Writing a simple assembly program for HiFive1

HiFive1 Rev B
5

Ok so I spent pretty much the whole day checking how “hello” works. I’m probably going to write a detailed blog about this so that others might find it useful, but overall:

  1. RISV-V GNU tools compiled to produce code for the RISC-V architecture (gcc, ld, libc,…et al).
  2. libwrap implements minimal memory handling (malloc/free) and syscalls required by many C functions (write,fstat,read…etc) that interface with file descriptors or underlying kernel (and there is none) so most of these syscalls are not implemented since there is no OS, but it helps build simple C programs.
  3. libwarp.mk Makefile uses --wrap for all the syscalls so that they get called instead of any others that might be defined in libc
  4. flash.lds maps the binary elf image sections to fit within the expected SPI EPROM memory map accepted by the FE310-G000 processor in the HiFive1 board. This sets all sections including stack, heap boundaries, rodata…etc
  5. start.S is where the execution begins. Here .data is loaded into loaded from 0x80000000 virtual addresses to physical addresses and .bss is cleared. Other boilerplate operations are executed as well before pushing the stack and calling main()

There is of course a lot more detail. At any rate, one thing I couldn’t fathom is the location of .rodata

From dasm output, the location is:

2040be20 <blanks.4376-0x1bc>:
2040be20:	6568                	flw	fa0,76(a0)
2040be22:	6c6c                	flw	fa1,92(s0)
2040be24:	6f77206f          	j	2047ed1a <__fini_array_end+0x72166>
2040be28:	6c72                	flw	fs8,28(sp)
2040be2a:	0a64                	addi	s1,sp,284

The above is “hello world\n”. Also, if you checkout main dasm, you see the following:

2040006c <main>:
2040006c:	1141                	addi	sp,sp,-16
2040006e:	0000c517          	auipc	a0,0xc
20400072:	db250513          	addi	a0,a0,-590 # 2040be20 <__clzsi2+0x44>
20400076:	c606                	sw	ra,12(sp)
20400078:	00000097          	auipc	ra,0x0
2040007c:	54c080e7          	jalr	1356(ra) # 204005c4 <printf>
20400080:	40b2                	lw	ra,12(sp)
20400082:	4501                	li	a0,0
20400084:	0141                	addi	sp,sp,16
20400086:	8082                	ret
20400088:	0000                	unimp

So it is loading a0 with 0x2040be20 which matches .rodata address above. However, when running the program on the board, I get different result:

          main:
2040006c:   addi    sp,sp,-16
5           printf("hello world\n");
2040006e:   auipc   a0,0xc
20400072:   addi    a0,a0,1150 # 0x2040c4ec
4         {
20400076:   sw      ra,12(sp)
5           printf("hello world\n");
20400078:   auipc   ra,0x0
2040007c:   jalr    2046(ra) # 0x20400876 <printf>
7         }
20400080:   lw      ra,12(sp)
20400082:   li      a0,0
20400084:   addi    sp,sp,16
20400086:   ret

Where does 0x2040c4ec come from? Looks like the .ro data was shifted down a bit, but why?

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