A bare metal Hello world in assembly on RISC-V with QEMU

I wanted to create a minimal, Hello world style, program, for RISC-V.

I wanted the program to run on Ubuntu, as well as on MacOS, and I wanted to use QEMU as RISC-V simulator.

QEMU – Ubuntu

For Ubuntu, I installed QEMU by following instructions on the QEMU site, which led to commands, as

git clone https://git.qemu.org/git/qemu.git
cd qemu
git submodule init
git submodule update --recursive
mkdir objdir
cd objdir
../configure --target-list=riscv32-softmmu --prefix=/home/$USER/prog/qemu_riscv

The configure step gave an error, as

ola@ola-VirtualBox:~/mw/qemu/objdir$ ../configure --target-list=riscv32-softmmu --prefix=/home/$USER/prog/qemu_riscv

ERROR: pkg-config binary 'pkg-config' not found

ola@ola-VirtualBox:~/mw/qemu/objdir$ 

I realized that this was due to lack of programs on my newly installed Ubuntu 18.04.3 on Virtualbox.

I installed pkg-config, by doing

sudo apt install pkg-config

after which I got more errors, which led to

sudo apt install libglib2.0-dev 
sudo apt install libpixman-1-dev

which in turn led to a successful configure command.

I could then build, using

make

and install, using

make install

The installation was then tested, as

ola@ola-VirtualBox:~/mw/qemu/objdir$ /home/$USER/prog/qemu_riscv/bin/qemu-system-riscv32 --help | head
QEMU emulator version 4.1.93 (v4.2.0-rc3-dirty)
Copyright (c) 2003-2019 Fabrice Bellard and the QEMU Project developers
usage: qemu-system-riscv32 [options] [disk_image]

'disk_image' is a raw hard disk image for IDE hard disk 0

Standard options:
-h or -help     display this help and exit
-version        display version information and exit
-machine [type=]name[,prop[=value][,...]]
ola@ola-VirtualBox:~/mw/qemu/objdir$ 

QEMU – macOS Catalina

For macOS, I had already done an installation of QEMU on macOS Mojave.

I followed the steps from this experience, but now on macOS Catalina, and changing the configure command from

../configure --target-list=riscv32-softmmu --prefix=/Users/$USER/qemu_riscv

to

../configure --target-list=riscv32-softmmu --prefix=/Users/$USER/prog/qemu_riscv

I also noted that a parallel make

make -j

was successful.

I checked by my installation, by doing.

objdir oladahl$ /Users/$USER/prog/qemu_riscv/bin/qemu-system-riscv32 --help | head
QEMU emulator version 4.1.94 (v4.2.0-rc4-2-g02f9c885ed)
Copyright (c) 2003-2019 Fabrice Bellard and the QEMU Project developers
usage: qemu-system-riscv32 [options] [disk_image]

'disk_image' is a raw hard disk image for IDE hard disk 0

Standard options:
-h or -help     display this help and exit
-version        display version information and exit
-machine [type=]name[,prop[=value][,...]]
objdir oladahl$ 

Toolchain

As the next step, I wanted to install a toolchain.

I decided to use this RISC-V toolchain.

Ubuntu

Following the instructions, I did, on Ubuntu

git clone https://github.com/riscv/riscv-gnu-toolchain
cd riscv-gnu-toolchain

followed by

git submodule update --init --recursive

which took some time.

I installed programs, as recommended, by doing

sudo apt-get install autoconf automake autotools-dev curl libmpc-dev libmpfr-dev libgmp-dev gawk build-essential bison flex texinfo gperf libtool patchutils bc zlib1g-dev libexpat-dev

A configure was then done, for a 32-bit instruction set, using information from the Github site, as

mkdir objdir
cd objdir/
../configure --prefix=/home/$USER/prog/gcc_riscv --with-arch=rv32gc --with-abi=ilp32d

I built the toolchain, at first trying with

make -j

which led to an out-of-memory situation, with prints such as

cc1plus: out of memory allocating 65536 bytes after a total of 2252800 bytes
virtual memory exhausted: Cannot allocate memory
virtual memory exhausted: Cannot allocate memory

cc1plus: out of memory allocating 240809 bytes after a total of 1343488 bytes
virtual memory exhausted: Cannot allocate memory
virtual memory exhausted: Cannot allocate memory
virtual memory exhausted: Cannot allocate memory

I changed to a sequential make

make

which was successful.

