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BeagleV-Ahead - RISC-V Assembly on Linux

javagoza
Beaglev-Ahead

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BeagleV-Ahead - RISC-V Assembly on Linux

The BeagleV-Ahead board is an excellent choice for diving into assembly programming with the RISC-V architecture. In this quick tutorial, I'll walk you through the steps to kickstart your RISC-V assembly programming journey on the Ubuntu image we loaded in the initial getting started tutorial. Let's get those assembly gears turning!

Linux SYSCALLs

We will use Linux syscall functions to display a welcome message on the console screen.

A Linux syscall, short for system call, is a fundamental interface between the operating system kernel and user-space programs. It provides a way for applications to request services or perform operations that require higher privileges and access to system resources. Syscalls serve as a bridge between the user-level applications and the kernel, allowing programs to interact with the underlying operating system.

Writing our first risc-v assembly program

Start a new ssh session

image

Using your preferred text editor, write the following code. In my case, I've chosen the nano editor, which is very user-friendly.

nano hello.s

Here is the RISC-V assembly code that prints "Hello to the element14 community!" to the console using Linux syscall functions:

# Risc-V Assembler program to print "Hello to the element14 community!"
# 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, 34              # 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 to the element14 community!\n"

This code initializes parameters to use the Linux syscall for writing (write) to standard output (stdout). After printing the message, it sets up parameters to exit the program using the Linux syscall for program termination (exit). The actual message "Hello to the element14 community!\n" is stored in the .data section.

Explanation:

  • .global _start: This declares the _start label as the entry point for the program, making it visible to the linker.

  • _start:: The actual start of the program, where the instructions begin.

  • addi a0, x0, 1: Sets up the file descriptor for standard output (stdout) in register a0.

  • la a1, helloworld: Loads the address of the helloworld string into register a1.

  • addi a2, x0, 34: Specifies the length of the string, which is 34 characters (including the newline).

  • addi a7, x0, 64: Sets up register a7 with the system call number for the Linux write system call.

  • ecall: Triggers a system call, invoking the Linux kernel to perform the specified action (in this case, write to stdout).

  • The subsequent block sets up parameters to exit the program using the Linux exit system call (command code 93).

  • Finally, the .data section contains the definition of the helloworld string.

image

Next, let's save the file.

In the nano editor, save the file by pressing Ctrl + O, then confirm the file name and press Enter. To exit, press Ctrl + X.

Assembling

We need to convert the source code written in assembly language into machine code or object code.

Write 

beagle@BeagleV:~$ as hello.s -o hello.o

The as command is the GNU assembler, and here it is used to assemble the RISC-V assembly source code file hello.s into an object file hello.o.

The -o flag specifies the output file.

Linking

Linking combines object files or libraries to produce a complete executable program
Write:

beagle@BeagleV:~$ ld hello.o -o hello

  • The ld command is the GNU linker, and it links the object file hello.o into an executable file named hello.
  • The -o flag specifies the output file.


After these commands, we have an executable file named hello.

We can execute it to see the "Hello to the element14 community!" message on the console.

Console output

beagle@BeagleV:~$ nano hello.s
beagle@BeagleV:~$ as hello.s -o hello.o
beagle@BeagleV:~$ ld hello.o -o hello
beagle@BeagleV:~$ ./hello
Hello to the element14 community!
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:       02200613                li      a2,34
   100f8:       04000893                li      a7,64
   100fc:       00000073                ecall
   10100:       00000513                li      a0,0
   10104:       05d00893                li      a7,93
   10108:       00000073                ecall
beagle@BeagleV:~$

Unlocking Binary Secrets: Getting Info with Objdump

objdump is a command-line utility that is part of the GNU Binutils suite. It is used to display information about one or more object files (binary files) including executable, shared object, and relocatable object files. objdump provides detailed information about the binary file's contents, such as disassembled machine code, section headers, symbols, and more.

image

Debugging without JTAG using `gdb`

GDB, short for GNU Debugger, is a powerful and versatile debugger that plays a pivotal role in software development. GDB is an essential tool  to identify, analyze, and fix bugs within the code. https://en.wikipedia.org/wiki/GNU_Debugger

To Install gdb: 

sudo apt install -y gdb

gdb help:

beagle@BeagleV:~$ gdb hello
GNU gdb (Ubuntu 13.1-2ubuntu2.1) 13.1
Copyright (C) 2023 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Type "show copying" and "show warranty" for details.
This GDB was configured as "riscv64-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<https://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
    <http://www.gnu.org/software/gdb/documentation/>.

