LLMs and BIOS INT 10h

Not a very fair test, but whatever. Just for fun I asked an LLM about some good old x86 assembler code. This is the prompt:

/no_think Find the bug:

mov ax, 0x3 ; switch into mcga graphics mode (320x200)
int 0x10

the bug being of course that the MCGA 320x200 graphics mode has number 0x13, not 0x3. Not so obvious to the LLM, which manages to produce this gem:

The code you've provided is a simple x86 assembly snippet that attempts to switch into MC68420 mode (a variant of the Motorola 68000) and call the BIOS interrupt 0x10 with the operand 0x3. However, there are several issues with this code:

1. Invalid Mode Switching:

• The value 0x3 is not a valid mode for the MC68420. The correct mode for MC68420 is 0x03 (or 0x03 in some implementations), which is used to switch into 320x200 mode.
• The instruction mov ax, 0x3 sets the mode register, but the actual mode switch is done using the mov ax, 0x03 instruction, not 0x3.

2. Incorrect Interrupt Usage:

• The BIOS interrupt 0x10 is used for system calls, but it is not the correct one for switching to MC68420 mode. The correct interrupt for mode switching is 0x10 with the operand 0x03 (or 0x03 depending on the system).
• The instruction int 0x10 is not the correct way to switch to MC68420 mode; it should be int 0x10 with the operand 0x03.

3. Corrected Code:

The correct code to switch into MC68420 mode and call the BIOS interrupt would be:

mov ax, 0x03      ; Set mode to 0x03 (MC68420)
int 0x10          ; Call BIOS interrupt

4. Additional Notes:

• The int 0x10 instruction is used for system calls, but in this context, it's being used to switch to a specific mode. The exact behavior may vary depending on the specific system and BIOS implementation.
• If you're trying to use this in a real system, you may need to refer to the specific documentation for your hardware and BIOS.

In short, the bug is in the mode setting and the interrupt call. The correct mode is 0x03, and the interrupt should be called with that value.

An earlier attempt without the /no_think failed because the program kept getting into an infinite loop.