Recent Posts

11

Example Code / Re: "Hello world" with a very simple encoding

« Last post by andyz74 on April 25, 2025, 05:48:11 PM »

A label bassed calculation of the opcodes to decode would really have been convinient, that's for sure. And easy to code too. But to be honest, I'm using micro as editor for programming, and there are shown the line numbers at the left side, so the substraction was really easy too...   

12

Programming with NASM / Re: how to switch video modes (switching to VESA video modes too) in protected mode

« Last post by marknoble on April 25, 2025, 04:37:33 PM »

For VESA in protected mode, you gotta use VBE functions, not BIOS interrupts. Check the VBE specs on OSDev wiki for mode info and framebuffer setup. It’s tricky but doable. What bootloader you using? Might help narrow it down.

13

Programming with NASM / Re: Printing of floating point values

« Last post by marknoble on April 25, 2025, 03:02:57 PM »

Ugh, that "no-PIE" error is such a pain! Been there. Instead of messing with recompiling GCC (total hassle), you could try using puts with sprintf to format your float into a string first. Something like:

Code: [Select]

char buffer[32];
sprintf(buffer, "%f", your_float);
puts(buffer);It’s not as slick as printf, but it sidesteps the linker drama. Another option is write to stdout if you’re feeling low-level, but you’d still need to convert the float to a string manually. I’ve had luck with this on Debian 12, no PIE nonsense required. Worth a shot?

14

Example Code / Re: "Hello world" with a very simple encoding

« Last post by marknoble on April 25, 2025, 03:00:24 PM »

Nice work messing around with this! I’ve dabbled in some assembly too, and it’s always a trip to see how low-level you can go. Your encoding trick with the +18h offset is a cool way to obfuscate things, and I like how you kept it simple but effective. I tried something similar a while back on a 32-bit setup, just playing with XOR encoding for the code section, and it was a pain to debug when I messed up the decoding loop, props for getting it running smoothly! One thing I’d maybe watch out for is that hardcoding the decode length (like 50 bytes) could bite you if you tweak the code later. Maybe a label-based size calc could save some headaches? Anyway, fun stuff, thanks for sharing!

15

Example Code / "Hello world" with a very simple encoding

« Last post by andyz74 on April 16, 2025, 09:23:11 AM »

Hello world! ;-)

Just out of curiosity I have tried this :  a simple Hello-World-program, step for step with ean easy encoding of code and the message.  This is none-professional stuff, but it works (for me // Debian Linux, 64-bit) .  The main thing is for sure the definition of the code-section as read, write and executable. We all know, this is evil sin, not to be done, but there seems to be no other way.
So, for testing purposes, have a look at the original program we will start with :

Code: [Select]

; 64-bit "Hello World!" uncodiert... 1 von 4
; by myself
; nasm -f elf64 hello-cod01.asm
; ld -s -o hello-cod01 hello-cod01.o


[bits 64]

global _start ; global entry point export for ld

section .mycode alloc exec write

_start:

jmp decode
db 90h
db 90h
db 90h
db 90h

sss:
mov eax,1
mov edi,1
mov rsi,msg
mov edx,msg_size
syscall

mov eax,60
xor rdi, rdi
syscall

msg db 'Hello world!',0xA
msg_size equ $-msg

decode:
jmp sss
db 90h
db 90h
db 90h
db 90h

As easy, as it looks. the db 90h are "nop"-commands, mostly for being able to find our relevant code inside the debugger or hex-dump.
After compiling and linking, the program should work, and we can do a "hexdump -C [progname]" to get the opcodes.

