x86-bare-metal-examples/common.h

/*
Using macros for now instead of functions because it simplifies the linker script.

But the downsides are severe:

-   no symbols to help debugging

-   impossible to step over method calls: you have to step into everything

-   larger output, supposing I can get linker gc for unused functions working,
    see --gc-section, which is for now uncertain.
    If I can get this working, I'll definitely move to function calls.

    The problem is that if I don't, every image will need a stage 2 loader.
    That is not too serious though, it could be added to BEGIN.

## Conventions

Every "function-like macro" should maintain GP register state
(flags currently not maintained).

%sp cannot be used to pass most arguments.

We don't care about setting %bp.
*/

/*
I really want this for the local labels.

The major downside is that every register passed as argument requires `<>`:
http://stackoverflow.com/questions/19776992/gas-altmacro-macro-with-a-percent-sign-in-a-default-parameter-fails-with-oper/
*/
.altmacro

/* Helpers */

/* Push registers ax, bx, cx and dx. Lightweight `pusha`. */
.macro PUSH_ADX
    push %ax
    push %bx
    push %cx
    push %dx
.endm

/*
Pop registers dx, cx, bx, ax. Inverse order from PUSH_ADX,
so this cancels that one.
*/
.macro POP_DAX
    pop %dx
    pop %cx
    pop %bx
    pop %ax
.endm

.macro PUSH_EADX
    push %eax
    push %ebx
    push %ecx
    push %edx
.endm

.macro POP_EDAX
    pop %edx
    pop %ecx
    pop %ebx
    pop %eax
.endm

/*
Convert the low nibble of a r8 reg to ASCII of 8-bit in-place.
reg: r8 to be converted
Output: stored in reg itself. Letters are uppercase.
*/
.macro HEX_NIBBLE reg
    LOCAL letter, end
    cmp $10, \reg
    jae letter
    add $'0, \reg
    jmp end
letter:
    /* 0x37 == 'A' - 10 */
    add $0x37, \reg
end:
.endm

/*
Convert a byte to hex ASCII value.
c: r/m8 byte to be converted
Output: two ASCII characters, is stored in `ah:al`
http://stackoverflow.com/questions/3853730/printing-hexadecimal-digits-with-assembly
*/
.macro HEX c
    mov \c, %al
    mov \c, %ah
    shr $4, %al
    HEX_NIBBLE <%al>
    and $0x0F, %ah
    HEX_NIBBLE <%ah>
.endm

/* Structural. */

/*
Setup a sane initial state.

Should be the first thing in every file.

Discussion of what is needed exactly: http://stackoverflow.com/a/32509555/895245


*/
.macro BEGIN
    .code16
    cli
    /* Set %cs to 0. TODO Is that really needed? */
    ljmp $0, $1f
    1:
    xor %ax, %ax
    /* We must zero %ds for any data access. */
    mov %ax, %ds
    /* TODO is it really need to clear all those segment registers, e.g. for BIOS calls? */
    mov %ax, %es
    mov %ax, %fs
    mov %ax, %gs
    /*
    TODO What to move into BP and SP?
    http://stackoverflow.com/questions/10598802/which-value-should-be-used-for-sp-for-booting-process
    */
    mov %ax, %bp
    /* Automatically disables interrupts until the end of the next instruction. */
    mov %ax, %ss
    /* We should set SP because BIOS calls may depend on that. TODO confirm. */
    mov %bp, %sp
.endm

/*
Load stage2 from disk to memory, and jump to it.

To be used when the program does not fit in the 512 bytes.

Sample usage:

    STAGE2
    Stage 2 code here.
*/
.macro STAGE2
    /* Defined in the linker script. */
    mov $__stage2_nsectors, %al
    mov $0x02, %ah
    mov $1f, %bx
    mov $0x0002, %cx
    mov $0x0080, %dx
    int $0x13
    jmp 1f
    .section .stage2
    1:
.endm

/*
Enter protected mode.