I checked that I could start gcc, using

ola@ola-VirtualBox:~/mw/riscv-gnu-toolchain/objdir$ /home/$USER/prog/gcc_riscv/bin/riscv32-unknown-elf-gcc -v
Using built-in specs.
COLLECT_GCC=/home/ola/prog/gcc_riscv/bin/riscv32-unknown-elf-gcc
COLLECT_LTO_WRAPPER=/home/ola/prog/gcc_riscv/libexec/gcc/riscv32-unknown-elf/9.2.0/lto-wrapper
Target: riscv32-unknown-elf
Configured with: /home/ola/mw/riscv-gnu-toolchain/objdir/../riscv-gcc/configure --target=riscv32-unknown-elf --prefix=/home/ola/prog/gcc_riscv --disable-shared --disable-threads --enable-languages=c,c++ --with-system-zlib --enable-tls --with-newlib --with-sysroot=/home/ola/prog/gcc_riscv/riscv32-unknown-elf --with-native-system-header-dir=/include --disable-libmudflap --disable-libssp --disable-libquadmath --disable-libgomp --disable-nls --disable-tm-clone-registry --src=../../riscv-gcc --disable-multilib --with-abi=ilp32d --with-arch=rv32gc --with-tune=rocket 'CFLAGS_FOR_TARGET=-Os   -mcmodel=medlow' 'CXXFLAGS_FOR_TARGET=-Os   -mcmodel=medlow'
Thread model: single
gcc version 9.2.0 (GCC) 
ola@ola-VirtualBox:~/mw/riscv-gnu-toolchain/objdir$ 
macOS Catalina

On macOS Catalina, I installed programs as

brew install gawk gnu-sed gmp mpfr libmpc isl zlib expat

Following the instructions, I cloned the repo, as

git clone https://github.com/riscv/riscv-gnu-toolchain
cd riscv-gnu-toolchain

followed by

git submodule update --init --recursive

which took some time.

A configure was done, for a 32-bit instruction set, using information from the Github site, as

mkdir objdir
cd objdir/
../configure --prefix=/Users/$USER/prog/gcc_riscv --with-arch=rv32gc --with-abi=ilp32d

I built the toolchain, using parallel make, as

make -j

which was successful.

I checked that I could start gcc, using

$ /Users/$USER/prog/gcc_riscv/bin/riscv32-unknown-elf-gcc -v
Using built-in specs.
COLLECT_GCC=/Users/oladahl/prog/gcc_riscv/bin/riscv32-unknown-elf-gcc
COLLECT_LTO_WRAPPER=/Users/oladahl/prog/gcc_riscv/libexec/gcc/riscv32-unknown-elf/9.2.0/lto-wrapper
Target: riscv32-unknown-elf
Configured with: /Users/oladahl/mw/riscv-gnu-toolchain/objdir/../riscv-gcc/configure --target=riscv32-unknown-elf --prefix=/Users/oladahl/prog/gcc_riscv --disable-shared --disable-threads --enable-languages=c,c++ --with-system-zlib --enable-tls --with-newlib --with-sysroot=/Users/oladahl/prog/gcc_riscv/riscv32-unknown-elf --with-native-system-header-dir=/include --disable-libmudflap --disable-libssp --disable-libquadmath --disable-libgomp --disable-nls --disable-tm-clone-registry --src=../../riscv-gcc --disable-multilib --with-abi=ilp32d --with-arch=rv32gc --with-tune=rocket 'CFLAGS_FOR_TARGET=-Os   -mcmodel=medlow' 'CXXFLAGS_FOR_TARGET=-Os   -mcmodel=medlow'
Thread model: single
gcc version 9.2.0 (GCC) 
$ 

A program

I decided to write an assembly program, and to use a simulated UART in QEMU as the means for printing.

Looking at the 32-bit instruction set, in the The RISC-V Instruction Set Manual for the Unprivileged ISA, available as one of the RISC-V Specifications, and also using information from this page for the rv8 simulator, I came up with a program as

    .global _start

_start:

    lui t0, 0x10010

    andi t1, t1, 0
    addi t1, t1, 72
    sw t1, 0(t0)

    andi t1, t1, 0
    addi t1, t1, 101
    sw t1, 0(t0)

    andi t1, t1, 0
    addi t1, t1, 108
    sw t1, 0(t0)

    andi t1, t1, 0
    addi t1, t1, 108
    sw t1, 0(t0)

    andi t1, t1, 0
    addi t1, t1, 111
    sw t1, 0(t0)

    andi t1, t1, 0
    addi t1, t1, 10
    sw t1, 0(t0)

finish:
    beq t1, t1, finish

Here, the UART address is indicated by the value 0x10010, which is used in the first instruction as

    lui t0, 0x10010

I found the UART address by first practising building of a hello world program from riscv-probe, where I also learned how to create a linker script.

