开机过程分析--BIOS阶段

// Indicator if POST phase has been started (and if it has completed).
int HaveRunPost VARFSEG;

src/entryfuncs.S:
        // Reset stack, transition to 32bit mode, and call a C function.
        .macro ENTRY_INTO32 cfunc
        xorw %dx, %dx 
        movw %dx, %ss 
        movl $ BUILD_STACK_ADDR , %esp
        movl $ \cfunc , %edx
        jmp transition32
        .endm

src/config.h：
// Various memory addresses used by the code.
#define BUILD_STACK_ADDR          0x7000

src/hw/rtc.h:
#define PORT_CMOS_INDEX        0x0070
#define PORT_CMOS_DATA         0x0071

// PORT_CMOS_INDEX nmi disable bit
#define NMI_DISABLE_BIT 0x80
...
#define CMOS_RESET_CODE          0x0f

// Dummy IDT that forces a machine shutdown if an irq happens in
// protected mode.
u8 dummy_IDT VARFSEG;

// Protected mode IDT descriptor
struct descloc_s pmode_IDT_info VARFSEG = {
    .length = sizeof(dummy_IDT) - 1,
    .addr = (u32)&dummy_IDT,
};

// GDT
u64 rombios32_gdt[] VARFSEG __aligned(8) = {
    // First entry can't be used.
    0x0000000000000000LL,
    // 32 bit flat code segment (SEG32_MODE32_CS)
    GDT_GRANLIMIT(0xffffffff) | GDT_CODE | GDT_B,
    // 32 bit flat data segment (SEG32_MODE32_DS)
    GDT_GRANLIMIT(0xffffffff) | GDT_DATA | GDT_B,
    // 16 bit code segment base=0xf0000 limit=0xffff (SEG32_MODE16_CS)
    GDT_LIMIT(BUILD_BIOS_SIZE-1) | GDT_CODE | GDT_BASE(BUILD_BIOS_ADDR),
    // 16 bit data segment base=0x0 limit=0xffff (SEG32_MODE16_DS)
    GDT_LIMIT(0x0ffff) | GDT_DATA,
    // 16 bit code segment base=0xf0000 limit=0xffffffff (SEG32_MODE16BIG_CS)
    GDT_GRANLIMIT(0xffffffff) | GDT_CODE | GDT_BASE(BUILD_BIOS_ADDR),
    // 16 bit data segment base=0 limit=0xffffffff (SEG32_MODE16BIG_DS)
    GDT_GRANLIMIT(0xffffffff) | GDT_DATA,
};

// GDT descriptor
struct descloc_s rombios32_gdt_48 VARFSEG = { 
    .length = sizeof(rombios32_gdt) - 1,
    .addr = (u32)rombios32_gdt,
};

src/x86.h:
// CR0 flags
#define CR0_PG (1<<31) // Paging
#define CR0_CD (1<<30) // Cache disable
#define CR0_NW (1<<29) // Not Write-through
#define CR0_PE (1<<0)  // Protection enable

src/config.h:
// Segment definitions in protected mode (see rombios32_gdt in misc.c)
#define SEG32_MODE32_CS    (1 << 3)
#define SEG32_MODE32_DS    (2 << 3)

src/post.c:
// Entry point for Power On Self Test (POST) - the BIOS initilization
// phase.  This function makes the memory at 0xc0000-0xfffff
// read/writable and then calls dopost().
void VISIBLE32FLAT
handle_post(void)
{
    if (!CONFIG_QEMU && !CONFIG_COREBOOT)
        return;

    serial_debug_preinit();
    debug_banner();

    // Check if we are running under Xen.
    xen_preinit();

    // Allow writes to modify bios area (0xf0000)
    make_bios_writable();

    // Now that memory is read/writable - start post process.
    dopost();
}

// Setup for code relocation and then relocate.
void VISIBLE32INIT
dopost(void)
{
    code_mutable_preinit();

    // Detect ram and setup internal malloc.
    qemu_preinit();
    coreboot_preinit();
    malloc_preinit();
    
    // Relocate initialization code and call maininit().
    reloc_preinit(maininit, NULL);
}

// Begin the boot process by invoking an int0x19 in 16bit mode.
void VISIBLE32FLAT
startBoot(void)
{
    // Clear low-memory allocations (required by PMM spec).
    memset((void*)BUILD_STACK_ADDR, 0, BUILD_EBDA_MINIMUM - BUILD_STACK_ADDR);

    dprintf(3, "Jump to int19\n");
    struct bregs br;
    memset(&br, 0, sizeof(br));
    br.flags = F_IF;
    call16_int(0x19, &br);
}

src/stacks.h:
#define call16_int(nr, callregs) do {                           \
        extern void irq_trampoline_ ##nr (void);                \
        __call16_int((callregs), (u32)&irq_trampoline_ ##nr );  \
    } while (0)

src/romlayout.S:
// IRQ trampolines
        .macro IRQ_TRAMPOLINE num 
        DECLFUNC irq_trampoline_0x\num
        irq_trampoline_0x\num :
        int $0x\num
        lretw
        .endm