Second part is, changing our program-code to opcodes. (For now, really only the code, not the message) We get the following :

Code: [Select]

; 64-bit "Hello World!" uncodiert, aber als opcodes... 2 von 4
; by myself
; nasm -f elf64 hello-cod02.asm
; ld -s -o hello-cod02 hello-cod02.o


[bits 64]

global _start ; global entry point export for ld

section .mycode exec write

_start:


jmp decode
db 90h
db 90h
db 90h
db 90h

sss:
db 0b8h
db 01h
db 00h
db 00h
db 00h
db 0bfh
db 01h
db 00h
db 00h
db 00h
db 48h
db 0beh
db 2bh
db 10h
db 40h
db 00h
db 00h
db 00h
db 00h
db 00h
db 0bah
db 0dh
db 00h
db 00h
db 00h
db 0fh
db 05h
db 0b8h
db 3ch
db 00h
db 00h
db 00h
db 48h
db 31h
db 0ffh
db 0fh
db 05h


msg db 'Hello world!',0xA
msg_size: equ $-msg


decode:
jmp sss
db 90h
db 90h
db 90h
db 90h


After compiling and linking, it should work as the first. So we can try a little very simple encoding (add a fix number to each opcode by hand and tell the "decoder" to subtract this, before jumping and executing the original code.
Let's try :

Code: [Select]

; 64-bit "Hello World!" codiert (nur Programm, nicht Message), als opcodes + 18h... 3 von 4
; by myself
; nasm -f elf64 hello-cod03.asm
; ld -s -o hello-cod03 hello-cod03.o


[bits 64]

global _start ; global entry point export for ld


section .mycode alloc exec write
_start:


jmp decode
db 90h
db 90h
db 90h
db 90h

sss:
zeugs db 0b8h + 18h
db 01h + 18h
db 00h + 18h
db 00h + 18h
db 00h + 18h
db 0bfh + 18h
db 01h + 18h
db 00h + 18h
db 00h + 18h
db 00h + 18h
db 48h + 18h
db 0beh + 18h
db 2bh + 18h
db 10h + 18h
db 40h + 18h
db 00h + 18h
db 00h + 18h
db 00h + 18h
db 00h + 18h
db 00h + 18h
db 0bah + 18h
db 0dh + 18h
db 00h + 18h
db 00h + 18h
db 00h + 18h
db 0fh + 18h
db 05h + 18h
db 0b8h + 18h
db 3ch + 18h
db 00h + 18h
db 00h + 18h
db 00h + 18h
db 48h + 18h
db 31h + 18h
db 0ffh + 18h
db 0fh + 18h
db 05h + 18h

msg db 'Hello world!',0xA
msg_size: equ $-msg


decode:
xor rcx, rcx
dec2:
sub byte [zeugs+rcx],18h
inc rcx
cmp rcx, 37
jne dec2
jmp sss
db 90h
db 90h
db 90h

You see, first line jumps to decoder , then decoding, then jump back to original start.

Now, finally encoding the message too.

Code: [Select]

; 64-bit "Hello World!" alles codiert, auch message, opcodes + 18h... 4 von 4
; by myself
; nasm -f elf64 hello-cod04.asm
; ld -s -o hello-cod04 hello-cod04.o


[bits 64]

global _start ; global entry point export for ld


section .mycode alloc exec write
_start:


jmp decode
db 90h
db 90h
db 90h
db 90h

sss:
zeugs db 0b8h + 18h
db 01h + 18h
db 00h + 18h
db 00h + 18h
db 00h + 18h
db 0bfh + 18h
db 01h + 18h
db 00h + 18h
db 00h + 18h
db 00h + 18h
db 48h + 18h
db 0beh + 18h
db 2bh + 18h
db 10h + 18h
db 40h + 18h
db 00h + 18h
db 00h + 18h
db 00h + 18h
db 00h + 18h
db 00h + 18h
db 0bah + 18h
db 0dh + 18h
db 00h + 18h
db 00h + 18h
db 00h + 18h
db 0fh + 18h
db 05h + 18h
db 0b8h + 18h
db 3ch + 18h
db 00h + 18h
db 00h + 18h
db 00h + 18h
db 48h + 18h
db 31h + 18h
db 0ffh + 18h
db 0fh + 18h
db 05h + 18h

