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4. linux/arch/i386/boot/setup.S

setup.S is responsible for getting the system data from the BIOS and putting them into appropriate places in system memory.

Other boot loaders, like GNU GRUB and LILO, can load bzImage too. Such boot loaders should load bzImage into memory and setup "real-mode kernel header", esp. type_of_loader, then pass control to bsetup directly. setup.S assumes:

4.1. Header

/* Signature words to ensure LILO loaded us right */
#define SIG1    0xAA55
#define SIG2    0x5A5A

INITSEG  = DEF_INITSEG          # 0x9000, we move boot here, out of the way
SYSSEG   = DEF_SYSSEG           # 0x1000, system loaded at 0x10000 (65536).
SETUPSEG = DEF_SETUPSEG         # 0x9020, this is the current segment
                                # ... and the former contents of CS

DELTA_INITSEG = SETUPSEG - INITSEG      # 0x0020

.code16
.text

///////////////////////////////////////////////////////////////////////////////
start:
{
        goto trampoline();              // skip the following header
}

# This is the setup header, and it must start at %cs:2 (old 0x9020:2)
                .ascii  "HdrS"          # header signature
                .word   0x0203          # header version number (>= 0x0105)
                                        # or else old loadlin-1.5 will fail)
realmode_swtch: .word   0, 0            # default_switch, SETUPSEG
start_sys_seg:  .word   SYSSEG
                .word   kernel_version  # pointing to kernel version string
                                        # above section of header is compatible
                                        # with loadlin-1.5 (header v1.5). Don't
                                        # change it.
// kernel_version defined below
type_of_loader: .byte   0               # = 0, old one (LILO, Loadlin,
                                        #      Bootlin, SYSLX, bootsect...)
                                        # See Documentation/i386/boot.txt for
                                        # assigned ids
# flags, unused bits must be zero (RFU) bit within loadflags
loadflags:
LOADED_HIGH     = 1                     # If set, the kernel is loaded high
CAN_USE_HEAP    = 0x80                  # If set, the loader also has set
                                        # heap_end_ptr to tell how much
                                        # space behind setup.S can be used for
                                        # heap purposes.
                                        # Only the loader knows what is free
#ifndef __BIG_KERNEL__
                .byte   0
#else
                .byte   LOADED_HIGH
#endif
setup_move_size: .word  0x8000          # size to move, when setup is not
                                        # loaded at 0x90000. We will move setup
                                        # to 0x90000 then just before jumping
                                        # into the kernel. However, only the
                                        # loader knows how much data behind
                                        # us also needs to be loaded.
code32_start:                           # here loaders can put a different
                                        # start address for 32-bit code.
#ifndef __BIG_KERNEL__
                .long   0x1000          #   0x1000 = default for zImage
#else
                .long   0x100000        # 0x100000 = default for big kernel
#endif
ramdisk_image:  .long   0               # address of loaded ramdisk image
                                        # Here the loader puts the 32-bit
                                        # address where it loaded the image.
                                        # This only will be read by the kernel.
ramdisk_size:   .long   0               # its size in bytes
bootsect_kludge:
                .word  bootsect_helper, SETUPSEG
heap_end_ptr:   .word   modelist+1024   # (Header version 0x0201 or later)
                                        # space from here (exclusive) down to
                                        # end of setup code can be used by setup
                                        # for local heap purposes.
// modelist is at the end of .text section
pad1:           .word   0
cmd_line_ptr:   .long 0                 # (Header version 0x0202 or later)
                                        # If nonzero, a 32-bit pointer
                                        # to the kernel command line.
                                        # The command line should be
                                        # located between the start of
                                        # setup and the end of low
                                        # memory (0xa0000), or it may
                                        # get overwritten before it
                                        # gets read.  If this field is
                                        # used, there is no longer
                                        # anything magical about the
                                        # 0x90000 segment; the setup
                                        # can be located anywhere in
                                        # low memory 0x10000 or higher.
ramdisk_max:    .long __MAXMEM-1        # (Header version 0x0203 or later)
                                        # The highest safe address for
                                        # the contents of an initrd

The __MAXMEM definition in linux/asm-i386/page.h:

/*
 * A __PAGE_OFFSET of 0xC0000000 means that the kernel has
 * a virtual address space of one gigabyte, which limits the
 * amount of physical memory you can use to about 950MB.
 */
#define __PAGE_OFFSET           (0xC0000000)

/*
 * This much address space is reserved for vmalloc() and iomap()
 * as well as fixmap mappings.
 */
#define __VMALLOC_RESERVE       (128 << 20)

#define __MAXMEM                (-__PAGE_OFFSET-__VMALLOC_RESERVE)
It gives __MAXMEM = 1G - 128M.

