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                        <h1><a class="header" href="#riscv" id="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><a class="header" href="#registers" id="registers">Registers</a></h2>
<ul>
<li>riscv32 =&gt; <code>XLEN=32</code></li>
<li>riscv64 =&gt; <code>XLEN=64</code></li>
</ul>
<h3><a class="header" href="#general-purpose-registers" id="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><a class="header" href="#asm-skeleton" id="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 &lt;asm/unistd.h&gt;     // syscall NRs

    .section .text, &quot;ax&quot;, @progbits
    .balign 4               // align code on 4byte boundary
    .global _start
_start:
    li a0, 2                // fd
    la a1, greeting         // buf
    ld a2, (greeting_len)   // &amp;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, &quot;a&quot;, @progbits
greeting:
    .asciz &quot;Hi ASM-World!\n&quot;
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">&gt; riscv64-linux-gnu-gcc -o greet greet.S -nostartfiles -nostdlib                \
    -Wl,--dynamic-linker=/usr/riscv64-linux-gnu/lib/ld-linux-riscv64-lp64d.so.1 \
  &amp;&amp; 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 &amp; link.</p>
</blockquote>
<h2><a class="header" href="#references" id="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>
</ul>

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