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| author | Christian Krinitsin <mail@krinitsin.com> | 2025-05-30 16:52:07 +0200 |
|---|---|---|
| committer | Christian Krinitsin <mail@krinitsin.com> | 2025-05-30 16:52:17 +0200 |
| commit | 9260319e7411ff8281700a532caa436f40120ec4 (patch) | |
| tree | 2f6bfe5f3458dd49d328d3a9eb508595450adec0 /gitlab/issues_text/target_missing/host_missing/accel_missing/2959 | |
| parent | 225caa38269323af1bfc2daadff5ec8bd930747f (diff) | |
| download | qemu-analysis-9260319e7411ff8281700a532caa436f40120ec4.tar.gz qemu-analysis-9260319e7411ff8281700a532caa436f40120ec4.zip | |
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Diffstat (limited to 'gitlab/issues_text/target_missing/host_missing/accel_missing/2959')
| -rw-r--r-- | gitlab/issues_text/target_missing/host_missing/accel_missing/2959 | 77 |
1 files changed, 77 insertions, 0 deletions
diff --git a/gitlab/issues_text/target_missing/host_missing/accel_missing/2959 b/gitlab/issues_text/target_missing/host_missing/accel_missing/2959 new file mode 100644 index 000000000..177f4e752 --- /dev/null +++ b/gitlab/issues_text/target_missing/host_missing/accel_missing/2959 @@ -0,0 +1,77 @@ +int 0x10 teletype output cuts final character in custom MBR on QEMU (i386 real mode) +Description of problem: +When using QEMU to test a custom bootloader in 16-bit real mode (i386), the BIOS interrupt `int 0x10` with AH=0x0E (teletype output) fails to display the last character of the printed message. For example, printing `"hello"` only renders `"hell"`. + +This happens only with this exact combination: + +real mode `int 0x10` teletype output + +message ends with `13, 10, 0` + +`QEMU` output cuts off the last character consistently + +All buffer and code logic has been verified to be correct. The same code, when run on Bochs or physical hardware, prints properly. +Steps to reproduce: +1.Assemble the following boot.asm: +```nasm +[org 0x7C00] +[BITS 16] + +_start: + cli + xor ax, ax + mov ds, ax + mov es, ax + mov ss, ax + mov sp, 0x7C00 + + mov si, msg + call print + + hlt + jmp $ + +print: + pusha +.loop: + lodsb + or al, al + jz .done + mov ah, 0x0E + int 0x10 + jmp .loop +.done: + popa + ret + +msg db 'hello', 13, 10, 0 +times 510 - ($ - $$) db 0 +dw 0xAA55 +``` + +2. Compile and run: +```bash +$ nasm -f bin boot.asm -o boot.img +$ qemu-system-i386 -nographic -boot a -drive format=raw,file=boot.img,index=0,if=floppy +``` + +3. Output will be: +```text +Booting from Floppy... +hell +``` +Expected output: +```text +Booting from Floppy... +hello +``` +Additional information: +- Adding padding (extra 13, 10) does not solve the problem. + +- Confirmed that boot.img includes all bytes (xxd dump is correct). + +- Tested on multiple machines with same QEMU version. + +- May relate to VGA character output buffer not flushing after last INT 0x10? + +- This makes QEMU inaccurate for BIOS-level debugging of bootloaders. |