regs
x10-x17 a0-a7 arg regs
x18-x27 s2-s11 saved regs
x28-x31 t3-t6 temp regs
```
## ASM skeleton
Small assembler skeleton, ready to use with following properties:
- use raw Linux syscalls (`man 2 syscall` for ABI)
- no `C runtime (crt)`
- gnu assembler [`gas`][gas_doc]
```mipsasm
// file: greet.S
#include <asm/unistd.h> // syscall NRs
.section .text, "ax", @progbits
.balign 4 // align code on 4byte boundary
.global _start
_start:
li a0, 2 // fd
la a1, greeting // buf
ld a2, (greeting_len) // &len
li a7, __NR_write // write(2) syscall
ecall
li a0, 42 // exit code
li a7, __NR_exit // exit(2) syscall
ecall
.balign 8 // align data on 8byte boundary
.section .rodata, "a", @progbits
greeting:
.asciz "Hi ASM-World!\n"
greeting_len:
.int .-greeting
```
> man gcc: `file.S` assembler code that must be preprocessed.
To cross-compile and run:
```bash
> riscv64-linux-gnu-gcc -o greet greet.S -nostartfiles -nostdlib \
-Wl,--dynamic-linker=/usr/riscv64-linux-gnu/lib/ld-linux-riscv64-lp64d.so.1 \
&& qemu-riscv64 ./greet
Hi ASM-World!
```
> Cross-compiling on `Ubuntu 20.04 (x86_64)`, paths might differ on other
> distributions. Explicitly specifying the dynamic linker should not be
> required when compiling natively on riscv.
>
> Select dynamic linker according to abi used during compile & link.
## References
- [GNU Assembler][gas_doc]
- [GNU Assembler Directives][gas_directives]
- [rv64i-linux-user-no-std][rvi64_nostd]
[gas_doc]: https://sourceware.org/binutils/docs/as
[gas_directives]: https://sourceware.org/binutils/docs/as/Pseudo-Ops.html#Pseudo-Ops
[rvi64_nostd]: https://github.com/johannst/rv64i-linux-user-no-std
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