The setup header must follow some layout pattern. Refer to linux/Documentation/i386/boot.txt:

Offset  Proto   Name            Meaning
/Size
0200/2  2.00+   jump            Jump instruction
0202/4  2.00+   header          Magic signature "HdrS"
0206/2  2.00+   version         Boot protocol version supported
0208/4  2.00+   realmode_swtch  Boot loader hook
020C/2  2.00+   start_sys       The load-low segment (0x1000) (obsolete)
020E/2  2.00+   kernel_version  Pointer to kernel version string
0210/1  2.00+   type_of_loader  Boot loader identifier
0211/1  2.00+   loadflags       Boot protocol option flags
0212/2  2.00+   setup_move_size Move to high memory size (used with hooks)
0214/4  2.00+   code32_start    Boot loader hook
0218/4  2.00+   ramdisk_image   initrd load address (set by boot loader)
021C/4  2.00+   ramdisk_size    initrd size (set by boot loader)
0220/4  2.00+   bootsect_kludge DO NOT USE - for bootsect.S use only
0224/2  2.01+   heap_end_ptr    Free memory after setup end
0226/2  N/A     pad1            Unused
0228/4  2.02+   cmd_line_ptr    32-bit pointer to the kernel command line
022C/4  2.03+   initrd_addr_max Highest legal initrd address

4.2. Check Code Integrity

As setup code may not be contiguous, we should check code integrity first.

///////////////////////////////////////////////////////////////////////////////
trampoline()
{
        start_of_setup();       // never return
        .space 1024;
}

///////////////////////////////////////////////////////////////////////////////
// check signature to see if all code loaded
start_of_setup()
{
        // Bootlin depends on this being done early, check bootlin:technic.doc
        int13/AH=15h(AL=0, DL=0x81);
        // int13/AH=15h: DISK - GET DISK TYPE

#ifdef SAFE_RESET_DISK_CONTROLLER
        int13/AH=0(AL=0, DL=0x80);
        // int13/AH=00h: DISK - RESET DISK SYSTEM
#endif

        DS = CS;
        // check signature at end of setup
        if (setup_sig1!=SIG1 || setup_sig2!=SIG2) {
                goto bad_sig;
        }
        goto goodsig1;
}

///////////////////////////////////////////////////////////////////////////////
// some small functions
prtstr();  /* print asciiz string at DS:SI */
prtsp2();  /* print double space */
prtspc();  /* print single space */
prtchr();  /* print ascii in AL */
beep();    /* print CTRL-G, i.e. beep */
Signature is checked to verify code integrity.

If signature is not found, the rest setup code may precede vmlinux at SYSSEG:0.

no_sig_mess: .string "No setup signature found ..."

goodsig1:
        goto goodsig;                           // make near jump

///////////////////////////////////////////////////////////////////////////////
// move the rest setup code from SYSSEG:0 to CS:0800
bad_sig()
        DELTA_INITSEG = 0x0020 (= SETUPSEG - INITSEG)
        SYSSEG = 0x1000
        word start_sys_seg = SYSSEG;            // defined in setup header
{
        DS = CS - DELTA_INITSEG;                // aka INITSEG
        BX = (byte)(DS:[497]);                  // i.e. setup_sects

        // first 4 sectors already loaded
        CX = (BX - 4) << 8;                     // rest code in word (2-bytes)
        start_sys_seg = (CX >> 3) + SYSSEG;     // real system code start
        move SYSSEG:0 to CS:0800 (CX*2 bytes);

        if (setup_sig1!=SIG1 || setup_sig2!=SIG2) {
no_sig:
                prtstr("No setup signature found ...");
no_sig_loop:
                hlt;
                goto no_sig_loop;
        }
}
"hlt" instruction stops instruction execution and places the processor in halt state. The processor generates a special bus cycle to indicate that halt mode has been entered. When an enabled interrupt (including NMI) is issued, the processor will resume execution after the "hlt" instruction, and the instruction pointer (CS:EIP), pointing to the instruction following the "hlt", will be saved to stack before the interrupt handler is called. Thus we need a "jmp" instruction after the "hlt" to put the processor back to halt state again.