Use the simplest GDT possible.
*/
.macro PROTECTED_MODE
    /* Must come before they are used. */
    .equ CODE_SEG, 8
    .equ DATA_SEG, gdt_data - gdt_start

    /* Tell the processor where our Global Descriptor Table is in memory. */
    lgdt gdt_descriptor

    /*
    Set PE (Protection Enable) bit in CR0 (Control Register 0),
    effectively entering protected mode.
    */
    mov %cr0, %eax
    orl $0x1, %eax
    mov %eax, %cr0

    ljmp $CODE_SEG, $protected_mode
/*
Our GDT contains:
- a null entry to fill the unusable entry 0:
  http://stackoverflow.com/questions/33198282/why-have-the-first-segment-descriptor-of-the-global-descriptor-table-contain-onl
- a code and data. Both are necessary, because:
  - it is impossible to write to the code segment
  - it is impossible execute the data segment
  Both start at 0 and span the entire memory,
  allowing us to access anything without problems.
A real OS might have 2 extra segments: user data and code.
This is the case for the Linux kernel.
This is better than modifying the privilege bit of the GDT
as we'd have to reload it several times, losing cache.
*/
gdt_start:
gdt_null:
    .long 0x0
    .long 0x0
gdt_code:
    .word 0xffff
    .word 0x0
    .byte 0x0
    .byte 0b10011010
    .byte 0b11001111
    .byte 0x0
gdt_data:
    .word 0xffff
    .word 0x0
    .byte 0x0
    .byte 0b10010010
    .byte 0b11001111
    .byte 0x0
gdt_end:
gdt_descriptor:
    .word gdt_end - gdt_start
    .long gdt_start
vga_current_line:
    .long 0
.code32
protected_mode:
    /*
    Setup the other segments.
    Those movs are mandatory because they update the descriptor cache:
    http://wiki.osdev.org/Descriptor_Cache
    */
    mov $DATA_SEG, %ax
    mov %ax, %ds
    mov %ax, %es
    mov %ax, %fs
    mov %ax, %gs
    mov %ax, %ss
    /*
    TODO detect the last memory address available properly.
    It depends on how much RAM we have.
    */
    mov $0X7000, %ebp
    mov %ebp, %esp
.endm

/*
Setup a single page directory, which give us 2^10 * 2^12 == 4MiB
of identity memory starting at address 0.
The currently executing code is inside that range, or else we'd jump somewhere and die.
*/
.equ page_directory, __end_align_4k
.equ page_table, __end_align_4k + 0x1000
.macro SETUP_PAGING_4M
    LOCAL page_setup_start page_setup_end
    PUSH_EADX

    /* Page directory steup. */
    /* Set the top 20 address bits. */
    mov $page_table, %eax
    /* Zero out the 4 low flag bits of the second byte (top 20 are address). */
    and $0xF000, %ax
    mov %eax, page_directory
    /* Set flags for the first byte. */
    mov $0b00100111, %al
    mov %al, page_directory

    /* Page table setup. */
    mov $0, %eax
    mov $page_table, %ebx
page_setup_start:
    cmp $0x400, %eax
    je page_setup_end
    /* Top 20 address bits. */
    mov %eax, %edx
    shl $12, %edx
    /*
    Set flag bits 0-7. We only set to 1:
    -   bit 0: Page present

    -   bit 1: Page is writable.
        Might work without this as the permission also depends on CR0.WP.
    */
    mov $0b00000011, %dl
    /* Zero flag bits 8-11 */
    and $0xF0, %dh
    mov %edx, (%ebx)
    inc %eax
    add $4, %ebx
    jmp page_setup_start
page_setup_end:
    POP_EDAX
.endm

/*
Turn paging on.
Registers are not saved because memory will be all messed up.
*/
.macro PAGING_ON
    /* Tell the CPU where the page directory is. */
    mov $page_directory, %eax
    mov %eax, %cr3

