bochs_vbe - Bochs VBE / QEMU BGA (DISPI) mode-setting

Header: kernel/include/kernel/bochs_vbe.h Source: arch/i386/display/bochs_vbe.c

The Bochs VBE / QEMU “Bochs Graphics Adapter” (DISPI) interface, accessible via I/O ports from protected mode — no real-mode BIOS calls. This is how Makar sets an arbitrary linear-framebuffer resolution at boot and at runtime on the QEMU-std / Bochs / VirtualBox-VGA adapters. VMware/VirtualBox SVGA II mode-sets go through that driver’s own registers instead (see video); DISPI is the fallback the framework reaches for when video_set_mode returns “can’t”.

API

Function	Role
`bochs_vbe_available()`	True if a Bochs-compatible VBE adapter is detected (DISPI id at port `0x1CE/0x1CF`).
`bochs_vbe_vram_bytes()`	Total video memory (DISPI `VIDEO_MEMORY_64K`); `0` if not reported.
`bochs_vbe_caps(&w,&h,&bpp)`	Max mode the adapter advertises (DISPI GETCAPS). Any out-param may be `0`.
`bochs_vbe_mode_supported(w,h,bpp)`	True if `w×h×bpp` fits both the dimension caps and the VRAM. Gates the boot-resolution pick and `setmode` so we never program a mode the adapter can’t scan out (black screen / FB overrun).
`bochs_vbe_set_mode(w,h,bpp)`	Switch the LFB to `w×h` at `bpp`. `LFB_ENABLED` keeps the framebuffer address constant. Call `vesa_update_geometry()` + `vesa_tty_init()` after.
`bochs_vbe_disable()`	Disable VBE graphics; QEMU reverts to VGA text mode 3 (80×25) and the 8×16 IBM VGA ROM font is uploaded to plane 2.
`vga_load_text_font_8x8()`	Replace the plane-2 font with the kernel’s 8×8 glyphs (for 80×50, where the CRTC reads only the first 8 bytes of each glyph).

Boot use

kernel_main picks the boot resolution from a vmode= cmdline request (or the 720p default), gated through bochs_vbe_mode_supported() so an unsupported ask falls back cleanly — 1280×720 → 1024×768 → 640×480 → the adapter max. The panic path calls bochs_vbe_disable() (plus video_svga2_to_vga()) to get back to legible VGA text before rendering. See vesa, video, and boot modes.