The setup code has been moved to correct place. Variable start_sys_seg points to where real system code starts. If "bad_sig" does not happen, start_sys_seg remains SYSSEG.

4.3. Check Loader Type

Check if the loader is compatible with the image.

///////////////////////////////////////////////////////////////////////////////
good_sig()
        char loadflags;                 // in setup header
        char type_of_loader;            // in setup header
        LOADHIGH = 1
{
        DS = CS - DELTA_INITSEG;        // aka INITSEG
        if ( (loadflags & LOADHIGH) && !type_of_loader ) {
                // Nope, old loader tries to load big-kernel
                prtstr("Wrong loader, giving up...");
                goto no_sig_loop;       // defined above in bad_sig()
        }
}

loader_panic_mess: .string "Wrong loader, giving up..."
Note that type_of_loader has been changed to 0x20 by bootsect_helper() when it loads bvmlinux.

4.4. Get Memory Size

Try three different memory detection schemes to get the extended memory size (above 1M) in KB.

First, try e820h, which lets us assemble a memory map; then try e801h, which returns a 32-bit memory size; and finally 88h, which returns 0-64M.

///////////////////////////////////////////////////////////////////////////////
// get memory size
loader_ok()
        E820NR  = 0x1E8
        E820MAP = 0x2D0
{
        // when entering this function, DS = CS-DELTA_INITSEG, aka INITSEG
        (long)DS:[0x1E0] = 0;

#ifndef STANDARD_MEMORY_BIOS_CALL
        (byte)DS:[0x1E8] = 0;                   // E820NR

        /* method E820H: see ACPI spec
         * the memory map from hell.  e820h returns memory classified into
         * a whole bunch of different types, and allows memory holes and
         * everything.  We scan through this memory map and build a list
         * of the first 32 memory areas, which we return at [E820MAP]. */
meme820:
        EBX = 0;
        DI = 0x02D0;                            // E820MAP
        do {
jmpe820:
                int15/EAX=E820h(EDX='SMAP', EBX, ECX=20, ES:DI=DS:DI);
                // int15/AX=E820h: GET SYSTEM MEMORY MAP
                if (failed || 'SMAP'!=EAX) break;
                // if (1!=DS:[DI+16]) continue; // not usable
good820:
                if (DS:[1E8]>=32) break;        // entry# > E820MAX
                DS:[0x1E8]++;                   // entry# ++;
                DI += 20;                       // adjust buffer for next
again820:
        } while (!EBX)                          // not finished
bail820:

        /* method E801H:
         * memory size is in 1k chunksizes, to avoid confusing loadlin.
         * we store the 0xe801 memory size in a completely different place,
         * because it will most likely be longer than 16 bits.
         * (use 1e0 because that's what Larry Augustine uses in his
         * alternative new memory detection scheme, and it's sensible
         * to write everything into the same place.) */
meme801:
        stc;            // to work around buggy BIOSes
        CX = DX = 0;
        int15/AX=E801h;
        /* int15/AX=E801h: GET MEMORY SIZE FOR >64M CONFIGURATIONS
         *   AX = extended memory between 1M and 16M, in K (max 3C00 = 15MB)
         *   BX = extended memory above 16M, in 64K blocks
         *   CX = configured memory 1M to 16M, in K
         *   DX = configured memory above 16M, in 64K blocks */
        if (failed) goto mem88;
        if (!CX && !DX) {
                CX = AX;
                DX = BX;
        }
e801usecxdx:
        (long)DS:[0x1E0] = ((EDX & 0xFFFF) << 6) + (ECX & 0xFFFF);      // in K
#endif

mem88:  // old traditional method
        int15/AH=88h;
        /* int15/AH=88h: SYSTEM - GET EXTENDED MEMORY SIZE
         *   AX = number of contiguous KB starting at absolute address 100000h */
        DS:[2] = AX;
}

4.5. Hardware Support

Check hardware support, like keyboard, video adapter, hard disk, MCA bus and pointing device.