    /* Turn paging on. */
    mov %cr0, %eax
    or $0x80000000, %eax
    mov %eax, %cr0
.endm

/* Turn paging off. */
.macro PAGING_OFF
    mov %cr0, %eax
    and $0x7FFFFFFF, %eax
    mov  %eax, %cr0
.endm

/* IDT */

.macro IDT_START
    idt_start:
.endm

.macro IDT_END
idt_end:
/* Exact same structure as gdt_descriptor. */
idt_descriptor:
    .word idt_end - idt_start
    .long idt_start
.endm

.macro IDT_ENTRY
    /*
    Low handler address.
    It is impossible to write:
    .word (handler & 0x0000FFFF)
    as we would like:
    http://stackoverflow.com/questions/18495765/invalid-operands-for-binary-and
    because this address has to be split up into two.
    So this must be done at runtime.
    Why this design choice from Intel?! Weird.
    */
    .word 0
    /* Segment selector: byte offset into the GDT. */
    .word CODE_SEG
    /* Reserved 0. */
    .byte 0
    /*
    Flags. Format:
    - 1 bit: present. If 0 and this happens, triple fault.
    - 2 bits: ring level we will be called from.
    - 5 bits: fixed to 0xE.
    */
    .byte 0x8E
    /* High word of base. */
    .word 0
.endm

/* Skip n IDT entries, usually to set the Nth one next. */
.macro IDT_SKIP n=1
    .skip n * 8
.endm

/*
- index: r/m/imm32 Index of the entry to setup.
- handler: r/m/imm32 Address of the handler function.
*/
.macro IDT_SETUP_ENTRY index, handler
    push %eax
    push %edx
    mov \index, %eax
    mov \handler, %edx
    mov %dx, idt_start(,%eax, 8)
    shr $16, %edx
    mov %dx, (idt_start + 6)(,%eax, 8)
    pop %edx
    pop %eax
.endm

/* Shamelessly copied from James Molloy's tutorial. */
.macro ISR_NOERRCODE i
    isr\()\i:
        cli
        push $0
        push $\i
        jmp interrupt_handler
.endm

.macro ISR_ERRCODE i
    isr\()\i:
        cli
        push $\i
        jmp interrupt_handler
.endm

/*
Entries and handlers.
48 = 32 processor built-ins + 16 PIC interrupts.
*/
.macro IDT_48_ENTRIES
    IDT_START
    .rept 48
    IDT_ENTRY
    .endr
    IDT_END

    .irp i, 0, 1, 2, 3, 4, 5, 6, 7
        ISR_NOERRCODE \i
    .endr
    ISR_ERRCODE   8
    ISR_NOERRCODE 9
    .irp i, 10, 11, 12, 13, 14
        ISR_ERRCODE \i
    .endr
    .irp i, 15, 16, 17, 18, 19, \
            20, 21, 22, 23, 24, 25, 26, 27, 28, 29, \
            30, 31, 32, 33, 34, 35, 36, 37, 38, 39, \
            40, 41, 42, 43, 44, 45, 46, 47, 48
        ISR_NOERRCODE \i
    .endr
.endm

.macro IDT_SETUP_48_ISRS
    .irp i,  0,  1,  2,  3,  4,  5,  6,  7,  8,  9, \
            10, 11, 12, 13, 14, 15, 16, 17, 18, 19, \
            20, 21, 22, 23, 24, 25, 26, 27, 28, 29, \
            20, 21, 22, 23, 24, 25, 26, 27, 28, 29, \
            30, 31, 32, 33, 34, 35, 36, 37, 38, 39, \
            40, 41, 42, 43, 44, 45, 46, 47, 48
        IDT_SETUP_ENTRY $\i, $isr\()\i
    .endr
    lidt idt_descriptor
.endm

/* BIOS */

.macro CURSOR_POSITION x=$0, y=$0
    PUSH_ADX
    mov $0x02, %ah
    mov $0x00, %bh
    mov \x, %dh
    mov \y, %dl
    int $0x10
    POP_DAX
.endm

/* Clear the screen, move to position 0, 0. */
.macro CLEAR
    PUSH_ADX
    mov $0x0600, %ax
    mov $0x7, %bh
    mov $0x0, %cx
    mov $0x184f, %dx
    int $0x10
    CURSOR_POSITION
    POP_DAX
.endm

/*
Print a 8 bit ASCII value at current cursor position.

