Add PCI register read utility

The newly added utility routine reads the content of a known PCI
device's register given the index assigned to the device during
enumeration and the index of the register (and the PCI driver area,
but that is implied).

i386/loader/src-asm/drivers/pci/utility.asm

@@ -0,0 +1,68 @@
+
+; 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