{
        // set the keyboard repeat rate to the max
        int16/AX=0305h(BX=0);
        // int16/AH=03h: KEYBOARD - SET TYPEMATIC RATE AND DELAY

        /* Check for video adapter and its parameters and
         *   allow the user to browse video modes. */
        video();                        // see video.S

        // get hd0 and hd1 data
        copy hd0 data (*int41) to CS-DELTA_INITSEG:0080 (16 bytes);
        // int41: SYSTEM DATA - HARD DISK 0 PARAMETER TABLE ADDRESS
        copy hd1 data (*int46) to CS-DELTA_INITSEG:0090 (16 bytes);
        // int46: SYSTEM DATA - HARD DISK 1 PARAMETER TABLE ADDRESS
        // check if hd1 exists
        int13/AH=15h(AL=0, DL=0x81);
        // int13/AH=15h: DISK - GET DISK TYPE
        if (failed || AH!=03h) {        // AH==03h if it is a hard disk
no_disk1:
                clear CS-DELTA_INITSEG:0090 (16 bytes);
        }
is_disk1:

        // check for Micro Channel (MCA) bus
        CS-DELTA_INITSEG:[0xA0] = 0;    // set table length to 0
        int15/AH=C0h;
        /* int15/AH=C0h: SYSTEM - GET CONFIGURATION
         *   ES:BX = ROM configuration table */
        if (failed) goto no_mca;
        move ROM configuration table (ES:BX) to CS-DELTA_INITSEG:00A0;
        // CX = (table length<14)? CX:16;    first 16 bytes only
no_mca:

        // check for PS/2 pointing device
        CS-DELTA_INITSEG:[0x1FF] = 0;   // default is no pointing device
        int11h();
        // int11h: BIOS - GET EQUIPMENT LIST
        if (AL & 0x04) {                // mouse installed
                DS:[0x1FF] = 0xAA;
        }
}

4.6. APM Support

Check BIOS APM support.

#if defined(CONFIG_APM) || defined(CONFIG_APM_MODULE)
{
        DS:[0x40] = 0;                  // version = 0 means no APM BIOS
        int15/AX=5300h(BX=0);
        // int15/AX=5300h: Advanced Power Management v1.0+ - INSTALLATION CHECK
        if (failed || 'PM'!=BX || !(CX & 0x02)) goto done_apm_bios;
        // (CX & 0x02) means 32 bit is supported
        int15/AX=5304h(BX=0);
        // int15/AX=5304h: Advanced Power Management v1.0+ - DISCONNECT INTERFACE
        EBX = CX = DX = ESI = DI = 0;
        int15/AX=5303h(BX=0);
        /* int15/AX=5303h: Advanced Power Management v1.0+
         *   - CONNECT 32-BIT PROTMODE INTERFACE */
        if (failed) {
no_32_apm_bios:                         // I moved label no_32_apm_bios here
                DS:[0x4C] &= ~0x0002;   // remove 32 bit support bit
                goto done_apm_bios;
        }
        DS:[0x42] = AX, 32-bit code segment base address;
        DS:[0x44] = EBX, offset of entry point;
        DS:[0x48] = CX, 16-bit code segment base address;
        DS:[0x4A] = DX, 16-bit data segment base address;
        DS:[0x4E] = ESI, APM BIOS code segment length;
        DS:[0x52] = DI, APM BIOS data segment length;
        int15/AX=5300h(BX=0);           // check again
        // int15/AX=5300h: Advanced Power Management v1.0+ - INSTALLATION CHECK
        if (success &&  'PM'==BX) {
                DS:[0x40] = AX, APM version;
                DS:[0x4C] = CX, APM flags;
        } else {
apm_disconnect:
                int15/AX=5304h(BX=0);
                /* int15/AX=5304h: Advanced Power Management v1.0+
                 *   - DISCONNECT INTERFACE */
        }
done_apm_bios:
}
#endif

4.7. Prepare for Protected Mode

// call mode switch
{
        if (realmode_swtch) {
                realmode_swtch();               // mode switch hook
        } else {
rmodeswtch_normal:
                default_switch() {
                        cli;                    // no interrupts allowed
                        outb(0x80, 0x70);       // disable NMI
                }
        }
rmodeswtch_end:
}

