VBE fn 0Ah returns wrong size for VBE2.0 PMI => Fixed in SVN

[RainerZufall]

I recently wrote a small application to display VBE/VESA BIOS mode information and noticed that in VirtualBox the reported size of the VBE Protected Mode Interface table is wrong.

My program reports:

Address of VBE 2.0 PM Interface Table : C000:4600 PM Interface Table Length : 47893 Bytes <= WAY TOO LARGE

This is apparently due to a double subtraction in /src/VBox/Devices/Graphics/BIOS/vberom.asm:

Here, after execution of VBE Function 0Ah cx should contain the table size.

In line 838 cx is loaded with (vesa_pm_end - vesa_pm_start) and thus already contains the table size, but then in line 839 (sub cx, di) di (=offset vesa_pm_start) is again subtracted from cx.

In effect, vesa_pm_start is subtracted twice from vesa_pm_end to calculate the size.

Line 839 (sub cx,di) should simply be removed.

822     ; Function 0Ah - Return VBE Protected Mode Interface
823	;
824	; Input:    AX   = 4F0Ah   VBE 2.0 Protected Mode Interface
825	;           BL   = 00h          Return protected mode table
826	; Output:   AX   =         Status
827	;           ES   =         Real Mode Segment of Table
828	;           DI   =         Offset of Table
829	;           CX   =         Length of Table including protected mode code
830	;                          (for copying purposes)
831	;
832	vbe_biosfn_return_protected_mode_interface:
833	  test bl, bl
834	  jnz _fail
835	  mov di, 0C000h
836	  mov es, di
837	  mov di, offset vesa_pm_start
838	  mov cx, vesa_pm_end - vesa_pm_start
839	  sub cx, di
840	  mov ax, 004Fh
841	  ret
842	_fail:
843	  mov ax, 014fh
844	  ret
845	
846	VGAROM  ends

[michaln]

Yes.