Running RISC-V Assembly

This is going to be a short introduction into getting assembly to run on my RISC-V emulation, going towards a RISC-V forth interpreter. My aim is to get basic assembly running, and be able to run a debugger on it well.

I started with looking at Ola's post on a hello world in assembly. Their hello world looks like this:

.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

Each character load clears the t0 register using andi (and immediate) and places the next character in using addi (add immediate). They load the UART address in as 0x10010 (check their post for explanation of that address), although I also have seen it looking through a small risc-v standard library. Each character is placed in the UART register, which will print it out on the QEMU prompt using the sw (store word) command.

The linker and makefile needed slight modifications. I simply changed the makefile for RISC-V 64:

hello: hello.o link.lds

        riscv64-unknown-elf-ld -T link.lds -o hello hello.o



hello.o: hello.s

        riscv64-unknown-elf-as -o hello.o hello.s



clean:

        rm hello hello.o

For the linker, I have OpenSBI act as the bootloader. It jumps to address 0x8020_0000 once SBI finishes.

OUTPUT_ARCH( "riscv" )



ENTRY( _start )



MEMORY

{

  ram   (wxa!ri) : ORIGIN = 0x80200000, LENGTH = 128M

}



PHDRS

{

  text PT_LOAD;

  data PT_LOAD;

  bss PT_LOAD;

}



SECTIONS

{

  .text : {

    PROVIDE(_text_start = .);

    *(.text.init) *(.text .text.*)

    PROVIDE(_text_end = .);

  } >ram AT>ram :text



  .rodata : {

    PROVIDE(_rodata_start = .);

    *(.rodata .rodata.*)

    PROVIDE(_rodata_end = .);

  } >ram AT>ram :text



  .data : {

    . = ALIGN(4096);

    PROVIDE(_data_start = .);

    *(.sdata .sdata.*) *(.data .data.*)

    PROVIDE(_data_end = .);

  } >ram AT>ram :data



  .bss :{

    PROVIDE(_bss_start = .);

    *(.sbss .sbss.*) *(.bss .bss.*)

    PROVIDE(_bss_end = .);

  } >ram AT>ram :bss



  PROVIDE(_memory_start = ORIGIN(ram));

  PROVIDE(_memory_end = ORIGIN(ram) + LENGTH(ram));

}

Now I can assemble code using make, and run in qemu using the command:

$ qemu-system-riscv64 -machine sifive_u -nographic -kernel hello

This prints the SBI preamble and then prints "Hello". Great!