// relocate code if necessary
{
        (long)code32 = code32_start;
        if (!(loadflags & LOADED_HIGH)) {       // low loaded zImage
                // 0x0100 <= start_sys_seg < CS-DELTA_INITSEG
do_move0:
                AX = 0x100;
                BP = CS - DELTA_INITSEG;        // aka INITSEG
                BX = start_sys_seg;
do_move:
                move system image from (start_sys_seg:0 .. CS-DELTA_INITSEG:0)
                        to 0100:0;              // move 0x1000 bytes each time
        }
end_move:
Note that code32_start is initialized to 0x1000 for zImage, or 0x100000 for bzImage. The code32 value will be used in passing control to linux/arch/i386/boot/compressed/head.S in Section 4.9. If we boot up zImage, it relocates vmlinux to 0100:0; If we boot up bzImage, bvmlinux remains at start_sys_seg:0. The relocation address must match the "-Ttext" option in linux/arch/i386/boot/compressed/Makefile. See Section 2.5.

Then it will relocate code from CS-DELTA_INITSEG:0 (bbootsect and bsetup) to INITSEG:0, if necessary.

        DS = CS;                // aka SETUPSEG
        // Check whether we need to be downward compatible with version <=201
        if (!cmd_line_ptr && 0x20!=type_of_loader && SETUPSEG!=CS) {
                cli;            // as interrupt may use stack when we are moving
                // store new SS in DX
                AX = CS - DELTA_INITSEG;
                DX = SS;
                if (DX>=AX) {   // stack frame will be moved together
                        DX = DX + INITSEG - AX; // i.e. SS-CS+SETUPSEG
                }
move_self_1:
                /* move CS-DELTA_INITSEG:0 to INITSEG:0 (setup_move_size bytes)
                 *   in two steps in order not to overwrite code on CS:IP
                 * move up (src < dest) but downward ("std") */
                move CS-DELTA_INITSEG:move_self_here+0x200
                  to INITSEG:move_self_here+0x200,
                  setup_move_size-(move_self_here+0x200) bytes;
                // INITSEG:move_self_here+0x200 == SETUPSEG:move_self_here
                goto SETUPSEG:move_self_here;   // CS=SETUPSEG now
move_self_here:
                move CS-DELTA_INITSEG:0 to INITSEG:0,
                  move_self_here+0x200 bytes;   // I mean old CS before goto
                DS = SETUPSEG;
                SS = DX;
        }
end_move_self:
}
Note again, type_of_loader has been changed to 0x20 by bootsect_helper() when it loads bvmlinux.

4.8. Enable A20

For A20 problem and solution, refer to A20 - a pain from the past.

        A20_TEST_LOOPS          =  32   # Iterations per wait
        A20_ENABLE_LOOPS        = 255   # Total loops to try
{
#if defined(CONFIG_MELAN)
        // Enable A20. AMD Elan bug fix.
        outb(0x02, 0x92);               // outb(val, port)
a20_elan_wait:
        while (!a20_test());            // test not passed
        goto a20_done;
#endif

a20_try_loop:
        // First, see if we are on a system with no A20 gate.
a20_none:
        if (a20_test()) goto a20_done;  // test passed

        // Next, try the BIOS (INT 0x15, AX=0x2401)
a20_bios:
        int15/AX=2401h;
        // Int15/AX=2401h: SYSTEM - later PS/2s - ENABLE A20 GATE
        if (a20_test()) goto a20_done;  // test passed

        // Try enabling A20 through the keyboard controller
a20_kbc:
        empty_8042();
        if (a20_test()) goto a20_done;  // test again in case BIOS delayed
        outb(0xD1, 0x64);               // command write
        empty_8042();
        outb(0xDF, 0x60);               // A20 on
        empty_8042();
        // wait until a20 really *is* enabled
a20_kbc_wait:
        CX = 0;
a20_kbc_wait_loop:
        do {
                if (a20_test()) goto a20_done;  // test passed
        } while (--CX)

        // Final attempt: use "configuration port A"
        outb((inb(0x92) | 0x02) & 0xFE, 0x92);
        // wait for configuration port A to take effect
a20_fast_wait:
        CX = 0;
a20_fast_wait_loop:
        do {
                if (a20_test()) goto a20_done;  // test passed
        } while (--CX)

        // A20 is still not responding. Try frobbing it again.
        if (--a20_tries) goto a20_try_loop;
        prtstr("linux: fatal error: A20 gate not responding!");
a20_die:
        hlt;
        goto a20_die;
}

a20_tries:
        .byte   A20_ENABLE_LOOPS                // i.e. 255
a20_err_msg:
        .ascii  "linux: fatal error: A20 gate not responding!"
        .byte   13, 10, 0
For I/O port operations, take a look at related reference materials in Section 4.11.