msg db 'H' + 18h
db 'e' + 18h
db 'l' + 18h
db 'l' + 18h
db 'o' + 18h
db ' ' + 18h
db 'w' + 18h
db 'o' + 18h
db 'r' + 18h
db 'l' + 18h
db 'd' + 18h
db '!' + 18h
db 0xA + 18h
msg_size: equ $-msg


decode:
xor rcx, rcx
dec2:
sub byte [zeugs+rcx],18h
inc rcx
cmp rcx, 50
jne dec2
jmp sss
db 90h
db 90h
db 90h


For me, all worked, hexdump shows no more plain text, and the original opcodes are others too. Program ran on my computer.

...and remember, this is none-professional stuff, just for fun and try. :-)

16

Programming with NASM / Re: how to switch video modes (switching to VESA video modes too) in protected mode

« Last post by fredericopissarra on April 13, 2025, 01:01:59 PM »

I ran into the same headache back when I was experimenting with writing a small OS kernel for fun. Switching to VESA modes in protected mode easily becomes a rabbit hole. I had to do a bunch of hacks just to get the BIOS to cooperate. The legacy BIOS interrupt (INT 0x10) just won't fly once you're out of real mode, so I remember relying on setting up a trampoline to call into real mode temporarily or using something like a VBE 2.0+ Linear Framebuffer approach after querying video modes beforehand.

Yep... OR you could setup and use Virtual 8086 mode (NOT available in x86-64 mode!) to do it... Anyway, it is a nightmare.

Another problems are: Getting the Linear Framebuffer Physical address from ModeBlockInfo strucuture... Often this field is zeroed in VMs like QEMU, for example... And page switching, for double buffering... another nightmare.

This and the Protect Mode Interface, described in VBE 3.0 documentation usually are not present (not even in Option-ROM that a nVidia card puts there)...

17

Programming with NASM / Re: how to switch video modes (switching to VESA video modes too) in protected mode

« Last post by ace012k8 on April 09, 2025, 01:00:47 PM »

I ran into the same headache back when I was experimenting with writing a small OS kernel for fun. Switching to VESA modes in protected mode easily becomes a rabbit hole. I had to do a bunch of hacks just to get the BIOS to cooperate. The legacy BIOS interrupt (INT 0x10) just won't fly once you're out of real mode, so I remember relying on setting up a trampoline to call into real mode temporarily or using something like a VBE 2.0+ Linear Framebuffer approach after querying video modes beforehand.

For videos and demo recording, I always used a lightweight editor to polish up footage or build quick walkthroughs. If you’re working on dev-related content and want to make clean quick edits, especially on Windows, I picked up a solid one here: https://www.movavi.com/imovie-for-windows/. Easy to use and doesn’t overcomplicate things.

18

Programming with NASM / Re: Printing of floating point values

« Last post by andyz74 on March 27, 2025, 02:07:47 PM »

I read a bit about the comiss command and the flags that are set, if there were failures, but to be honest, I am absolutely not sure, if this is the right way.

19

Programming with NASM / Re: Printing of floating point values

« Last post by fredericopissarra on March 25, 2025, 09:44:06 PM »

Ah, I got it. You decrease the fractional part after each loop and check with the
Comiss xmm0, xmm3
If it gets no more smaller...   This is nice. :-)

Of... since it seems you didn't got the three potential bugs there here they are:

• What will happen if the value is negative?
• What will happen if the value is too big (like bigger then 2⁶⁴-1?
• Whay will happen if the value is a NaN?

There is a simple way to fix this... I'll leave it to you...

20

Programming with NASM / Re: Printing of floating point values

« Last post by andyz74 on March 23, 2025, 04:42:04 PM »

Ah, I got it. You decrease the fractional part after each loop and check with the
Comiss xmm0, xmm3
If it gets no more smaller...   This is nice. :-)