Initial commit: Schunix as it stands before the SchOS rename

An x86-64 operating system built from xv6, now largely rewritten: SMP,
a copy-on-write root filesystem (SchFS), PCI/USB/virtio drivers, a TTF
rasteriser, and a Linux-numbered syscall ABI.

No history before this point -- the tree was never under version control.
This commit exists so the rename that follows has something to undo to.

boot/stage2.S

@@ -0,0 +1,129 @@
+# Stage 2 — loaded by stage1 at 0x7E00, still in 16-bit real mode. Does the
+# VBE mode setup, A20 line, and real->protected mode switch that used to
+# live directly in the boot sector, then hands off to stage2main() (C) to
+# draw a splash and load the actual kernel. Not sector-size-constrained —
+# only bounded by STAGE2_SECTORS in bootasm.S / the Makefile.
+#include "../include/asm.h"
+#include "../include/memlayout.h"
+#include "../include/mmu.h"
+
+.code16
+.global stage2start
+stage2start:
+  # Segments are already zeroed by stage1; %sp is inherited too.
+
+  movw    $bootmsg, %si
+  call    print16
+
+  # VBE Setup. Mode info block goes at 0x30000, not 0x9000 — stage2 itself
+  # is loaded starting at 0x7E00 and, with the boot-menu/progress-bar code,
+  # is now big enough to reach 0x9000, so writing the VBE block there would
+  # clobber stage2's own tail end. 0x30000 is clear of stage2 (ends well
+  # under 0xA000), the multiboot kernel-search buffer (0x10000-0x14000),
+  # and the kernel's own load address (0x100000).
+  movw    $0x3000, %ax
+  movw    %ax, %es
+  xorw    %di, %di            # ES:DI = 3000:0000 = flat 0x30000
+  movw    $0x0143, %cx
+  movw    $0x4F01, %ax
+  int     $0x10
+
+  cmpw    $0x004F, %ax
+  jne     e820
+  movw    $0x4143, %bx
+  movw    $0x4F02, %ax
+  int     $0x10
+
+  # BIOS E820 memory map — the standard firmware interface every serious
+  # loader (GRUB, Linux's own boot stub, the BSDs) uses to learn the real
+  # RAM layout instead of guessing. Entry count goes to flat 0x30200,
+  # 24-byte entries start at 0x30208 (both inside the 0x3000 segment the
+  # VBE block already claimed; verbose flag sits at +0x100). Capped at 32
+  # entries so a pathological BIOS can't run us past the buffer.
+e820:
+  movw    $0x3000, %ax
+  movw    %ax, %es
+  xorl    %ebx, %ebx          # continuation cookie, 0 = first call
+  movw    $0x208, %di
+  xorl    %ebp, %ebp          # entry count
+e820loop:
+  movl    $0xE820, %eax
+  movl    $0x534D4150, %edx   # 'SMAP'
+  movl    $24, %ecx
+  int     $0x15
+  jc      e820done            # carry = end of map (or unsupported BIOS)
+  cmpl    $0x534D4150, %eax
+  jne     e820done
+  addw    $24, %di
+  incl    %ebp
+  cmpl    $32, %ebp
+  jae     e820done
+  testl   %ebx, %ebx
+  jnz     e820loop
+e820done:
+  movl    %ebp, %es:(0x200)
+  xorw    %ax, %ax
+  movw    %ax, %es
+
+seta20.1:
+  inb     $0x64, %al
+  testb   $0x2, %al
+  jnz     seta20.1
+  movb    $0xd1, %al
+  outb    %al, $0x64
+seta20.2:
+  inb     $0x64, %al
+  testb   $0x2, %al
+  jnz     seta20.2
+  movb    $0xdf, %al
+  outb    %al, $0x60
+
+  lgdt    gdtdesc
+  movl    %cr0, %eax
+  orl     $CR0_PE, %eax
+  movl    %eax, %cr0
+
+  ljmp    $(SEG_KCODE<<3), $start32
+
+# Print null-terminated string at DS:SI via BIOS teletype (real mode only).
+# Clobbers AX, BX, SI.
+print16:
+.Lp16loop:
+  lodsb
+  testb   %al, %al
+  jz      .Lp16done
+  movb    $0x0E, %ah
+  movw    $0x0007, %bx
+  int     $0x10
+  jmp     .Lp16loop
+.Lp16done:
+  ret
+
+bootmsg:
+  .asciz "Schunix booting...\r\n"
+
+.code32
+start32:
+  movw    $(SEG_KDATA<<3), %ax
+  movw    %ax, %ds
+  movw    %ax, %es
+  movw    %ax, %ss
+  xorw    %ax, %ax
+  movw    %ax, %fs
+  movw    %ax, %gs
+
+  movl    $0x7c00, %esp
+  call    stage2main
+
+spin:
+  jmp     spin
+
+.p2align 2
+gdt:
+  SEG_NULLASM
+  SEG_ASM(STA_X|STA_R, 0x0, 0xffffffff)
+  SEG_ASM(STA_W, 0x0, 0xffffffff)
+
+gdtdesc:
+  .word   (gdtdesc - gdt - 1)
+  .long   gdt