4.9. Switch to Protected Mode

To ensure code compatibility with all 32-bit IA-32 processors, perform the following steps to switch to protected mode:

  1. Prepare GDT with a null descriptor in the first GDT entry, one code segment descriptor and one data segment descriptor;

  2. Disable interrupts, including maskable hardware interrupts and NMI;

  3. Load the base address and limit of the GDT to GDTR register, using "lgdt" instruction;

  4. Set PE flag in CR0 register, using "mov cr0" (Intel 386 and up) or "lmsw" instruction (for compatibility with Intel 286);

  5. Immediately execute a far "jmp" or a far "call" instruction.

The stack can be placed in a normal read/write data segment, so no dedicated descriptor is required.

a20_done:
{
        lidt    idt_48;         // load idt with 0, 0;

        // convert DS:gdt to a linear ptr
        *(long*)(gdt_48+2) = DS << 4 + &gdt;
        lgdt    gdt_48;

        // reset coprocessor
        outb(0, 0xF0);
        delay();
        outb(0, 0xF1);
        delay();

        // reprogram the interrupts
        outb(0xFF, 0xA1);       // mask all interrupts
        delay();
        outb(0xFB, 0x21);       // mask all irq's but irq2 which is cascaded

        // protected mode!
        AX = 1;
        lmsw ax;                // machine status word, bit 0 thru 15 of CR0
                                // only affects PE, MP, EM & TS flags
        goto flush_instr;

flush_instr:
        BX = 0;                                 // flag to indicate a boot
        ESI = (CS - DELTA_INITSEG) << 4;        // pointer to real-mode code
        /* NOTE: For high loaded big kernels we need a
         * jmpi    0x100000,__KERNEL_CS
         *
         * but we yet haven't reloaded the CS register, so the default size
         * of the target offset still is 16 bit.
         * However, using an operand prefix (0x66), the CPU will properly
         * take our 48 bit far pointer. (INTeL 80386 Programmer's Reference
         * Manual, Mixing 16-bit and 32-bit code, page 16-6) */

        // goto __KERNEL_CS:[(uint32*)code32]; */
        .byte   0x66, 0xea
code32: .long   0x1000          // overwritten in Section 4.7
        .word   __KERNEL_CS     // segment 0x10
        // see linux/arch/i386/boot/compressed/head.S:startup_32
}
The far "jmp" instruction (0xea) updates CS register. The contents of the remaining segment registers (DS, SS, ES, FS and GS) should be reloaded later. The operand-size prefix (0x66) is used to enforce "jmp" to be executed upon the 32-bit operand code32. For operand-size prefix details, check IA-32 Manual (Vol.1. Ch.3.6. Operand-size and Address-size Attributes, and Vol.3. Ch.17. Mixing 16-bit and 32-bit Code).

Control is passed to linux/arch/i386/boot/compressed/head.S:startup_32. For zImage, it is at address 0x1000; For bzImage, it is at 0x100000. See Section 5.

ESI points to the memory area of collected system data. It is used to pass parameters from the 16-bit real mode code of the kernel to the 32-bit part. See linux/Documentation/i386/zero-page.txt for details.

For mode switching details, refer to IA-32 Manual Vol.3. (Ch.9.8. Software Initialization for Protected-Mode Operation, Ch.9.9.1. Switching to Protected Mode, and Ch.17.4. Transferring Control Among Mixed-Size Code Segments).

4.10. Miscellaneous

The rest are supporting functions and variables.

