Use PCI BIOS Functions for Device Enumeration

Topic Overview

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

Underneath INT 0x1A, every config access is a write to the CONFIG_ADDRESS I/O port that packs bus, device, function, and offset into one dword: 0x80000000 | bus<<16 | dev<<11 | func<<8 | (offset & 0xFC). You will build and decode that dword in both directions: the same bit-packing the PCI BIOS performs before pushing the value to port 0xCF8.

Your Task

Legacy PCI configuration space is reached by writing a 32-bit CONFIG_ADDRESS to I/O port 0xCF8 and then reading or writing the data at 0xCFC. Build and decode that dword:

Input

One command:

• pack BUS DEV FUNC OFFSET: bus, device and function in decimal, the offset in hex (for example 0x3C).
• unpack ADDRESS: a CONFIG_ADDRESS in hex.

Output

• pack: CONFIG_ADDRESS 0x80001010 for pack 0 2 0 0x10, as 8 uppercase hex digits.
• unpack: two lines, for example for unpack 0x8000F818:

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The offset is printed as 2 uppercase hex digits after masking with 0xFC.