blog_os: QEMU Bug in macOS 14: No "Hello, World" in `post-02` - just a black screen.

The line width is different (160, not 80 as the code in post 3 assumes), and the first line is invisible. If you change the function to

#[no_mangle]
pub extern "C" fn _start() -> ! {
    let vga_buffer = 0xb8000 as *mut u8;

    for (i, &byte) in HELLO.iter().enumerate() {
        unsafe {
            let line_offset: isize = 160 * 2;
            let char_offset_within_line: isize = i as isize * 2;
            let color_offset_within_line: isize = i as isize * 2 + 1;
            let char_offset = char_offset_within_line + line_offset;
            let color_offset = color_offset_within_line + line_offset;


            *vga_buffer.offset(char_offset) = byte;
            *vga_buffer.offset(color_offset) = 0xb;
        }
    }

    loop {}
}

You get the first fully visible line

And:

let line_offset: isize = 160 * 24;

gives

Looking at the code of post 3 I see that it starts writing from column 23. So yeah, that’s the problem. Maybe qemu has an ability to specify different settings for VGA, but it seems that it just doesn’t display the 0 line with in the current setup

To anybody that stumbled upon this via a google search or because of a pinned comment, here’s a quick rundown:

• If you are not on MacOS, this should not affect you
• If you are not using Qemu, this should not affect you
• If you are on MacOS version 13 and bellow, this should not affect you

Now, if you are on MacOS newer than than version 14, and you are using Qemu, listen up. There is a window-sizing bug in Qemu that will shift text up, preventing you from being able to see some of it. It is fixed in Qemu 9.0. Bellow are package managers and latest version of Qemu available:

• Homebrew (9.0.0)
• MacPorts (9.0.0)

Edit: The package managers have updated their packages. Both of them are now up to date. To fix the big, just update Qemu!

I have opened this issue: https://gitlab.com/qemu-project/qemu/-/issues/1994

It's an API difference between Sonoma and earlier macOS versions NSView documented in https://developer.apple.com/documentation/macos-release-notes/appkit-release-notes-for-macos-14.

Sonoma set NSView.clipsToBounds to false by default and it was true in earlier versions.

DrDonk seems to he working on it, it will hopefully be in the upstream Qemu soon…

Looks like both homebrew and macPorts provide v0.9 now, do I see that right?

The line width is different (160, not 80 as the code in post 3 assumes), and the first line is invisible. If you change the function to

#[no_mangle]
pub extern "C" fn _start() -> ! {
    let vga_buffer = 0xb8000 as *mut u8;

    for (i, &byte) in HELLO.iter().enumerate() {
        unsafe {
            let line_offset: isize = 160 * 2;
            let char_offset_within_line: isize = i as isize * 2;
            let color_offset_within_line: isize = i as isize * 2 + 1;
            let char_offset = char_offset_within_line + line_offset;
            let color_offset = color_offset_within_line + line_offset;


            *vga_buffer.offset(char_offset) = byte;
            *vga_buffer.offset(color_offset) = 0xb;
        }
    }

    loop {}
}

You get the first fully visible line

And:

let line_offset: isize = 160 * 24;

gives

Looking at the code of `post 3` I see that it starts writing from column `23`. So yeah, that's the problem. Maybe qemu has an ability to specify different settings for VGA, but it seems that it just doesn't display the `0` line with in the current setup

Works for me, thanks!

macOS Sonoma 14.1.1

Latest stable QEMU build also has the first 20-ish pixels at the top of the output window hidden by the window’s title bar:

What was the OS, QEMU & rustc versions on the M2 you used? - perhaps something is different there

I’m also using 8.1.2 installed through homebrew. I just tried it again with the latest rustc nightly (2023-11-20), but I still see the expected output on the post-02 branch. I’m on macOS 13.5.2.

I’ve got the same problem, just figured out that the first line with “Hello world” is visible in full screen mode.

Interesting! Maybe it’s just a window sizing bug then. Are you all on MacOS 14?

Quite interesting… so this verifies that it is not the os code. It is an issue related to Qemu on MacOS.