/* macros created by linux/Makefile targets:
 *   include/linux/compile.h and include/linux/version.h */
kernel_version: .ascii  UTS_RELEASE
                .ascii  " ("
                .ascii  LINUX_COMPILE_BY
                .ascii  "@"
                .ascii  LINUX_COMPILE_HOST
                .ascii  ") "
                .ascii  UTS_VERSION
                .byte   0

///////////////////////////////////////////////////////////////////////////////
default_switch() { cli; outb(0x80, 0x70); } /* disable interrupts and NMI */
bootsect_helper(ES:BX); /* see Section 3.7 */

///////////////////////////////////////////////////////////////////////////////
a20_test()
{
        FS = 0;
        GS = 0xFFFF;
        CX = A20_TEST_LOOPS;                    // i.e. 32
        AX = FS:[0x200];
        do {
a20_test_wait:
                FS:[0x200] = ++AX;
                delay();
        } while (AX==GS:[0x210] && --CX);
        return (AX!=GS[0x210]);
        // ZF==0 (i.e. NZ/NE, a20_test!=0) means test passed
}

///////////////////////////////////////////////////////////////////////////////
// check that the keyboard command queue is empty
empty_8042()
{
        int timeout = 100000;

        for (;;) {
empty_8042_loop:
                if (!--timeout) return;
                delay();
                inb(0x64, &AL);                 // 8042 status port
                if (AL & 1) {                   // has output
                        delay();
                        inb(0x60, &AL);         // read it
no_output:      } else if (!(AL & 2)) return;   // no input either
        }
}

///////////////////////////////////////////////////////////////////////////////
// read the CMOS clock, return the seconds in AL, used in video.S
gettime()
{
        int1A/AH=02h();
        /* int1A/AH=02h: TIME - GET REAL-TIME CLOCK TIME
         * DH = seconds in BCD */
        AL = DH & 0x0F;
        AH = DH >> 4;
        aad;
}

///////////////////////////////////////////////////////////////////////////////
delay() { outb(AL, 0x80); }                     // needed after doing I/O

// Descriptor table
gdt:
        .word   0, 0, 0, 0                      # dummy
        .word   0, 0, 0, 0                      # unused
        // segment 0x10, __KERNEL_CS
        .word   0xFFFF                          # 4Gb - (0x100000*0x1000 = 4Gb)
        .word   0                               # base address = 0
        .word   0x9A00                          # code read/exec
        .word   0x00CF                          # granularity = 4096, 386
                                                #  (+5th nibble of limit)
        // segment 0x18, __KERNEL_DS
        .word   0xFFFF                          # 4Gb - (0x100000*0x1000 = 4Gb)
        .word   0                               # base address = 0
        .word   0x9200                          # data read/write
        .word   0x00CF                          # granularity = 4096, 386
                                                #  (+5th nibble of limit)
idt_48:
        .word   0                               # idt limit = 0
        .word   0, 0                            # idt base = 0L
/* [gdt_48] should be 0x0800 (2048) to match the comment,
 *   like what Linux 2.2.22 does. */
gdt_48:
        .word   0x8000                          # gdt limit=2048,
                                                #  256 GDT entries
        .word   0, 0                            # gdt base (filled in later)

#include "video.S"

// signature at the end of setup.S:
{
setup_sig1:     .word   SIG1                    // 0xAA55
setup_sig2:     .word   SIG2                    // 0x5A5A
modelist:
}

Video setup and detection code in video.S:

ASK_VGA = 0xFFFD  // defined in linux/include/asm-i386/boot.h
///////////////////////////////////////////////////////////////////////////////
video()
{
        pushw DS;               // use different segments
        FS = DS;
        DS = ES = CS;
        GS = 0;
        cld;
        basic_detect();         // basic adapter type testing (EGA/VGA/MDA/CGA)
#ifdef CONFIG_VIDEO_SELECT
        if (FS:[0x01FA]!=ASK_VGA) {     // user selected video mode
                mode_set();
                if (failed) {
                        prtstr("You passed an undefined mode number.\n");
                        mode_menu();
                }
        } else {
vid2:           mode_menu();
        }
vid1:
#ifdef CONFIG_VIDEO_RETAIN
        restore_screen();               // restore screen contents
#endif /* CONFIG_VIDEO_RETAIN */
#endif /* CONFIG_VIDEO_SELECT */
        mode_params();                  // store mode parameters
        popw ds;                        // restore original DS
}
/* TODO: video() details */

4.11. Reference