Foundation of RISC-V Assembly Programming

[RISC-V Assembly manual], [RISC-V ELF spec], [RISC-V Calling Conventions], [GNU Assembler Directives]

Flow from assembly program to the corresponding binary executable is depicted in the figure below.

Prerequisites

Let's setup tools to compile and debug the RISC-V Assembly programs.

⚠️ These instructions are applicable for Linux Debian distribution.

Qemu Emulator: Qemu Emulator to run and simulate RISC-V programs.

  sudo apt-get install qemu-system-misc qemu-user-static binfmt-support opensbi u-boot-qemu

Cross Compiler Toolchain: GNU Compiler Toolchain to compile the assembly programs.
```
  sudo apt-get install gcc-riscv64-linux-gnu
```
Debugger: In order to debug the assembly programs install GDB-multiarch.
```
  sudo apt-get install gdb-multiarch
```

Getting Started

Let's take an example assembly program as follows

.text
.global _start

_start:
    addi x10, x0, 7
    addi x17, x0, 93
    add  t0, x10,x17
    ecall

Compiling the assembly program

Creating Object File from assembly by running:

 riscv64-linux-gnu-as -o example.o example.S

To obtain executable from the object file using the toolchain (linux) linker file, run the following command
```
 riscv64-linux-gnu-ld -o example example.o
```
The disassembly of the executable can be obtained by running.
```
 riscv64-linux-gnu-objdump -sd example
```
💡 riscv64-linux-gnu-objdump --H for more dump options
Emulating and Debugging
```
 qemu-riscv64-static -g 1234 example
```
After running the above command, leave the terminal window as it is, open another terminal window and run
```
 gdb-multiarch example
```
The debugger will open. Now, run the following command to connect the debugger with the qemu port 1234
```
 (gdb) target remote :1234
```
Now, it's connected. In order to look at the code on terminal for debugging, obtain chucnk of source code on the terminal window by specifying the number of lines.
```
 (gdb) display /3i $pc
```
Here, 3 is the number of lines of source code and i represents the instructions.

Other gdb options:

si to step each instruction for debugging.
c to continue to iterate through the whole program.
q to quit the debugger.

GNU Binary Utilities for RISC-V Asm

[GNU Bin Utilities]

Below are some of the widely GNU binary utilities by RISC-V toolchain. For all the available utilities, visit the documentation.

prefix: riscv64-linux-gnu or riscv64-unknown-elf

(prefix)-objdump [--help, --info]
(prefix)-objcopy [--help, --info]
(prefix)-readelf
elf2hex

Full system Emulation

To emulate a full system, a further step is necessary. A whole system needs a boot and system image to start up. You can download pre-baked images from DQIB. Download the file for riscv64-virt and extract the file. There is helpful information in a readme.txt. However, you should be able to emulate the system by typing this command: