Use PCI BIOS Functions for Device Enumeration

PCI BIOS INT 0x1A

Before OS PCI drivers exist, the PCI BIOS extensions (INT 0x1A, AX=0xB101) let real-mode code enumerate devices. You supply a bus/device/function triple and a register offset; the call returns configuration dword data. For example, reading offset 0x00 on bus 0 dev 0 fn 0 yields vendor ID 0x8086 for many Intel host bridges.

Configuration space access

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• AX=0xB101: Read configuration dword
• AX=0xB102: Write configuration dword
• AX=0xB103: Generate special cycle (rare in boot code)
• Not every platform implements PCI BIOS; modern UEFI uses MMIO ECAM instead

Vendor ID 0xFFFF means no device present at that slot.

Availability caveats

Many UEFI-only machines omit PCI BIOS entirely; your code should detect failure via carry and fall back to MMIO ECAM if present. Function numbers above zero appear on multifunction cards like USB controllers. Always enable I/O and memory decode in the command register before expecting BAR accesses to work on real hardware after enumeration.

Why for this exercise

You will enumerate PCI functions and print vendor/device IDs using INT 0x1A. This exercise requires packing bus, device, and function into BX and interpreting the configuration dword the BIOS returns.