add RISC-V intro

intro/beagle101/riscv.rst

@@ -3,4 +3,115 @@
 Introduction to RISC-V
 ######################
 
-.. todo:: Place-holder for RISC-V tutorial
+`RISC-V`_ is an open standard `instruction set architecture (ISA) <ISA>`_, meaning that the details of the standard are publicly available and free
+to license. Companies that make `CPUs`_ that utilize the RISC-V ISA are able to do so in a compatible way without paying royalty.
+
+Why RISC-V?
+***********
+
+.. figure:: figures/why-riscv.png
+
+   Why RISC-V?
+
+There are many benefits, but one that stands out is the opportunity for diversity. CPU vendors are able to create custom
+CPUs with performance and features suited for various applications, making their own power, performance and cost tradeoffs,
+while still maintaining compatibility. The ability to run tens of thousands of pre-built binary packages in a Linux
+distribution is just an example.
+
+Hardware
+********
+
+Check out :ref:`beaglev-fire-home` and :ref:`beaglev-ahead-home` for suitable hardware for learning more about RISC-V.
+
+Hello World
+***********
+
+Stephen Smith wrote a great `RISC-V Assembly Language Hello World blog post <https://smist08.wordpress.com/2019/09/07/risc-v-assembly-language-hello-world/>`_ that
+runs well on BeagleV boards.
+
+hello.s
+=======
+
+Once you've logged into your BeagleV, save the :ref:`hello.s <hello_s_source>` file.
+
+.. _hello_s_source:
+
+.. code-block::
+   :filename: hello.s
+
+   # Risc-V Assembler program to print "Hello World!"
+   # to stdout.
+   #
+   # a0-a2 - parameters to linux function services
+   # a7 - linux function number
+
+   .global _start     # Provide program starting address to linker
+
+   # Setup the parameters to print hello world
+   # and then call Linux to do it.
+
+   _start: addi  a0, x0, 1     # 1 = StdOut
+           la    a1, helloworld # load address of helloworld
+           addi  a2, x0, 13    # length of our string
+           addi  a7, x0, 64    # linux write system call
+           ecall               # Call linux to output the string
+
+   # Setup the parameters to exit the program
+   # and then call Linux to do it.
+
+           addi    a0, x0, 0   # Use 0 return code
+           addi    a7, x0, 93  # Service command code 93 terminates
+           ecall               # Call linux to terminate the program
+
+   .data
+   helloworld:      .ascii "Hello World!\n"
+
+Running hello.s
+===============
+
+Then, you can assemble, link and run :ref:`hello.s <hello_s_source>`.
+
+.. code-block:: shell-session
+
+  beagle@BeagleV:~$ as hello.s -o hello.o
+  beagle@BeagleV:~$ ld hello.o -o hello
+  beagle@BeagleV:~$ ./hello
+  Hello World!
+
+
+If you are curious, you can take a better look at what the executable has in it using :ref:`objdump <objdump-example>`.
+
+.. _objdump-example:
+
+.. code-block:: shell-session
+
+  beagle@BeagleV:~$ objdump -d hello
+  hello: 	file format elf64-littleriscv
+
+  Disassembly of section .text:
+
+  00000000000100e8 <_start>:
+     100e8:    00100513   li    a0,1
+     100ec:    00001597   auipc    a1,0x1
+     100f0:    02058593   add    a1,a1,32 # 1110c <__DATA_BEGIN__>
+     100f4:    00d00613   li    a2,13
+     100f8:    04000893   li    a7,64
+     100fc:    00000073   ecall
+     10100:    00000513   li    a0,0
+     10104:    05d00893   li    a7,93
+     10108:    00000073   ecall
+
+Recommended Resources
+*********************
+
+* `RISC-V Assembly Manual <https://github.com/riscv-non-isa/riscv-asm-manual/blob/master/riscv-asm.md>`_
+* `Digikey Webinar on BeagleV-Fire <https://www.beagleboard.org/blog/2024-03-13-webinar-on-beaglev-fire-powered-by-polarfire-soc-risc-v-and-fpga>`_
+
+.. _RISC-V:
+   https://en.wikipedia.org/wiki/RISC-V
+
+.. _ISA:
+   https://en.wikipedia.org/wiki/Instruction_set_architecture>
+
+.. _CPUs:
+   https://en.wikipedia.org/wiki/Central_processing_unit