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Bochs VBE Extensions: Difference between revisions
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== Overview ==
The Bochs emulated graphics hardware (henceforth called BGA for Bochs Graphics Adaptor) is accessed via two 16-bit IO-ports. The first one is an index port, the second one a data port (comparable to how the VGA handles its sets of registers). Via these ports it is possible to enable or disable the VBE extensions, change the screen resolution and bit depth, and manage a larger virtual screen. There are five versions of the BGA (0xB0C0 through 0xB0C4), but if you use the latest version of Bochs you only need to concern yourself with the latest one (0xB0C4).
The Bochs sources define in vga.h, located in the subdirectory iodev/, a number of defines that are useful for programming the BGA. The names of these defines all start with VBE_DISPI. They are used in the sections below, with their numerical value between parentheses.
== BGA versions ==
[TODO]
== Programming the BGA ==
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=== Reading registers ===
To read a register, first write the index value to VBE_DISPI_IOPORT_INDEX (0x01CE), then read the 16-bit value from VBE_DISPI_IOPORT_DATA (0x01CF). The value returned depends on the specific register that is queried. [TODO: check and describe, especially _ID and _ENABLE, others seem trivial]
=== Checking availability ===
To check whether the BGA is available, read the value from VBE_DISPI_INDEX_ID (0). If it equals VBE_DISPI_ID4 (0xB0C4) the latest version of the BGA is present. If it returns a value of 0xB0C0 through 0xB0C3, you have an old version of Bochs and/or the Bochs VGA BIOS.
If for some reason you want Bochs to emulate an older version of the BGA, you can write the desired version to VBE_DISPI_INDEX_ID (0). If succesful, reading the register again will return the value just set. This is used by the Bochs VGA BIOS to ensure it is run with the right version of Bochs. If done from an application (or your OS), this will break compatability with the Bochs VBE BIOS, which expects the latest version.
=== Setting display resolution and bit depth ===
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