- c  r/m/imm8  ASCII value to be printed.

Usage:

    PUTC $'a

prints `'a'` to the screen.
*/
.macro PUTC c=$0x20
    push %ax
    mov \c, %al
    mov $0x0E, %ah
    int $0x10
    pop %ax
.endm

/*
Print a byte as two hexadecimal digits.

- reg: 1 byte register.
*/
.macro PRINT_HEX reg=<%al>
    push %ax
    HEX <\reg>
    PUTC <%al>
    PUTC <%ah>
    pop %ax
.endm

/*
Print a 16-bit number

- in: r/m/imm16
*/
.macro PRINT_WORD_HEX in=<%ax>
    push %ax
    mov \in, %ax
    PRINT_HEX <%ah>
    PRINT_HEX <%al>
    pop %ax
.endm

.macro PRINT_NEWLINE
    PUTC $'\n
    PUTC $'\r
.endm

/*
Print a null terminated string.

Use as:

        PRINT_STRING $s
        hlt
    s:
        .asciz "string"
*/
.macro PRINT_STRING s
    LOCAL end, loop
    mov s, %si
    mov $0x0e, %ah
    cld
loop:
    lodsb
    or %al, %al
    jz end
    int $0x10
    jmp loop
end:
.endm

/*
Dump memory.

- s: starting address
- n: number of bytes to dump
*/
.macro PRINT_BYTES s, n=$16
    LOCAL end, loop, no_newline
    PUSH_ADX
    push %di
    mov s, %si
    mov \n, %cx
    mov $0, %di
    cld
loop:
    cmp $0, %cx
    je end
    dec %cx
    lodsb
    PRINT_HEX
    PUTC
    /* Print a newline for every 8 bytes. */
    mov $0, %dx
    mov %di, %ax
    mov $8, %bx
    div %bx
    cmp $7, %dx
    jne no_newline
    PRINT_NEWLINE
no_newline:
    inc %di
    jmp loop
end:
    pop %di
    POP_DAX
.endm

/* VGA */

/*
Print a NULL terminated string to position 0 in VGA.

s: 32-bit register or memory containing the address of the string to print.

Clobbers: none.

Uses and updates vga_current_line to decide the current line.
Loops around the to the top.
*/
.macro VGA_PRINT_STRING s
    LOCAL loop, end
    PUSH_EADX
    mov \s, %ecx
    mov vga_current_line, %eax
    mov $0, %edx
    /* Number of horizontal lines. */
    mov $25, %ebx
    div %ebx
    mov %edx, %eax
    /* 160 == 80 * 2 == line width * bytes per character on screen */
    mov $160, %edx
    mul %edx
    /* 0xb8000 == magic video memory address which shows on the screen. */
    lea 0xb8000(%eax), %edx
    /* White on black. */
    mov $0x0f, %ah
loop:
    mov (%ecx), %al
    cmp $0, %al
    je end
    mov %ax, (%edx)
    add $1, %ecx
    add $2, %edx
    jmp loop
end:
    incl vga_current_line
    POP_EDAX
.endm

/*
Print a 32-bit r/m/immm in hex.

