arch.x86.bda¶

BIOS Data Area (BDA) structures and system tick counter management.

Overview¶

This unit maps the x86 BIOS Data Area, which is a region of conventional memory at physical address 0x400 that the BIOS populates with hardware configuration data. It also defines a software tick counter (TCounters) that is incremented by the timer ISR on every system tick. The BDA pointer is mapped to the virtual address $C0000400 (physical $400 plus the kernel virtual base), which is valid once paging is active.

Dependencies¶

arch.x86.memory.virtual (implementation)

Types¶

TBDA / PBDA¶

TBDA = bitpacked record
    COM1            : uint16;
    COM2            : uint16;
    COM3            : uint16;
    COM4            : uint16;
    LPT1            : uint16;
    LPT2            : uint16;
    LPT3            : uint16;
    EBDA            : uint16;
    Hardware_Flags  : uint16;
    Keyboard_Flags  : uint16;
    Keyboard_Buffer : ARRAY[0..31] OF uint8;
    Display_Mode    : uint8;
    BaseIO          : uint16;
    Ticks           : uint16;
    HDD_Count       : uint8;
    Keyboard_Start  : uint16;
    Keyboard_End    : uint16;
    Keyboard_State  : uint8;
end;

Maps selected fields of the BIOS Data Area. Notable fields:

Field	Description
COM1–COM4	I/O port base addresses for serial COM ports
LPT1–LPT3	I/O port base addresses for parallel ports
EBDA	Segment address of the Extended BIOS Data Area
Hardware_Flags	Hardware presence flags set by the BIOS POST
Keyboard_Flags	Modifier key state (Shift, Ctrl, Alt, etc.)
Keyboard_Buffer	BIOS keyboard input ring buffer
Display_Mode	Current display mode number
Ticks	Low 16-bit system tick counter incremented by the BIOS timer ISR (INT 8); also updated by `tick_update`
HDD_Count	Number of hard drives detected by the BIOS
Keyboard_Start	Offset of the start of the keyboard buffer within the BDA
Keyboard_End	Offset of the end of the keyboard buffer within the BDA
Keyboard_State	Additional keyboard state flags

TMCFG / PMCFG¶

TMCFG = bitpacked record
    Signature        : Array[0..3] of Char;
    Table_Length     : uint32;
    Revision         : Byte;
    Checksum         : Byte;
    OEM_ID           : Array[0..5] of Byte;
    OEM_Table_ID     : uint64;
    OEM_Revision     : uint32;
    Creator_ID       : uint32;
    Creator_Revision : uint32;
    Reserved         : uint64;
end;

Describes the MCFG ACPI table header, used for PCI Express base address discovery. Defined here for convenience alongside other BIOS-area structures.

TCounters¶

TCounters = record
    c16 : uint16;
    c32 : uint32;
    c64 : uint64;
end;

Software tick counters maintained by tick_update. Provides 16-bit, 32-bit, and 64-bit monotonic counters all incremented synchronously on each timer interrupt.

Constants¶

BDA¶

const BDA : PBDA = PBDA($C0000400);

Pointer to the BIOS Data Area, mapped at the kernel virtual address corresponding to physical address 0x400.

Variables¶

Counters¶

var Counters : TCounters;

Global software tick counters. All three fields (c16, c32, c64) are incremented by tick_update on every timer interrupt.

Functions and Procedures¶

tick_update¶

procedure tick_update(data : void);

Timer ISR callback. Increments BDA^.Ticks, Counters.c16, Counters.c32, and Counters.c64 by one. Intended to be registered as a hook on the timer 0 ISR (arch.x86.isr.tmr0).

Notes¶

The BDA pointer is valid only after paging is enabled and the first 4 MiB of physical memory is mapped to KERNEL_VIRTUAL_BASE. Accessing BDA before paging is active will cause a fault.
Ticks in the BDA is a 16-bit field and will wrap after 65 535 ticks. Code that requires longer time spans should use Counters.c32 or Counters.c64.