rubenvannieuwpoort / riscv-flat-assembler

RISC-V flat assembler

This repo contains some code to compile 32-bit RISC-V to a flat binary. This can be useful for testing simple RISC-V emulators or CPU's.

First you should build a GCC cross compiler as described on https://wiki.osdev.org/GCC_Cross-Compiler. After that, you should be able to simply run make in this repo and a main.bin should be generated.

main.bin is just a flat binary that is assumed to be loaded at address 0. You can check the disassembly with riscv-elf-objdump -D main.bin -b binary -mriscv. The data in main.bin should be the same as the data in main.elf, with the difference that the last one is a proper ELF file instead of just a flat binary file.

$ riscv-elf-objdump -D main.bin -b binary -mriscv

main.bin:     file format binary


Disassembly of section .data:

0000000000000000 <.data>:
   0:   0040006f                j       0x4
   4:   00500593                li      a1,5
   8:   00400513                li      a0,4
   c:   0040006f                j       0x10
  10:   40455793                sra     a5,a0,0x4
  14:   0045f593                and     a1,a1,4
  18:   0047f793                and     a5,a5,4
  1c:   02b787b3                mul     a5,a5,a1
  20:   40855513                sra     a0,a0,0x8
  24:   00457513                and     a0,a0,4
  28:   00a78533                add     a0,a5,a0
  2c:   00008067                ret
  30:   0201                    add     tp,tp,0 # 0x0
  32:   06050403                lb      s0,96(a0)
  36:   02010807                .4byte  0x2010807
  3a:   06050403                lb      s0,96(a0)
  3e:   0a090807                .4byte  0xa090807
  42:   0e0d0c0b                .4byte  0xe0d0c0b
  46:   0f 10                   Address 0x46 is out of bounds.

The disassembler gets a bit confused because there's both data and code in the one section, but we can see all the code is in there.

• _start starts at 0x0
• main starts at 0x4
• test starts at 0x10
• roarray starts at 0x30
• dataarray starts at 0x38
• bssarray starts at 0x48 (it is not included in the file because it is zero-initialized)