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<h1 id="riscv"><a class="header" href="#riscv">riscv</a></h1>
<p>keywords: rv32, rv64</p>
<ul>
<li>ISA type: <code>RISC</code></li>
<li>Endianness: <code>little</code>, <code>big</code></li>
</ul>
<h2 id="registers"><a class="header" href="#registers">Registers</a></h2>
<ul>
<li>riscv32 => <code>XLEN=32</code></li>
<li>riscv64 => <code>XLEN=64</code></li>
</ul>
<h3 id="general-purpose-registers"><a class="header" href="#general-purpose-registers">General purpose registers</a></h3>
<pre><code class="language-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
</code></pre>
<h2 id="asm-skeleton"><a class="header" href="#asm-skeleton">ASM skeleton</a></h2>
<p>Small assembler skeleton, ready to use with following properties:</p>
<ul>
<li>use raw Linux syscalls (<code>man 2 syscall</code> for ABI)</li>
<li>no <code>C runtime (crt)</code></li>
<li>gnu assembler <a href="https://sourceware.org/binutils/docs/as"><code>gas</code></a></li>
</ul>
<pre><code class="language-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
</code></pre>
<blockquote>
<p>man gcc: <code>file.S</code> assembler code that must be preprocessed.</p>
</blockquote>
<p>To cross-compile and run:</p>
<pre><code class="language-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!
</code></pre>
<blockquote>
<p>Cross-compiling on <code>Ubuntu 20.04 (x86_64)</code>, paths might differ on other
distributions. Explicitly specifying the dynamic linker should not be
required when compiling natively on riscv.</p>
<p>Select dynamic linker according to abi used during compile & link.</p>
</blockquote>
<h2 id="references"><a class="header" href="#references">References</a></h2>
<ul>
<li><a href="https://sourceware.org/binutils/docs/as">GNU Assembler</a></li>
<li><a href="https://sourceware.org/binutils/docs/as/Pseudo-Ops.html#Pseudo-Ops">GNU Assembler Directives</a></li>
<li><a href="https://github.com/johannst/rv64i-linux-user-no-std">rv64i-linux-user-no-std</a></li>
</ul>
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