        IRQ_TRAMPOLINE 02
        IRQ_TRAMPOLINE 05
        IRQ_TRAMPOLINE 10
        IRQ_TRAMPOLINE 13
        IRQ_TRAMPOLINE 15
        IRQ_TRAMPOLINE 16
        IRQ_TRAMPOLINE 18
        IRQ_TRAMPOLINE 19
        IRQ_TRAMPOLINE 1b
        IRQ_TRAMPOLINE 1c
        IRQ_TRAMPOLINE 4a

src/post.c:
    // Initialize software handlers.
    SET_IVT(0x02, FUNC16(entry_02));
    SET_IVT(0x05, FUNC16(entry_05));
    SET_IVT(0x10, FUNC16(entry_10));
    SET_IVT(0x11, FUNC16(entry_11));
    SET_IVT(0x12, FUNC16(entry_12));
    SET_IVT(0x13, FUNC16(entry_13_official));
    SET_IVT(0x14, FUNC16(entry_14));
    SET_IVT(0x15, FUNC16(entry_15_official));
    SET_IVT(0x16, FUNC16(entry_16));
    SET_IVT(0x17, FUNC16(entry_17));
    SET_IVT(0x18, FUNC16(entry_18));
    SET_IVT(0x19, FUNC16(entry_19_official));
    SET_IVT(0x1a, FUNC16(entry_1a_official));
    SET_IVT(0x40, FUNC16(entry_40));

src/romlayout.S:
        // int 18/19 are special - they reset stack and call into 32bit mode.
        DECLFUNC entry_19
entry_19:
        ENTRY_INTO32 _cfunc32flat_handle_19
...
        .global entry_19_official
entry_19_official:
        jmp entry_19

src/boot.c:
// Determine next boot method and attempt a boot using it.
static void 
do_boot(int seq_nr)
{
    if (! CONFIG_BOOT)
        panic("Boot support not compiled in.\n");

    if (seq_nr >= BEVCount)
        boot_fail();

    // Boot the given BEV type.
    struct bev_s *ie = &BEV[seq_nr];
    switch (ie->type) {
    case IPL_TYPE_FLOPPY:
        printf("Booting from Floppy...\n");
        boot_disk(0x00, CheckFloppySig);
        break;
    case IPL_TYPE_HARDDISK:
        printf("Booting from Hard Disk...\n");
        boot_disk(0x80, 1);
        break;
    case IPL_TYPE_CDROM:
        boot_cdrom((void*)ie->vector);
        break;
    case IPL_TYPE_CBFS:
        boot_cbfs((void*)ie->vector);
        break;
    case IPL_TYPE_BEV:
        boot_rom(ie->vector);
        break;
    case IPL_TYPE_HALT:
        boot_fail();
        break;
    }    

    // Boot failed: invoke the boot recovery function
    struct bregs br;
    memset(&br, 0, sizeof(br));
    br.flags = F_IF;
    call16_int(0x18, &br);
}

// INT 19h Boot Load Service Entry Point
void VISIBLE32FLAT
handle_19(void)
{
    debug_enter(NULL, DEBUG_HDL_19);
    BootSequence = 0; 
    do_boot(0);
}

src/boot.c
// Boot from a disk (either floppy or harddrive)
static void 
boot_disk(u8 bootdrv, int checksig)
{
    u16 bootseg = 0x07c0;

    // Read sector
    struct bregs br;
    memset(&br, 0, sizeof(br));
    br.flags = F_IF;
    br.dl = bootdrv;
    br.es = bootseg;
    br.ah = 2; 
    br.al = 1; 
    br.cl = 1; 
    call16_int(0x13, &br);

    if (br.flags & F_CF) {
        printf("Boot failed: could not read the boot disk\n\n");
        return;
    }    

    if (checksig) {
        struct mbr_s *mbr = (void*)0;
        if (GET_FARVAR(bootseg, mbr->signature) != MBR_SIGNATURE) {
            printf("Boot failed: not a bootable disk\n\n");
            return;
        }    
    }    

    tpm_add_bcv(bootdrv, MAKE_FLATPTR(bootseg, 0), 512);

    /* Canonicalize bootseg:bootip */
    u16 bootip = (bootseg & 0x0fff) << 4;
    bootseg &= 0xf000;

    call_boot_entry(SEGOFF(bootseg, bootip), bootdrv);
}