Bochs VBE Extensions: Difference between revisions

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Line 1: The [[Bochs]] VGA BIOS supports, to an extent, the [[VBE]] specification. Since Bochs only emulates a VGA card down to the hardware level (and a Cirrus graphics card if you enable it, but that is not tied in with the Bochs VBE extensions), it emulates very simple graphics hardware that the VBE BIOS can drive. The advantage of this is that if you are running your OS in Bochs (or QEMU, which uses the Bochs VGA BIOS, or even VirtualBox), you can use this emulated hardware to directly set video modes without using VBE (which would require real mode or v86). == Overview == Line 12: * 0xB0C2 - 15, 16, 24 and 32 BPP modes, support for linear frame buffer, support for retaining memory contents on mode switching * 0xB0C3 - support for getting capabilities, support for using 8 bit DAC * 0xB0C4 - ~~[TODO:~~VRAM ~~check~~increased ~~and~~to ~~describe]~~8 MB * 0xB0C5 - [http://marc.info/?l=bochs-cvs&m=123287460501516 VRAM increased to 16 MB?] [TODO: verify and check for other changes] * 0xB0C5 - [TODO: check and describe] [TODO: if and when 4BPP modes are supported] [TODO: if and when VGA attribute controller (AC) is supported] Line 37: === Checking availability === To check whether the BGA is available, read the value from VBE_DISPI_INDEX_ID (0). If it equals ~~VBE_DISPI_ID4~~VBE_DISPI_ID5 (~~0xB0C4~~0xB0C5) 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 compatibility with the Bochs VBE BIOS, which expects the latest version. Line 84: === Using a linear frame buffer (LFB) === When using a linear ~~frame buffer~~framebuffer, the BGA exposes all of the ~~graphics~~video memory in a ~~linear~~single ~~fashion.~~linearly Inaddressable ~~older versions~~section of ~~Bochs and QEMU the location is fixed at 0xE0000000 (VBE_DISPI_LFB_PHYSICAL_ADDRESS)~~memory. ~~However, when Bochs is configured to emulate a PCI video card rather than an ISA one, the~~The address of the framebuffer is ~~no longer~~not fixed, ~~but~~and ~~can~~must be read from the first [[PCI]] base address register (BAR 0 of device 0x1234:0x1111~~<!--[TODO: check behaviour for 4 BPP modes and QEMU]-->~~). To enable the ~~LFB~~linear framebuffer, use the VBE_DISPI_LFB_ENABLED flag (0x40) when enabling the ~~VBE~~BGA ~~extensions~~in ~~(so~~conjunction ~~write~~with athe ~~value~~VBE_DISPI_ENABLED offlag. ~~VBE_DISPI_ENABLED~~<!--[TODO: \|check ~~VBE_DISPI_LFB_ENABLED~~behaviour ~~(0x41)).~~for 4 BPP modes]--> Unlike Bochs, QEMU does not necessarily pay attention to the VBE_DISPI_LFB_ENABLED flag. ~~Both~~with respect to banked memory access, allowing both the linear ~~frame buffer~~framebuffer and banked memory ~~bank~~to ~~are~~be ~~available~~used at all times. Bochs ~~pays~~will ~~attention to the flag. In LFB mode it~~not ~~ignores~~honour requests to change the memory bank ~~and nothing happens~~ when ~~you~~the ~~write~~linear toframebuffer ~~the~~is ~~bank~~enabled, ~~memory.~~and Init ~~banked~~will ~~mode~~similarly ~~nothing~~ignore ~~happens~~any ~~when~~writes ~~you write~~made to the ~~LFB~~memory ~~memory~~bank. '''Note:''' In older versions of Bochs and QEMU, the framebuffer was fixed at 0xE0000000, and modern versions will use that address when emulating ISA-only systems. '''It is highly inadvisable to make assumptions about the address of the linear framebuffer.''' It should always be read from the BGA's PCI BAR0. === Clearing display memory === Line 116 ⟶ 118: === Example code === <~~source~~syntaxhighlight lang="c"> void BgaWriteRegister(unsigned short IndexValue, unsigned short DataValue) { Line 150 ⟶ 152: } </syntaxhighlight> ~~</source>~~ ==External Links== [http://cvs.savannah.nongnu.org/viewvc/checkout/vgabios/vgabios/vbe_display_api.txt?revision=1.14 Specification] [[Category:Video]]