Changing VBE Configuration in SeaBIOS using I/O Ports and MMIO in QEMU
QEMU's standard VGA, which uses the PCI interface, provides I/O ports and Memory Mapped Input/Output (MMIO) to configure VBE (Video Electronics Standards Association BIOS Extensions). SeaBIOS, the default BIOS for QEMU, uses the default VBE configuration. However, you may want to modify the VGA BIOS code in SeaBIOS to change the VBE configuration.
Understanding I/O Ports and MMIO
I/O ports and MMIO are two methods used to communicate with hardware devices in a computer system. I/O ports are memory-mapped addresses that are used to send and receive data to and from hardware devices. MMIO, on the other hand, is a technique where a portion of the physical memory is reserved for use by hardware devices. This memory can be accessed directly by the CPU, eliminating the need for I/O port access.
Configuring VBE using I/O Ports and MMIO
To configure VBE in QEMU, you can use the -vga std option to enable the standard VGA device, which provides I/O ports and MMIO for VBE configuration. The VBE specification defines a set of I/O ports and MMIO registers that can be used to configure the VBE mode, set the display start address, and enable/disable the protected mode VBE interface.
$ qemu-system-x86_64 -vga std -display vgabios=seabios.bin
In the above command, the seabios.bin file is the SeaBIOS binary that you want to use. Once QEMU is running, you can use the VBE I/O ports and MMIO registers to configure the VBE mode and other settings.
Modifying VGA BIOS Code in SeaBIOS
If you want to change the default VBE configuration in SeaBIOS, you can modify the VGA BIOS code in the SeaBIOS source code. The VGA BIOS code is responsible for initializing the VGA hardware and providing the VBE interface. To modify the VGA BIOS code, you will need to have a good understanding of x86 assembly language and the VBE specification.
$ git clone git://git.qemu.org/seabios.git
$ cd seabios
$ make
The above commands will clone the SeaBIOS source code from the QEMU git repository and build the SeaBIOS binary. Once you have built the SeaBIOS binary, you can modify the VGA BIOS code and rebuild the binary. The VGA BIOS code is located in the src/vgabios/ directory.
- QEMU's standard VGA provides I/O ports and MMIO for VBE configuration.
- SeaBIOS uses the default VBE configuration.
- You can modify the VGA BIOS code in SeaBIOS to change the VBE configuration.
- Modifying the VGA BIOS code requires a good understanding of x86 assembly language and the VBE specification.