For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from hello...
(No debugging symbols found in hello)
(gdb) help
List of classes of commands:

aliases -- User-defined aliases of other commands.
breakpoints -- Making program stop at certain points.
data -- Examining data.
files -- Specifying and examining files.
internals -- Maintenance commands.
obscure -- Obscure features.
running -- Running the program.
stack -- Examining the stack.
status -- Status inquiries.
support -- Support facilities.
text-user-interface -- TUI is the GDB text based interface.
tracepoints -- Tracing of program execution without stopping the program.
user-defined -- User-defined commands.

Type "help" followed by a class name for a list of commands in that class.
Type "help all" for the list of all commands.
Type "help" followed by command name for full documentation.
Type "apropos word" to search for commands related to "word".
Type "apropos -v word" for full documentation of commands related to "word".
Command name abbreviations are allowed if unambiguous.
(gdb)

A typical GDB Debugging Session

In this GDB session we will debug the "hello" executable.

  • We start GDB with the command: gdb hello
  • Then GDB provides information about its version, configuration, and licensing. And it indicates that no debugging symbols are found in the "hello" program.
  • We start the program execution with the starti command.
  • The program stops at the _start function.
  • With the disas command we disassemble the _start function, revealing the assembly code.
  • Then we set a breakpoint at the instruction address *0x00000000000100fc.
  • We use the continue command to resume execution.
  • The program hits the breakpoint at address 0x00000000000100fc, corresponding to the _start function.
  • We use the info registers command to display the values of various registers, providing insight into the program state.
  • The program continues its execution until completion, displaying the message "Hello to the element14 community!".
  • We exit gdb with the exit command.

Session log:

beagle@BeagleV:~$ gdb hello
GNU gdb (Ubuntu 13.1-2ubuntu2.1) 13.1
Copyright (C) 2023 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Type "show copying" and "show warranty" for details.
This GDB was configured as "riscv64-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<https://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
    <http://www.gnu.org/software/gdb/documentation/>.

For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from hello...
(No debugging symbols found in hello)
(gdb) starti
Starting program: /home/beagle/hello

Program stopped.
0x00000000000100e8 in _start ()
(gdb) disas
Dump of assembler code for function _start:
=> 0x00000000000100e8 <+0>:     li      a0,1
   0x00000000000100ec <+4>:     auipc   a1,0x1
   0x00000000000100f0 <+8>:     add     a1,a1,32 # 0x1110c
   0x00000000000100f4 <+12>:    li      a2,34
   0x00000000000100f8 <+16>:    li      a7,64
   0x00000000000100fc <+20>:    ecall
   0x0000000000010100 <+24>:    li      a0,0
   0x0000000000010104 <+28>:    li      a7,93
   0x0000000000010108 <+32>:    ecall
End of assembler dump.
(gdb) break *0x00000000000100fc
Breakpoint 1 at 0x100fc
(gdb) continue
Continuing.

Breakpoint 1, 0x00000000000100fc in _start ()
(gdb) info registers
ra             0x2aaaaeeb1e     0x2aaaaeeb1e
sp             0x3ffffff3c0     0x3ffffff3c0
gp             0x2aaab95408     0x2aaab95408
tp             0x3ff7e65780     0x3ff7e65780
t0             0x3ff7f8f9a0     274743228832
t1             0x2aaaadc10c     183252140300
t2             0xb1c51557703218e4       -5637075893295638300
fp             0x2aaabaf8d0     0x2aaabaf8d0
s1             0x2aaabaf890     183253006480
a0             0x1      1
a1             0x1110c  69900
a2             0x22     34
a3             0x0      0
a4             0x0      0
a5             0x0      0
a6             0x48     72
a7             0x40     64
s2             0x2aaabaf8d0     183253006544
s3             0x0      0
s4             0x2aaabaf890     183253006480
s5             0x3ff7ffdd70     274743680368
s6             0x2aaabacf10     183252995856
s7             0x2aaab94e60     183252897376
s8             0x0      0
s9             0x0      0
s10            0x63     99
s11            0x2aaab9d010     183252930576
t3             0x3ff7efbf2c     274742624044
t4             0x2aaabad        44739501
--Type <RET> for more, q to quit, c to continue without paging--
t5             0x2aaabad        44739501
t6             0x2aaabadd50     183252999504
pc             0x100fc  0x100fc <_start+20>
(gdb) continue
Continuing.
Hello to the element14 community!
[Inferior 1 (process 2313) exited normally]
(gdb) exit
beagle@BeagleV:~$

Complete session:

image

References

BeagleV-Ahead Getting Started Blog Series

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