RISC-V from scratch 2: Hardware layouts, linker scripts, and C runtimes

[utterances-bot]

RISC-V from scratch 2: Hardware layouts, linker scripts, and C runtimes

A post describing how C programs get to the main function. Devicetree layouts, linker scripts, minimal C runtimes, GDB and QEMU, basic RISC-V assembly, and other topics are reviewed along the way.

https://twilco.github.io/riscv-from-scratch/2019/04/27/riscv-from-scratch-2.html

[yichnal]

grep doesn't need cat. At all. It's called UUOC.

[twilco]

@StephaneFr - fixed with 9c4840e. Thanks for the feedback!

[alicelyy]

Please keep them coming!

[twilco]

@limslarmo, work has officially begun on part three - stay tuned. 😄

[yashomer1994]

hey!, I am not able to use tub option on macOS Mojave can you help me?

[twilco]

Hey again @yashomer1994. I'm not sure what you mean by "tub option" - could you give me some more information? What command are you trying to run, and what error are you getting?

[yashomer1994]

Tui*option in gdb

On Thu, 1 Aug 2019 at 1:14 AM, Tyler Wilcock ***@***.***> wrote: Hey again @yashomer1994 <https://github.com/yashomer1994>. I'm not sure what you mean by "tub option" - could you give me some more information? What command are you trying to run, and what error are you getting? — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#5?email_source=notifications&email_token=AEKGT3VRPQGPPBP5QX4TCR3QCHTS5A5CNFSM4HN3WXJKYY3PNVWWK3TUL52HS4DFVREXG43VMVBW63LNMVXHJKTDN5WW2ZLOORPWSZGOD3ILB5Q#issuecomment-516993270>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AEKGT3VD2ELTVBZIUTLA2RTQCHTS5ANCNFSM4HN3WXJA> .

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[twilco]

Is this the command you are trying to run?

riscv64-unknown-elf-gdb --tui a.out

The riscv64-unknown-elf-gdb executable should support the --tui flag. If this indeed the command you are trying to run, could you give me the version of your GDB installation? Here is mine:

riscv64-unknown-elf-gdb --version
GNU gdb (GDB) 8.2.90.20190228-git

[yashomer1994]

Hey, Yeah its done working. I jus relinked my riscv64-unkown-elf-gdb And its working fine.

On Thu, 1 Aug 2019 at 9:45 AM, Tyler Wilcock ***@***.***> wrote: Is this the command you are trying to run? riscv64-unknown-elf-gdb --tui a.out The riscv64-unknown-elf-gdb executable should support the --tui flag. If this indeed the command you are trying to run, could you give me the version of your GDB installation? Here is mine: riscv64-unknown-elf-gdb --version GNU gdb (GDB) 8.2.90.20190228-git — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#5?email_source=notifications&email_token=AEKGT3UMBAN75U6WD7SKEIDQCJPPHA5CNFSM4HN3WXJKYY3PNVWWK3TUL52HS4DFVREXG43VMVBW63LNMVXHJKTDN5WW2ZLOORPWSZGOD3JIARQ#issuecomment-517111878>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AEKGT3RKVH2IGW3V45FMDYTQCJPPHANCNFSM4HN3WXJA> .

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[chillancezen]

hi Tyler,

I got a way to allocate the initial stack without having to define a symbol in the linker script:

.section .text
guest_kernel_entry:
    la sp, __stack_top
    //ebreak
    //j init_main
    j infinite_loop


infinite_loop:
    //wfi
    j infinite_loop




// allocate an area of memory to serve as initial stack
// 8K is supposed to be enough
.section .bss
.space 1024*8
.align 4
__stack_top:

[twilco]

Hey @chillancezen,

Looks promising — nice work!

Not sure if this is applicable in this context, but you may need greater than a 4-byte alignment (.align 4). Stacks tend to be aligned to the largest byte-size datatype that can be pushed to the stack, which for RISC-V is the long double weighing in at 16 bytes. Also, the RISC-V calling convention calls for the stack pointer to be 16-byte aligned.

For more discussion on this subject, look here in part four.

[chillancezen]

Not sure if this is applicable in this context, but you may need greater than a 4-byte alignment (.align 4). Stacks tend to be align

sure, you are right, it must be 16-bytes aligned.

[adeaarm]

Hi, is there any way to use gdb from within Eclipse to attach to the QEMU gdbserver?

[adeaarm]

Also, it would be nice to have some documentation stating what is the content of the ROM of the virt machine, placed at 0x1000.