After this, I looked in the QEMU source code, where I found the file hw/riscv/sifive_u.c, in which the UART address is found in a struct, defined (with values for entries not related to UART0 omitted) as

static const struct MemmapEntry {
     hwaddr base;
     hwaddr size;
} sifive_u_memmap[] = {
...
    [SIFIVE_U_UART0] =    { 0x10010000,     0x1000 },
...
};

Considering that the lui instruction loads its immediate value into the upper 20 bits of the register used in the instruction, we see that the value 0x10010000 can be loaded in to a register, in this case t0, by doing

    lui t0, 0x10010

Using a makefile, as

hello: hello.o link.lds
	riscv32-unknown-elf-ld -T link.lds -o hello hello.o

hello.o: hello.s
	riscv32-unknown-elf-as -o hello.o hello.s

clean:
	rm hello hello.o

where the file link.lds has the same content as the riscv-probe linker script, I could build and run the program, on Ubuntu, as

export PATH=/home/$USER/prog/gcc_riscv/bin/:$PATH
make
/home/$USER/prog/qemu_riscv/bin/qemu-system-riscv32 -machine sifive_u -nographic -kernel hello

with printouts as

$ /home/$USER/prog/qemu_riscv/bin/qemu-system-riscv32 -machine sifive_u -nographic -kernel hello
qemu-system-riscv32: warning: No -bios option specified. Not loading a firmware.
qemu-system-riscv32: warning: This default will change in a future QEMU release. Please use the -bios option to avoid breakages when this happens.
qemu-system-riscv32: warning: See QEMU's deprecation documentation for details.
Hello
Hello
QEMU: Terminated
$ 

On macOS Catalina, I did

export PATH=/Users/$USER/prog/gcc_riscv/bin/:$PATH
make
/Users/$USER/prog/qemu_riscv/bin/qemu-system-riscv32 -machine sifive_u -nographic -kernel hello

with printouts as

$ /Users/$USER/prog/qemu_riscv/bin/qemu-system-riscv32 -machine sifive_u -nographic -kernel hello
qemu-system-riscv32: warning: No -bios option specified. Not loading a firmware.
qemu-system-riscv32: warning: This default will change in a future QEMU release. Please use the -bios option to avoid breakages when this happens.
qemu-system-riscv32: warning: See QEMU's deprecation documentation for details.
Hello
Hello
QEMU: Terminated
$ 

In both cases (Ubuntu and macOS Catalina), I terminated QEMU using Ctrl-A x. Also, I don’t know why the Hello string was printed twice.

Installing NASM

NASM is an assembler for x86-based computers. Here I describe my experiences from installing NASM on Linux and on Mac Mountain Lion.

Ubuntu Linux

NASM was installed using the command

sudo apt-get install nasm

Linux

Starting from the NASM site I navigated to the NASM download page.

I downloaded NASM version 2.10.05 in source format, by downloading the file

nasm-2.10.05.tar.bz2

I unpacked the file and navigated to the newly created directory, using the commands

tar xvjf nasm-2.10.05.tar.bz2
cd nasm-2.10.05

As a preparation for building NASM, a configure command shall be issued. Since I was not root for the machine, I used a prefix, indicating the place where I wanted NASM to be installed. The command used was

./configure --prefix=/nobackup/local/prog/nasm

Then I could build and install NASM, using the commands

make
make install

I also added the following changes

export PATH=/nobackup/local/prog/nasm/bin:$PATH

to my setup script, where I also set up other environment variables.

Mac Mountain Lion

Starting from the NASM site I navigated to the NASM download page. From there I continued to the Mac OS download page, from where I downloaded the file

nasm-2.10.05-macosx.zip

Unzipping this file resulted in a directory named nasm-2.10.05 being created.

I now noticed that I already had nasm on my computer, as could be seen from

Olas-MacBook-Air:nasm-2.10.05 oladahl$ nasm -v
NASM version 0.98.40 (Apple Computer, Inc. build 11) compiled on Aug 4 2012

That version was however older than the newly downloaded, as could be seen by doing

Olas-MacBook-Air:nasm-2.10.05 oladahl$ ./nasm -v
NASM version 2.10.05 compiled on Sep 9 2012

I decided to put the downloaded nasm in a directory called prog/nasm inside my home directory. I moved it there using

mkdir -p ~/prog/nasm
mv * ~/prog/nasm

I then added the following changes

export PATH=~/prog/nasm:$PATH

to my setup script, where I also set up other environment variables. After having rerun the setup script I could use the downloaded nasm, after first having verified that it was found using

Olas-MacBook-Air:i1_bare_metal oladahl$ nasm -v
NASM version 2.10.05 compiled on Sep 9 2012