Add option to disable Double Buffering by setting refresh rate < 4fps

README.md

@@ -75,7 +75,7 @@ This table lists the parameters you can specify in the `[VBESVGA.DRV]` section o
 |`Depth` | 8 - 24 | Significant bits per pixel of the desired video mode ("significant" means that padding bits are excluded, so for example if you choose 24, both 24-bit and 32-bit modes will qualify) | 24 |
 |`fontsize` | `small` or `large` | Choose whether to use 96dpi or 120dpi fonts | `small` |
 |`dacdepth` | 6 or 8 | Significant bits to use per colour in 256-colour modes | 6 |
-|`DoubleBufRefreshRate` | 4 - 255 | Number of times per second to swap buffers if [double-buffering](#linear-modes-and-double-buffering) is enabled | 60 |
+|`DoubleBufRefreshRate` | 0 - 255 | Number of times per second to swap buffers if [double-buffering](#linear-modes-and-double-buffering) is enabled; specifying a value less than 4 **disables** double-buffering | 60 |
 |`PMIDcheck` | `disable`, `none`, `sum` or `sanity` | See [below](#protected-mode-interface) | `sum` |
 
 ### Protected-Mode Interface
@@ -108,6 +108,8 @@ When using a linear framebuffer, the driver also attempts to use [Double Bufferi
 
 Basically, if you're using 386 Enhanced Mode (or Standard Mode without `EMM386`), with a modern graphics card and a decent amount of system RAM, then the driver will probably enable Double Buffering. In that case, you can adjust the refresh rate using the `DoubleBufRefreshRate=` setting in `SYSTEM.INI`!
 
+If you suspect there are problems with Double Buffering, you can force-disable it by setting `DoubleBufRefreshRate=0` (or any value less than 4). This can **significantly** degrade performance for certain operations on large screens, but may be useful for debugging...
+
 ## Mode selection
 
 When Windows boots, the driver queries the BIOS for available modes, and automatically selects the first one which fulfills the following criteria:

VBESVGA/src/DBLBUF.ASM

@@ -537,6 +537,11 @@ cBegin
 	jz	@F
 	jmp	setup_done
 
+@@:	; has user force-disabled double buffering?
+	cmp	[desired_fps],4
+	jnb	@F
+	jmp	setup_done
+
 @@:	; Calculate the relevant period
 	mov	ax,1000		; milliseconds per second
 	div	[desired_fps]	; AL := milliseconds per frame, AH := remainder

VBESVGA/src/ENABLE.ASM

@@ -97,7 +97,9 @@ fail_reason	dw	offset general_fail
 bad_width	db	'Width < 640 is not allowed',13,10,'$'
 bad_height	db	'Height < 480 is not allowed',13,10,'$'
 bad_depth	db	'Depth < 8 is not allowed',13,10,'$'
-bad_framerate	db	'Refresh rate must be in range 4-255 fps inclusive',13,10,'$'
+;bad_framerate	db	'Refresh rate must be in range 4-255 fps inclusive',13,10,'$'
+bad_framerate	db	'Refresh rate must be in less than 256 fps',13,10
+		db	'(Note that values less than 4 fps disable double buffering)',13,10,'$'
 general_fail	db	'Unsuitable settings for current driver / hardware',13,10,'$'
 press_key	db	'Press any key to abort booting Windows...',13,10,'$'
 
@@ -403,8 +405,8 @@ enable_read_profile	PROC	NEAR
 	lea	bx,fps_member
 	lea	cx,aszSystemIni 		;ptr to "system.ini" string
         cCall   GetPrivateProfileInt,<ds,ax,ds,bx,60,ds,cx>
-	cmp	al,4
-	rcl	ah,1	
+	;cmp	al,4
+	;rcl	ah,1	
 	test	ah,ah	; AH is non-zero if user went over 255 OR below 4
 	jz	@F
 	mov	[fail_reason],offset bad_framerate