Sample usage:

    mov $12345678, %eax
    VGA_PRINT_HEX_4 <%eax>

Expected output on screen:

    12345678
*/
.macro VGA_PRINT_HEX_4 reg=<%eax>
    LOCAL loop
    PUSH_EADX
    /* Null terminator. */
    mov \reg, %ecx

    /* Write ASCII representation to memory. */
    push $0
    mov $2, %ebx
loop:
    HEX <%cl>
    mov %ax, %dx
    shl $16, %edx
    HEX <%ch>
    mov %ax, %dx
    push %edx
    shr $16, %ecx
    dec %ebx
    cmp $0, %ebx
    jne loop

    /* Print it. */
    mov %esp, %edx
    VGA_PRINT_STRING <%edx>

    /* Restore the stack. We have pushed 3 * 4 bytes. */
    add $12, %esp
    POP_EDAX
.endm

/*
Dump memory.

- s: starting address
- n: number of bytes to dump

TODO implement
*/
.macro VGA_PRINT_BYTES s, n=$16
    LOCAL end, loop, no_newline
    PUSH_ADX
    push %edi
    mov s, %esi
    mov \n, %ecx
    mov $0, %edi
    cld
loop:
    cmp $0, %ecx
    je end
    dec %ecx
    lodsb
    PRINT_HEX
    PUTC
    /* Print a newline for every 8 bytes. */
    mov $0, %edx
    mov %di, %eax
    mov $8, %ebx
    div %ebx
    cmp $7, %edx
    jne no_newline
    /*VGA_PRINT_NEWLINE*/
no_newline:
    inc %edi
    jmp loop
end:
    pop %di
    POP_DAX
.endm

/* IO ports. */

.macro OUTB value, port
    push %ax
    mov \value, %al
    out %al, \port
    pop %ax
.endm

#define PORT_PIC_MASTER_CMD $0x20
#define PORT_PIC_MASTER_DATA $0x21
#define PORT_PIT_CHANNEL0 $0x40
#define PORT_PIT_MODE $0x43
#define PORT_PIC_SLAVE_CMD $0xA0
#define PORT_PIC_SLAVE_DATA $0xA1

/* PIC */

#define PIC_CMD_RESET $0x20
#define PIC_ICR_ADDRESS $0xFEE00300

/* EOI End Of Interrupt: PIC it will not fire again unless we reset it. */
.macro PIC_EOI
    OUTB PIC_CMD_RESET, PORT_PIC_MASTER_CMD
.endm

.macro REMAP_PIC_32
    /*
    Remap the PIC interrupts to start at 32.
    TODO understand.
    */
    OUTB $0x11, PORT_PIC_MASTER_CMD
    OUTB $0x11, PORT_PIC_SLAVE_CMD
    OUTB $0x20, PORT_PIC_MASTER_DATA
    OUTB $0x28, PORT_PIC_SLAVE_DATA
    OUTB $0x04, PORT_PIC_MASTER_DATA
    OUTB $0x02, PORT_PIC_SLAVE_DATA
    OUTB $0x01, PORT_PIC_MASTER_DATA
    OUTB $0x01, PORT_PIC_SLAVE_DATA
    OUTB $0x00, PORT_PIC_MASTER_DATA
    OUTB $0x00, PORT_PIC_SLAVE_DATA
.endm

/* PIT */

/*
Set the minimum possible PIT frequency.
This is a human friendly frequency: you can see individual events,
but you don't have to wait much for each one.
*/
.macro PIT_SET_MIN_FREQ
    mov $0xFF, %al
    out %al, PORT_PIT_CHANNEL0
    out %al, PORT_PIT_CHANNEL0
.endm

/* IVT */

#define IVT_PIT 8
#define IVT_HANDLER_SIZE 4
#define IVT_CODE_OFFSET 2

/* Setup interrupt handler 8: this is where the PIC maps IRQ 0 to. */
.macro IVT_PIT_SETUP
    movw $handler, IVT_PIT * IVT_HANDLER_SIZE
    mov %cs, IVT_PIT * IVT_HANDLER_SIZE + IVT_CODE_OFFSET
.endm