; Arguments:
;   [FURTHER FROM EBP]
;     3.  U16           Register Index
;     2.  U16           Device Index in device list
;     1.  Ptr32         Pointer to PCI driver area
;   [NEAREST TO EBP]
pci_read_register:
    push dword esi
    sub esp, 64
    mov esi, esp

    mov [esi + (64 - 8)], ebx
    mov [esi + (64 - 12)], ecx
    mov [esi + (64 - 16)], edx
    mov [esi + (64 - 20)], edi

.check_device_index:
    ; todo: Boundary checks

.get_device:

    xor         ebx,                    ebx
    mov         bx,                     [ebp - 6]
    shl         ebx,                    4
    ; Point into the driver's device area
    add         ebx,                    [ebp - 4]
    add         ebx,                    PCI_DEVICE_AREA_OFFSET
    mov         [esi],                  ebx

    xor         eax,                    eax
    mov         ax,                     [ebx]
    mov         [esi + 4],              ah
    mov         [esi + 5],              al

.make_address:
    xor eax, eax
    ; Enable Bit
    or al, 1
    ; Make space for 8 Reserved Bits and 8 Bytes of bus number.
    shl eax, 15
    ; Bus Number
    mov al, [esi + 5]
    ; Device Number
    shl eax, 5 ; Make space for device number
    mov dl, [esi + 4]
    and dl, 0x1f
    or al, dl
    shl eax, 9
    or al, [ebp - 8]
    shl eax, 2

.input_output:
    mov dx, PCI_CONFIG_ADDRESS_PORT
    out dx, eax

    mov dx, PCI_CONFIG_DATA_PORT
    in eax, dx

.epilog:
    mov         ebx,                    [esi + (64 - 8)]
    mov         ecx,                    [esi + (64 - 12)]
    mov         edx,                    [esi + (64 - 16)]
    mov         edi,                    [esi + (64 - 20)]

    add         esp,                    64
    pop         dword esi
    ret